# kb

id: rot-proof-20260505-0
file: data/en.wikipedia.org/wiki/Rot-proof-0.md
category: reference
title: Rot-proof
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Rot-proof
date_saved: 2026-05-05T10:53:19.567660+00:00
instance: kb-cron
parent: rot-proof-20260505-0
summary:
Rot-proof or rot resistant is a condition of preservation or protection, by a process or treatment of materials used in industrial manufacturing or production to prevent biodegradation and chemical decomposition. Decomposition is a factor in which organic matter breaks down over time. It is commonly caused by fungus, mold or mildew. There are natural conditions where the environment is inhospitable to animals, bacteria and fungus, for example in high altitude and the freezing subzero temperatures of the Arctic and Antarctic, which creates a similar suspension. The proofing of materials may also prevent dry rot and wet rot. == See also == Dust resistant Fireproofing Rustproofing Thermal resistant Toughness Waterproofing


id: smoke-point-20260505-0
file: data/en.wikipedia.org/wiki/Smoke_point-0.md
category: reference
title: Smoke point
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Smoke_point
date_saved: 2026-05-05T10:53:20.743224+00:00
instance: kb-cron
parent: smoke-point-20260505-0
summary:
The smoke point, also referred to as the burning point, is the temperature at which an oil or fat begins to produce a continuous bluish smoke that becomes clearly visible, dependent upon specific and defined conditions. This happens when one or multiple substances in the oil start to chemically react with oxygen and burn, which can include the oil itself, proteins, sugars, or other organic material. It is distinct from the flash point and fire point, which denote the temperatures at which the oil itself (specifically, vaporized oil, which is distinct from the smoke produced at the smoke point) begins to burn. Smoke point values can vary greatly. The most important factor determining the smoke point of an oil is the amount of proteins and free fatty acids (FFAs). Higher quantities of these lower the smoke point. The FFA content typically represents less than 1% of the total oil and consequently renders smoke point a poor indicator of the capacity of a fat or oil to withstand heat, in a non-cuisine related sense. Virgin (raw) oils, which contain various flavorful organic compounds, have lower smoke points than refined oils because the organic compounds burn. Animal-based fats and oils tend to


id: solubility-20260505-0
file: data/en.wikipedia.org/wiki/Solubility-0.md
category: reference
title: Solubility
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Solubility
date_saved: 2026-05-05T10:53:21.932559+00:00
instance: kb-cron
parent: solubility-20260505-0
summary:
In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution. The extent of the solubility of a substance in a specific solvent is generally measured as the concentration of the solute in a saturated solution, one in which no more solute can be dissolved. At this point, the two substances are said to be at the solubility equilibrium. For some solutes and solvents, there may be no such limit, in which case the two substances are said to be "miscible in all proportions" (or just "miscible"). The solute can be a solid, a liquid, or a gas, while the solvent is usually solid or liquid. Both may be pure substances, or may themselves be solutions. Gases are always miscible in all proportions, except in very extreme situations, and a solid or liquid can be "dissolved" in a gas only by passing into the gaseous state first. The solubility mainly depends on the composition of solute and solvent (including their pH and the presence of other dissolved substances) as well as on temperature and pressure. The


id: solubility-20260505-1
file: data/en.wikipedia.org/wiki/Solubility-1.md
category: reference
title: Solubility
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Solubility
date_saved: 2026-05-05T10:53:21.932559+00:00
instance: kb-cron
parent: solubility-20260505-0
summary:
=== Conversion of solubility values === Conversion between these various ways of measuring solubility may not be trivial, since it may require knowing the density of the solution—which is often not measured, and cannot be predicted. While the total mass is conserved by dissolution, the final volume may be different from both the volume of the solvent and the sum of the two volumes. Moreover, many solids (such as acids and salts) will dissociate in non-trivial ways when dissolved; conversely, the solvent may form coordination complexes with the molecules or ions of the solute. In those cases, the sum of the moles of molecules of solute and solvent is not really the total moles of independent particles solution. To sidestep that problem, the solubility per mole of solution is usually computed and quoted as if the solute does not dissociate or form complexes—that is, by pretending that the mole amount of solution is the sum of the mole amounts of the two substances. == Qualifiers used to describe extent of solubility == The extent of solubility ranges widely, from infinitely soluble (without limit, i.e. miscible) such as ethanol in water, to essentially insoluble, such as titanium dioxide in water. A


id: solubility-20260505-2
file: data/en.wikipedia.org/wiki/Solubility-2.md
category: reference
title: Solubility
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Solubility
date_saved: 2026-05-05T10:53:21.932559+00:00
instance: kb-cron
parent: solubility-20260505-0
summary:
For most solids and liquids, their solubility increases with temperature because their dissolution reaction is endothermic (ΔH > 0). In liquid water at high temperatures, (e.g. that approaching the critical temperature), the solubility of ionic solutes tends to decrease due to the change of properties and structure of liquid water; the lower dielectric constant results in a less polar solvent and in a change of hydration energy affecting the ΔG of the dissolution reaction. Gaseous solutes exhibit more complex behavior with temperature. As the temperature is raised, gases usually become less soluble in water (exothermic dissolution reaction related to their hydration) (to a minimum, which is below 120 °C for most permanent gases), but more soluble in organic solvents (endothermic dissolution reaction related to their solvation). The chart shows solubility curves for some typical solid inorganic salts in liquid water (temperature is in degrees Celsius, i.e. kelvins minus 273.15). Many salts behave like barium nitrate and disodium hydrogen arsenate, and show a large increase in solubility with temperature (ΔH > 0). Some solutes (e.g. sodium chloride in water) exhibit solubility that is fairly independent of temperature (ΔH ≈ 0). A few, such as calcium sulfate (gypsum) and cerium(III) sulfate, become


id: solubility-20260505-3
file: data/en.wikipedia.org/wiki/Solubility-3.md
category: reference
title: Solubility
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Solubility
date_saved: 2026-05-05T10:53:21.932559+00:00
instance: kb-cron
parent: solubility-20260505-0
summary:
== Polarity == A popular aphorism used for predicting solubility is "like dissolves like" also expressed in the Latin language as "Similia similibus solventur". This statement indicates that a solute will dissolve best in a solvent that has a similar chemical structure to itself, based on favorable entropy of mixing. This view is simplistic, but it is a useful rule of thumb. The overall solvation capacity of a solvent depends primarily on its polarity. For example, a very polar (hydrophilic) solute such as urea is very soluble in highly polar water, less soluble in fairly polar methanol, and practically insoluble in non-polar solvents such as benzene. In contrast, a non-polar or lipophilic solute such as naphthalene is insoluble in water, fairly soluble in methanol, and highly soluble in non-polar benzene. In even more simple terms a simple ionic compound (with positive and negative ions) such as sodium chloride (common salt) is easily soluble in a highly polar solvent (with some separation of positive (δ+) and negative (δ-) charges in the covalent molecule) such as water, as thus the sea is salty as it accumulates dissolved salts since early geological ages. The solubility is favored by entropy of mixing (ΔS) and


id: solubility-20260505-4
file: data/en.wikipedia.org/wiki/Solubility-4.md
category: reference
title: Solubility
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Solubility
date_saved: 2026-05-05T10:53:21.932559+00:00
instance: kb-cron
parent: solubility-20260505-0
summary:
== Applications == Solubility is of fundamental importance in a large number of scientific disciplines and practical applications, ranging from ore processing and nuclear reprocessing to the use of medicines, and the transport of pollutants. Solubility is often said to be one of the "characteristic properties of a substance", which means that solubility is commonly used to describe the substance, to indicate a substance's polarity, to help to distinguish it from other substances, and as a guide to applications of the substance. For example, indigo is described as "insoluble in water, alcohol, or ether but soluble in chloroform, nitrobenzene, or concentrated sulfuric acid". Solubility of a substance is useful when separating mixtures. For example, a mixture of salt (sodium chloride) and silica may be separated by dissolving the salt in water, and filtering off the undissolved silica. The synthesis of chemical compounds, by the milligram in a laboratory, or by the ton in industry, both make use of the relative solubilities of the desired product, as well as unreacted starting materials, byproducts, and side products to achieve separation. Another example of this is the synthesis of benzoic acid from phenylmagnesium bromide and dry ice. Benzoic acid is more soluble in


id: solubility-20260505-5
file: data/en.wikipedia.org/wiki/Solubility-5.md
category: reference
title: Solubility
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Solubility
date_saved: 2026-05-05T10:53:21.932559+00:00
instance: kb-cron
parent: solubility-20260505-0
summary:
== Solid solution == This term is often used in the field of metallurgy to refer to the extent that an alloying element will dissolve into the base metal without forming a separate phase. The solvus or solubility line (or curve) is the line (or lines) on a phase diagram that give the limits of solute addition. That is, the lines show the maximum amount of a component that can be added to another component and still be in solid solution. In the solid's crystalline structure, the 'solute' element can either take the place of the matrix within the lattice (a substitutional position; for example, chromium in iron) or take a place in a space between the lattice points (an interstitial position; for example, carbon in iron). In microelectronic fabrication, solid solubility refers to the maximum concentration of impurities one can place into the substrate. In solid compounds (as opposed to elements), the solubility of a solute element can also depend on the phases separating out in equilibrium. For example, amount of Sn soluble in the ZnSb phase can depend significantly on whether the phases separating out in equilibrium are (Zn4Sb3+Sn(L)) or (ZnSnSb2+Sn(L)). Besides these, the ZnSb compound with Sn


id: solubility-20260505-6
file: data/en.wikipedia.org/wiki/Solubility-6.md
category: reference
title: Solubility
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Solubility
date_saved: 2026-05-05T10:53:21.932559+00:00
instance: kb-cron
parent: solubility-20260505-0
summary:
log 10 ⁡ ( S ) = 0.5 − log 10 ⁡ ( P ) − 0.01 ( melting point − 25 ) {\displaystyle \log _{10}(S)=0.5-\log _{10}(P)-0.01({\text{melting point}}-25)} These equations are founded on the principles of the fusion cycle. == See also == Apparent molar property – Difference in properties of one mole of substance in a mixture vs. an ideal solution Biopharmaceutics Classification System – System to differentiate drugs on the basis of their solubility and permeability Dühring's rule – Rule in thermodynamics Fajans–Paneth–Hahn Law – Chemistry rule concerning co-precipitation and adsorption Flexible SPC water model – Aspect of computational chemistryPages displaying short descriptions of redirect targets Henry's law – Gas law regarding proportionality of dissolved gas Hot water extraction – Method of carpet cleaning Hydrotrope – Chemical substance Micellar solubilization – Process of incorporating the solubilizate into or onto micelles Raoult's law – Law of thermodynamics for vapour pressure of a mixture Rate of solution – Capacity of a substance to dissolve in a homogeneous wayPages displaying short descriptions of redirect targets Solubility equilibrium – Thermodynamic equilibrium between a solid and a solution of the same compound Van 't Hoff equation – Relation between temperature and the equilibrium


id: solvation-shell-20260505-0
file: data/en.wikipedia.org/wiki/Solvation_shell-0.md
category: reference
title: Solvation shell
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Solvation_shell
date_saved: 2026-05-05T10:53:23.113007+00:00
instance: kb-cron
parent: solvation-shell-20260505-0
summary:
A solvation shell or solvation sheath is the solvent interface of any chemical compound or biomolecule that constitutes the solute in a solution. When the solvent is water it is called a hydration shell or hydration sphere. The number of solvent molecules surrounding each unit of solute is called the hydration number of the solute. A classic example is when water molecules arrange around a metal ion. If the metal ion is a cation, the electronegative oxygen atom of the water molecule would be attracted electrostatically to the positive charge on the metal ion. The result is a solvation shell of water molecules that surround the ion. This shell can be several molecules thick, dependent upon the charge of the ion, its distribution and spatial dimensions. A number of molecules of solvent are involved in the solvation shell around anions and cations from a dissolved salt in a solvent. Metal ions in aqueous solutions form metal aquo complexes. This number can be determined by various methods like compressibility and NMR measurements among others. == Relation to activity coefficient of an electrolyte and its solvation shell number == The solvation shell number of a dissolved electrolyte can be linked to the


id: specific-rotation-20260505-0
file: data/en.wikipedia.org/wiki/Specific_rotation-0.md
category: reference
title: Specific rotation
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Specific_rotation
date_saved: 2026-05-05T10:53:24.353865+00:00
instance: kb-cron
parent: specific-rotation-20260505-0
summary:
In chemistry, specific rotation ([α]) is a property of a chiral chemical compound. It is defined as the change in orientation of monochromatic plane-polarized light, per unit distance–concentration product, as the light passes through a sample of a compound in solution. Compounds which rotate the plane of polarization of a beam of plane polarized light clockwise are said to be dextrorotary, and correspond with positive specific rotation values, while compounds which rotate the plane of polarization of plane polarized light counterclockwise are said to be levorotary, and correspond with negative values. If a compound is able to rotate the plane of polarization of plane-polarized light, it is said to be “optically active”. Specific rotation is an intensive property, distinguishing it from the more general phenomenon of optical rotation. As such, the observed rotation (α) of a sample of a compound can be used to quantify the enantiomeric excess of that compound, provided that the specific rotation ([α]) for the enantiopure compound is known. The variance of specific rotation with wavelength—a phenomenon known as optical rotatory dispersion—can be used to find the absolute configuration of a molecule. The concentration of bulk sugar solutions is sometimes determined by comparison of the observed


id: speed-of-sound-20260505-0
file: data/en.wikipedia.org/wiki/Speed_of_sound-0.md
category: reference
title: Speed of sound
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Speed_of_sound
date_saved: 2026-05-05T10:53:25.551804+00:00
instance: kb-cron
parent: speed-of-sound-20260505-0
summary:
The speed of sound is the distance travelled per unit of time by a sound wave as it propagates through an elastic medium. More simply, the speed of sound is how fast vibrations travel. At 20 °C (68 °F), the speed of sound in air is about 343 m/s (1,125 ft/s; 1,235 km/h; 767 mph; 667 kn), or 1 km in 2.92 s or one mile in 4.69 s. It depends strongly on temperature as well as the medium through which a sound wave is propagating. At 0 °C (32 °F), the speed of sound in dry air (sea level 14.7 psi) is about 331 m/s (1,086 ft/s; 1,192 km/h; 740 mph; 643 kn). The speed of sound in an ideal gas depends only on its temperature and composition. The speed has a weak dependence on frequency and pressure in dry air, deviating slightly from ideal behavior. In colloquial speech, speed of sound refers to the speed of sound waves in air. The speed of sound varies from substance to substance, however: typically, sound travels most slowly in gases, faster in liquids, and fastest in solids. For example, while sound travels at 343 m/s in air, it travels in fresh


id: speed-of-sound-20260505-1
file: data/en.wikipedia.org/wiki/Speed_of_sound-1.md
category: reference
title: Speed of sound
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Speed_of_sound
date_saved: 2026-05-05T10:53:25.551804+00:00
instance: kb-cron
parent: speed-of-sound-20260505-0
summary:
== History == The Pythagorean Archytas taught that higher pitched sound travels faster, an opinion accepted by some subsequent philosophers, such as those of the Academy and the Peripatos, including possibly Aristotle. Sir Isaac Newton's 1687 Principia includes a computation of the speed of sound in air as 979 feet per second (298 m/s). This is too low by about 15%. The discrepancy is due primarily to neglecting the (then unknown) effect of rapidly fluctuating temperature in a sound wave (in modern terms, sound wave compression and expansion of air is an adiabatic process, not an isothermal process). Newton then invented various fudge factors, such as the "crassitude of the solid particles of the air", until the number agreed with the experimental measurement. Lagrange and Euler both attempted and failed to explain the discrepancy. This discrepancy was finally correctly explained by Pierre-Simon Laplace. In Traité de mécanique céleste, he used the result from the Clément-Desormes experiment of 1819, which measured the heat capacity ratio of air to be 1.35. This produced a near agreement between theory and experiment for the speed of sound. The modern value of 1.40 was found some years later, leading to complete agreement. During the 17th


id: speed-of-sound-20260505-2
file: data/en.wikipedia.org/wiki/Speed_of_sound-2.md
category: reference
title: Speed of sound
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Speed_of_sound
date_saved: 2026-05-05T10:53:25.551804+00:00
instance: kb-cron
parent: speed-of-sound-20260505-0
summary:
== Compression and shear waves == In a gas or liquid, sound consists of compression waves. In solids, waves propagate as two different types. A longitudinal wave is associated with compression and decompression in the direction of travel, and is the same process in gases and liquids, with an analogous compression-type wave in solids. Only compression waves are propagated through fluids (gases and liquids). An additional type of wave, the transverse wave, also called a shear wave, occurs only in solids because only solids support elastic deformations. It is due to elastic deformation of the medium perpendicular to the direction of wave travel; the direction of shear deformation is called the "polarization" of this type of wave. In general, transverse waves occur as a pair of orthogonal polarizations. These different waves (compression waves and the different polarizations of shear waves) may have different speeds at the same frequency. Therefore, they arrive at an observer at different times, an extreme example being an earthquake, where sharp compression waves arrive first and rocking transverse waves seconds later. The speed of a compression wave in a fluid is determined by the medium's compressibility and density. In solids, the compression waves are analogous to


id: speed-of-sound-20260505-3
file: data/en.wikipedia.org/wiki/Speed_of_sound-3.md
category: reference
title: Speed of sound
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Speed_of_sound
date_saved: 2026-05-05T10:53:25.551804+00:00
instance: kb-cron
parent: speed-of-sound-20260505-0
summary:
== Dependence on the properties of the medium == The speed of sound is variable and depends on the properties of the substance through which the wave is travelling. In solids, the speed of transverse (or shear) waves depends on the shear deformation under shear stress (called the shear modulus), and the density of the medium. Longitudinal (or compression) waves in solids depend on the same two factors with the addition of a dependence on compressibility. In fluids, only the medium's compressibility and density are the important factors, since fluids do not transmit shear stresses. In heterogeneous fluids, such as a liquid filled with gas bubbles, the density of the liquid and the compressibility of the gas affect the speed of sound in an additive manner, as demonstrated in the hot chocolate effect. In gases, adiabatic compressibility is directly related to pressure through the heat capacity ratio (adiabatic index), while pressure and density are inversely related to the temperature and molecular weight, thus making only the completely independent properties of temperature and molecular structure important (heat capacity ratio may be determined by temperature and molecular structure, but simple molecular weight is not sufficient to determine it). Sound propagates faster in


id: speed-of-sound-20260505-4
file: data/en.wikipedia.org/wiki/Speed_of_sound-4.md
category: reference
title: Speed of sound
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Speed_of_sound
date_saved: 2026-05-05T10:53:25.551804+00:00
instance: kb-cron
parent: speed-of-sound-20260505-0
summary:
c i d e a l = γ p ρ = γ R T M = γ k T m = 1.380649 ⋅ 10 − 23 ⋅ γ T m ≈ 8.314 γ T M { = 1.9329086 ⋅ 10 − 23 ⋅ T m ≈ 11.640 T M , γ = 7 5 ≈ 2.301 ⋅ 10 − 23 ⋅ T m ≈ 13.857 T M , γ = 5 3 ≈ 1.840 ⋅ 10 − 23 ⋅ T m ≈ 11.086 T M , γ = 4 3 c d r y a i r ≈ 20.04687087513010149970678963 T {\displaystyle {\begin{aligned}c_{\mathrm {ideal} }&={\sqrt {\frac {\gamma p}{\rho }}}={\sqrt {\frac {\gamma RT}{M}}}={\sqrt {\frac {\gamma kT}{m}}}\\&={\sqrt {\frac {1.380649\cdot 10^{-23}\cdot \gamma T}{m}}}\approx {\sqrt {\frac {8.314\gamma T}{M}}}\\&{\begin{cases}&={\sqrt {\frac {1.9329086\cdot 10^{-23}\cdot T}{m}}}\approx {\sqrt {\frac {11.640T}{M}}},&&\gamma ={\frac {7}{5}}\\&\approx {\sqrt {\frac {2.301\cdot 10^{-23}\cdot T}{m}}}\approx {\sqrt {\frac {13.857T}{M}}},&&\gamma ={\frac {5}{3}}\\&\approx {\sqrt {\frac {1.840\cdot 10^{-23}\cdot T}{m}}}\approx {\sqrt {\frac {11.086T}{M}}},&&\gamma ={\frac {4}{3}}\\\end{cases}}\\c_{\mathrm {dry\ air} }&\approx 20.04687087513010149970678963{\sqrt {T}}\end{aligned}}} where cideal is the speed of sound in an ideal gas; p is the pressure; ρ is the density; γ (gamma) is the adiabatic index. At room temperature, where thermal energy is fully partitioned into rotation (rotations are fully excited) but quantum effects


id: speed-of-sound-20260505-5
file: data/en.wikipedia.org/wiki/Speed_of_sound-5.md
category: reference
title: Speed of sound
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Speed_of_sound
date_saved: 2026-05-05T10:53:25.551804+00:00
instance: kb-cron
parent: speed-of-sound-20260505-0
summary:
=== Effects due to wind shear === The speed of sound varies with temperature. Since temperature and sound velocity normally decrease with increasing altitude, sound is refracted upward, away from listeners on the ground, creating an acoustic shadow at some distance from the source. Wind shear of 4 m/(s · km) can produce refraction equal to a typical temperature lapse rate of 7.5 °C/km. Higher values of wind gradient will refract sound downward toward the surface in the downwind direction, eliminating the acoustic shadow on the downwind side. This will increase the audibility of sounds downwind. This downwind refraction effect occurs because there is a wind gradient; the fact that sound is carried along by the wind is not important. For sound propagation, the exponential variation of wind speed with height can be defined as follows: U ( h ) = U ( 0 ) h ζ , d U d h ( h ) = ζ U ( h ) h , {\displaystyle {\begin{aligned}U(h)&=U(0)h^{\zeta },\\{\frac {\mathrm {d} U}{\mathrm {d} h}}(h)&=\zeta {\frac {U(h)}{h}},\end{aligned}}} where U(h) is the speed of the wind at height h; ζ is the exponential coefficient based on ground surface roughness, typically between 0.08 and 0.52; dU/dh(h)


id: speed-of-sound-20260505-6
file: data/en.wikipedia.org/wiki/Speed_of_sound-6.md
category: reference
title: Speed of sound
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Speed_of_sound
date_saved: 2026-05-05T10:53:25.551804+00:00
instance: kb-cron
parent: speed-of-sound-20260505-0
summary:
=== Practical application to air === By far, the most important factor influencing the speed of sound in air is temperature. The speed is proportional to the square root of the absolute temperature, giving an increase of about 0.6 m/s per degree Celsius. For this reason, the pitch of a musical wind instrument increases as its temperature increases. The speed of sound is raised by humidity. The difference between 0% and 100% humidity is about 1.5 m/s at standard pressure and temperature, but the size of the humidity effect increases dramatically with temperature. The dependence on frequency and pressure are normally insignificant in practical applications. In dry air, the speed of sound increases by about 0.1 m/s as the frequency rises from 10 Hz to 100 Hz. For audible frequencies above 100 Hz it is relatively constant. Standard values of the speed of sound are quoted in the limit of low frequencies, where the wavelength is large compared to the mean free path. As shown above, the approximate value 1000/3 = 333.33... m/s is exact a little below 5 °C and is a good approximation for all "usual" outside temperatures (in temperate climates, at least), hence the usual rule of


id: speed-of-sound-20260505-7
file: data/en.wikipedia.org/wiki/Speed_of_sound-7.md
category: reference
title: Speed of sound
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Speed_of_sound
date_saved: 2026-05-05T10:53:25.551804+00:00
instance: kb-cron
parent: speed-of-sound-20260505-0
summary:
==== Three-dimensional solids ==== In a solid, there is a non-zero stiffness both for volumetric deformations and shear deformations. Hence, it is possible to generate sound waves with different velocities dependent on the deformation mode. Sound waves generating volumetric deformations (compression) and shear deformations (shearing) are called pressure waves (longitudinal waves) and shear waves (transverse waves), respectively. In earthquakes, the corresponding seismic waves are called P-waves (primary waves) and S-waves (secondary waves), respectively. The sound velocities of these two types of waves propagating in a homogeneous 3-dimensional solid are respectively given by c solid,p = K + 4 3 G ρ = E ( 1 − ν ) ρ ( 1 + ν ) ( 1 − 2 ν ) , {\displaystyle c_{\text{solid,p}}={\sqrt {\frac {K+{\frac {4}{3}}G}{\rho }}}={\sqrt {\frac {E(1-\nu )}{\rho (1+\nu )(1-2\nu )}}},} c solid,s = G ρ , {\displaystyle c_{\text{solid,s}}={\sqrt {\frac {G}{\rho }}},} where K is the bulk modulus of the elastic materials; G is the shear modulus of the elastic materials; E is the Young's modulus; ρ is the density; ν is Poisson's ratio. The last quantity is not an independent one, as E = 3K(1 − 2ν). The speed of pressure waves depends both on the pressure


id: speed-of-sound-20260505-8
file: data/en.wikipedia.org/wiki/Speed_of_sound-8.md
category: reference
title: Speed of sound
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Speed_of_sound
date_saved: 2026-05-05T10:53:25.551804+00:00
instance: kb-cron
parent: speed-of-sound-20260505-0
summary:
with check value 1550.744 m/s for T = 25 °C, S = 35 parts per thousand, z = 1,000 m. This equation has a standard error of 0.070 m/s for salinity between 25 and 40 ppt. See Technical Guides - Speed of sound in sea water for an online calculator. (The Sound Speed vs. Depth graph does not correlate directly to the MacKenzie formula. This is due to the fact that the temperature and salinity varies at different depths. When T and S are held constant, the formula itself is always increasing with depth.) Other equations for the speed of sound in sea water are accurate over a wide range of conditions, but are far more complicated, e.g., that by V. A. Del Grosso and the Chen-Millero-Li Equation. === Speed of sound in plasma === The speed of sound in a plasma for the common case that the electrons are hotter than the ions (but not too much hotter) is given by the formula (see here) c s = ( γ Z k T e m i ) 1 / 2 = ( γ Z T e μ ) 1 / 2 × 90.85 m / s , {\displaystyle c_{s}=\left({\frac


id: standard-atomic-weight-20260505-0
file: data/en.wikipedia.org/wiki/Standard_atomic_weight-0.md
category: reference
title: Standard atomic weight
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Standard_atomic_weight
date_saved: 2026-05-05T10:53:26.735765+00:00
instance: kb-cron
parent: standard-atomic-weight-20260505-0
summary:
The standard atomic weight of a chemical element (symbol Ar°(E) for element "E") is the weighted arithmetic mean of the relative isotopic masses of all isotopes of that element weighted by each isotope's abundance on Earth. For example, isotope 63Cu (Ar = 62.929) constitutes 69% of the copper on Earth, the rest being 65Cu (Ar = 64.927), so A r ° ( 29 Cu ) = 0.69 × 62.929 + 0.31 × 64.927 = 63.55. {\displaystyle A_{\text{r}}{\text{°}}(_{\text{29}}{\text{Cu}})=0.69\times 62.929+0.31\times 64.927=63.55.} Relative isotopic mass is dimensionless, and so is the weighted average. It can be converted into a measure of mass (with dimension M) by multiplying it with the atomic mass constant dalton. Among various variants of the notion of atomic weight (Ar, also known as relative atomic mass) used by scientists, the standard atomic weight (Ar°) is the most common and practical. The standard atomic weight of each chemical element is determined and published by the Commission on Isotopic Abundances and Atomic Weights (CIAAW) of the International Union of Pure and Applied Chemistry (IUPAC) based on natural, stable, terrestrial sources of the element. The definition specifies the use of samples from many representative sources from the Earth, so that the value


id: standard-atomic-weight-20260505-1
file: data/en.wikipedia.org/wiki/Standard_atomic_weight-1.md
category: reference
title: Standard atomic weight
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Standard_atomic_weight
date_saved: 2026-05-05T10:53:26.735765+00:00
instance: kb-cron
parent: standard-atomic-weight-20260505-0
summary:
=== Causes of uncertainty on Earth === Famously, the published atomic weight value comes with an uncertainty. This uncertainty (and related: precision) follows from its definition, the source being "terrestrial and stable". Systematic causes for uncertainty are: Measurement limits. As always, the physical measurement is never finite. There is always more detail to be found and read. This applies to every single, pure isotope found. For example, today the mass of the main natural fluorine isotope (fluorine-19) can be measured to the accuracy of eleven decimal places: 18.998403163(6). But a still more precise measurement system could become available, producing more decimals. Imperfect mixtures of isotopes. In the samples taken and measured the mix (relative abundance) of those isotopes may vary. For example, copper. While in general its two isotopes make out 69.15% and 30.85% each of all copper found, the natural sample being measured can have had an incomplete 'stirring' and so the percentages are different. The precision is improved by measuring more samples of course, but there remains this cause of uncertainty. (Example: lead samples vary so much, it can not be noted more precise than four figures: 207.2) Earthly sources with a different history. A source is the


id: standard-atomic-weight-20260505-2
file: data/en.wikipedia.org/wiki/Standard_atomic_weight-2.md
category: reference
title: Standard atomic weight
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Standard_atomic_weight
date_saved: 2026-05-05T10:53:26.735765+00:00
instance: kb-cron
parent: standard-atomic-weight-20260505-0
summary:
== Published values == IUPAC, through its Commission on Isotopic Abundances and Atomic Weights (CIAAW), publishes a formal value, called the standard atomic weight for each element that has at least one stable isotope or a well-characterised terrestrial isotopic composition. Updates are made only when justified by new high-precision measurements; they are no longer issued on a fixed biennial schedule. The most recent revisions (January 2025) affected three elements: gadolinium, lutetium, and zirconium. The published value may be given with an uncertainty, e.g. neon: 20.1797(6), or as an interval, e.g. boron: [10.806, 10.821]. Currently 83 elements have an assigned standard atomic weight. In addition to the full-precision values, CIAAW publishes abridged standard atomic weights (to five significant digits) and, for the twelve elements whose atomic weights are expressed as intervals, single-number conventional atomic-weight values for use when isotopic variation is unimportant. The symbol Ar(E) denotes the relative atomic mass of a particular sample of element E, whereas the standard atomic weight is denoted Ar°(E) (the circle ° indicating the defined standard-state composition). === Abridged atomic weight === The abridged atomic weight, also published by CIAAW, is derived from the standard atomic weight, reducing the numbers to five digits (five significant


id: standard-molar-entropy-20260505-0
file: data/en.wikipedia.org/wiki/Standard_molar_entropy-0.md
category: reference
title: Standard molar entropy
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Standard_molar_entropy
date_saved: 2026-05-05T10:53:27.947075+00:00
instance: kb-cron
parent: standard-molar-entropy-20260505-0
summary:
In chemistry, the standard molar entropy is the entropy content of one mole of pure substance at a standard state of pressure and any temperature of interest. These are often (but not necessarily) chosen to be the standard temperature and pressure. The standard molar entropy at pressure = P 0 {\displaystyle P^{0}} is usually given the symbol S°, and has units of joules per mole per kelvin (J⋅mol−1⋅K−1). Unlike standard enthalpies of formation, the value of S° is absolute. That is, an element in its standard state has a definite, nonzero value of S at room temperature. The entropy of a pure crystalline structure can be 0 J⋅mol−1⋅K−1 only at 0 K, according to the third law of thermodynamics. However, this assumes that the material forms a 'perfect crystal' without any residual entropy. This can be due to crystallographic defects, dislocations, and/or incomplete rotational quenching within the solid, as originally pointed out by Linus Pauling. These contributions to the entropy are always present, because crystals always grow at a finite rate and at temperature. However, the residual entropy is often quite negligible and can be accounted for when it occurs using statistical mechanics. == Thermodynamics == If a mole of


id: superhydrophilicity-20260505-0
file: data/en.wikipedia.org/wiki/Superhydrophilicity-0.md
category: reference
title: Superhydrophilicity
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Superhydrophilicity
date_saved: 2026-05-05T10:53:29.121794+00:00
instance: kb-cron
parent: superhydrophilicity-20260505-0
summary:
Superhydrophilicity refers to the phenomenon of excess hydrophilicity, or attraction to water; in superhydrophilic materials, the contact angle of water is equal to zero degrees. This effect was discovered in 1995 by the Research Institute of Toto Ltd. for titanium dioxide irradiated by sunlight. Under light irradiation, water dropped onto titanium dioxide forms no contact angle (almost 0 degrees). Superhydrophilic material has various advantages. For example, it can defog glass, and it can also enable oil spots to be swept away easily with water. Such materials are already commercialized as door mirrors for cars, coatings for buildings, self-cleaning glass, etc. Several mechanisms of this superhydrophilicity have been proposed by researchers. One is the change of the surface structure to a metastable structure, and another is cleaning the surface by the photodecomposition of dirt such as organic compounds adsorbed on the surface, after either of which water molecules can adsorb to the surface. The mechanism is still controversial, and it is too soon to decide which suggestion is correct. To decide, atomic scale measurements and other studies will be necessary. == See also == Superhydrophobicity, the opposite phenomenon == References == == Further reading == Kommireddy, Dinesh S.; Patel, Amish A.;


id: tactoid-20260505-0
file: data/en.wikipedia.org/wiki/Tactoid-0.md
category: reference
title: Tactoid
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Tactoid
date_saved: 2026-05-05T10:53:30.321955+00:00
instance: kb-cron
parent: tactoid-20260505-0
summary:
Tactoids are liquid crystal microdomains nucleated in isotropic phases, which can be distinguished as spherical or spindle-shaped birefringent microdroplets under polarized light microscopy. Tactoids are a transition state between isotropic and macroscopic liquid crystalline phases. The first observation of tactoids was made by Zocher in 1925, when he studied the nematic phase formed in vanadium pentoxide sols. After that, tactoids have been found in the phase transition processes in many lyotropic liquid crystalline substances, such as tobacco mosaic virus, polypeptides, and cellulose nanocrystals. == In biology == It has been shown that filamin causes actin to condense into tactoids. The filamentous phage Pf4 generates a tactoid shell around host P. aeruginosa cells that confer antibiotic resistance. == References ==


id: list-of-thermal-conductivities-20260505-0
file: data/en.wikipedia.org/wiki/List_of_thermal_conductivities-0.md
category: reference
title: List of thermal conductivities
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/List_of_thermal_conductivities
date_saved: 2026-05-05T10:53:33.094384+00:00
instance: kb-cron
parent: list-of-thermal-conductivities-20260505-0
summary:
In heat transfer, the thermal conductivity of a substance, k, is an intensive property that indicates its ability to conduct heat. For most materials, the amount of heat conducted varies (usually non-linearly) with temperature. Thermal conductivity is often measured with laser flash analysis. Alternative measurements are also established. Mixtures may have variable thermal conductivities due to composition. Note that for gases in usual conditions, heat transfer by advection (caused by convection or turbulence for instance) is the dominant mechanism compared to conduction. This table shows thermal conductivity in SI units of watts per metre-kelvin (W·m−1·K−1). Some measurements use the imperial unit BTUs per foot per hour per degree Fahrenheit (1 BTU h−1 ft−1 F−1 = 1.728 W·m−1·K−1). == Sortable list == This concerns materials at atmospheric pressure and around 293 K (20 °C). == Analytical list == Thermal conductivities have been measured with longitudinal heat flow methods where the experimental arrangement is so designed to accommodate heat flow in only the axial direction, temperatures are constant, and radial heat loss is prevented or minimized. For the sake of simplicity the conductivities that are found by that method in all of its variations are noted as L conductivities, those that are


id: transferability-chemistry-20260505-0
file: data/en.wikipedia.org/wiki/Transferability_(chemistry)-0.md
category: reference
title: Transferability (chemistry)
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Transferability_(chemistry)
date_saved: 2026-05-05T10:53:34.363641+00:00
instance: kb-cron
parent: transferability-chemistry-20260505-0
summary:
In chemistry, transferability is the assumption that a chemical property that is associated with an atom or a functional group in a molecule will have a similar (but not identical) value in a variety of different circumstances. Examples of transferable properties include: Electronegativity Nucleophilicity Chemical shifts in NMR spectroscopy Characteristic frequencies in Infrared spectroscopy Bond length and bond angle Bond energy Transferable properties are distinguished from conserved properties, which are assumed to always have the same value whatever the chemical situation, e.g. standard atomic weight. == References ==


id: triple-point-20260505-0
file: data/en.wikipedia.org/wiki/Triple_point-0.md
category: reference
title: Triple point
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Triple_point
date_saved: 2026-05-05T10:53:35.612527+00:00
instance: kb-cron
parent: triple-point-20260505-0
summary:
In thermodynamics, the triple point of a substance is the temperature and pressure at which the three phases (gas, liquid, and solid) of that substance coexist in thermodynamic equilibrium. It is that temperature and pressure at which the sublimation, fusion, and vapourisation curves meet. For example, the triple point of mercury occurs at a temperature of −38.8 °C (−37.8 °F) and a pressure of 0.165 mPa. In addition to the triple point for solid, liquid, and gas phases, a triple point may involve more than one solid phase, for substances with multiple polymorphs. Helium-4 is unusual in that it has no sublimation/deposition curve and therefore no triple points where its solid phase meets its gas phase. Instead, it has a vapor-liquid-superfluid point, a solid-liquid-superfluid point, a solid-solid-liquid point, and a solid-solid-superfluid point. None of these should be confused with the lambda point, which is not any kind of triple point. The first mention of the term "triple point" was on August 3, 1871 by James Thomson, brother of Lord Kelvin. The triple points of several substances are used to define points in the ITS-90 international temperature scale, ranging from the triple point of hydrogen (13.8033 K) to the triple point


id: true-vapor-pressure-20260505-0
file: data/en.wikipedia.org/wiki/True_vapor_pressure-0.md
category: reference
title: True vapor pressure
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/True_vapor_pressure
date_saved: 2026-05-05T10:53:36.887782+00:00
instance: kb-cron
parent: true-vapor-pressure-20260505-0
summary:
True vapor pressure (TVP) is a common measure of the volatility of petroleum distillate fuels. It is defined as the equilibrium partial pressure exerted by a volatile organic liquid as a function of temperature as determined by the test method ASTM D 2879. The true vapor pressure (TVP) at 100 °F differs slightly from the Reid vapor pressure (RVP) (per definition also at 100 °F), as it excludes dissolved fixed gases such as air. Conversions between the two can be found in AP 42, Fifth Edition, Volume I Chapter 7: Liquid Storage Tanks (p 7.1-54 and onwards) == References == == External links == ASTM D2879 - 97(2007) Standard Test Method for Vapor Pressure-Temperature Relationship and Initial Decomposition Temperature of Liquids by Isoteniscope USA's Environmental Protection Agency (EPA) publication AP-42, Compilation of Air Pollutant Emissions. Chapter 7


id: ultrahydrophobicity-20260505-0
file: data/en.wikipedia.org/wiki/Ultrahydrophobicity-0.md
category: reference
title: Ultrahydrophobicity
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Ultrahydrophobicity
date_saved: 2026-05-05T10:53:38.147986+00:00
instance: kb-cron
parent: ultrahydrophobicity-20260505-0
summary:
In chemistry and materials science, ultrahydrophobic (or superhydrophobic) surfaces are highly hydrophobic, i.e., extremely difficult to wet. The contact angles of a water droplet on an ultrahydrophobic material exceed 150°. This is also referred to as the lotus effect, after the superhydrophobic leaves of the lotus plant. A droplet striking these kinds of surfaces can fully rebound like an elastic ball. Interactions of bouncing drops can be further reduced using special superhydrophobic surfaces that promote symmetry breaking, pancake bouncing or waterbowl bouncing. == Theory == In 1805, Thomas Young defined the contact angle θ by analysing the forces acting on a fluid droplet resting on a smooth solid surface surrounded by a gas. γ S G = γ S L + γ L G cos ⁡ θ {\displaystyle \gamma _{SG}\ =\gamma _{SL}+\gamma _{LG}\cos {\theta }} where γ S G {\displaystyle \gamma _{SG}\ } = Interfacial tension between the solid and gas γ S L {\displaystyle \gamma _{SL}\ } = Interfacial tension between the solid and liquid γ L G {\displaystyle \gamma _{LG}\ } = Interfacial tension between the liquid and gas θ can be measured using a contact angle goniometer. Wenzel determined that when the liquid is in intimate contact


id: ultrahydrophobicity-20260505-1
file: data/en.wikipedia.org/wiki/Ultrahydrophobicity-1.md
category: reference
title: Ultrahydrophobicity
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Ultrahydrophobicity
date_saved: 2026-05-05T10:53:38.147986+00:00
instance: kb-cron
parent: ultrahydrophobicity-20260505-0
summary:
θ a = λ p ( θ a , 0 + w ) + ( 1 − λ p ) θ a i r {\displaystyle \theta _{a}=\lambda _{p}(\theta _{a,0}+w)+(1-\lambda _{p})\theta _{air}} θ r = λ p θ r , 0 + ( 1 − λ p ) θ a i r {\displaystyle \theta _{r}=\lambda _{p}\theta _{r,0}+(1-\lambda _{p})\theta _{air}} with also the Wenzel state: θ a = λ p ( θ a , 0 + w ) + ( 1 − λ p ) θ a , 0 {\displaystyle \theta _{a}=\lambda _{p}(\theta _{a,0}+w)+(1-\lambda _{p})\theta _{a,0}} θ r = λ p ( θ r , 0 − w ) + ( 1 − λ p ) θ r , 0 {\displaystyle \theta _{r}=\lambda _{p}(\theta _{r,0}-w)+(1-\lambda _{p})\theta _{r,0}} where λp = linear fraction of contact line on the asperities θr,0 = receding contact angle of a smooth substrate θair = contact angle between liquid and air (typically assumed to be 180°) == Unitary versus hierarchical roughness structures == M. Nosonovsky and B. Bhushan studied the effect of unitary (non-hierarchical) structures of micro and nano roughness, and hierarchical structures (micro roughness covered with nano roughness). They found that hierarchical structure was not only necessary


id: ultrahydrophobicity-20260505-2
file: data/en.wikipedia.org/wiki/Ultrahydrophobicity-2.md
category: reference
title: Ultrahydrophobicity
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Ultrahydrophobicity
date_saved: 2026-05-05T10:53:38.147986+00:00
instance: kb-cron
parent: ultrahydrophobicity-20260505-0
summary:
Dettre and Johnson discovered in 1964 that the superhydrophobic lotus effect phenomenon was related to rough hydrophobic surfaces, and they developed a theoretical model based on experiments with glass beads coated with paraffin or TFE telomer. The self-cleaning property of superhydrophobic micro-nanostructured surfaces was reported in 1977. Perfluoroalkyl, perfluoropolyether and RF plasma formed superhydrophobic materials were developed, used for electrowetting and commercialized for bio-medical applications between 1986 and 1995. Other technology and applications have emerged since the mid-1990s. A durable superhydrophobic hierarchical composition, applied in one or two steps, was disclosed in 2002 comprising nano-sized particles ≤ 100 nanometers overlaying a surface having micrometer-sized features or particles ≤ 100 µm. The larger particles were observed to protect the smaller particles from mechanical abrasion. Durable, optically transparent superhydrophobic and oleophobic coatings were developed in 2012 comprising nano particles in the 10 to 100 nm size range. Research in superhydrophobicity recently accelerated with a letter that reported man-made superhydrophobic samples produced by allowing alkylketene dimer (AKD) to solidify into a nanostructured fractal surface. Many papers have since presented fabrication methods for producing superhydrophobic surfaces including particle deposition, sol-gel techniques, plasma treatments, vapor deposition, and casting techniques. Current opportunity for research impact lies


id: ultrahydrophobicity-20260505-3
file: data/en.wikipedia.org/wiki/Ultrahydrophobicity-3.md
category: reference
title: Ultrahydrophobicity
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Ultrahydrophobicity
date_saved: 2026-05-05T10:53:38.147986+00:00
instance: kb-cron
parent: ultrahydrophobicity-20260505-0
summary:
== Potential applications == Active recent research on superhydrophobic materials might eventually lead to industrial applications. Some attempts at fabricating a superhydrophobic surface include mimicking a lotus leaf surface, namely the two-tiered characteristic. This requires micro-scale surfaces with typically nanoscale features on top of them. For example, a simple routine of coating cotton fabric with silica or titania particles by sol-gel technique has been reported, which protects the fabric from UV light and makes it superhydrophobic. Similarly, silica nanoparticles can be deposited on top of already hydrophobic carbon fabric. The carbon fabric by itself is identified as inherently hydrophobic, but not distinguished as superhydrophobic since its contact angle is not higher than 150°. With the adhesion of silica nanoparticles, contact angles as high as 162° are achieved. Using silica nano-particles is also of interest to develop transparent hydrophobic materials for car windshields and self-cleaning windows. By coating an already transparent surface with nano-silica with about 1 wt%, droplet contact angles can be raised up to 168° with a 12° sliding angle. An efficient routine has been reported for making linear low-density polyethylene superhydrophobic and thus self-cleaning; 99% of dirt deposited on such a surface is easily washed away. Patterned superhydrophobic


id: valence-chemistry-20260505-0
file: data/en.wikipedia.org/wiki/Valence_(chemistry)-0.md
category: reference
title: Valence (chemistry)
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Valence_(chemistry)
date_saved: 2026-05-05T10:53:39.330202+00:00
instance: kb-cron
parent: valence-chemistry-20260505-0
summary:
In chemistry, the valence (US spelling) or valency (British spelling) of an atom is a measure of its combining capacity with other atoms when it forms chemical compounds or molecules. Valence is generally understood to be the number of chemical bonds that each atom of a given chemical element typically forms. Double bonds are considered to be two bonds, triple bonds to be three, quadruple bonds to be four, quintuple bonds to be five and sextuple bonds to be six. In most compounds, the valence of hydrogen is 1, of oxygen is 2, of nitrogen is 3, and of carbon is 4. Valence is not to be confused with the related concepts of the coordination number, the oxidation state, or the number of valence electrons for a given atom. == Description == The valence is the combining capacity of an atom of a given element, determined by the number of hydrogen atoms that it combines with. In methane, carbon has a valence of 4; in ammonia, nitrogen has a valence of 3; in water, oxygen has a valence of 2; and in hydrogen chloride, chlorine has a valence of 1. Chlorine, as it has a valence of one, can be


id: valence-chemistry-20260505-1
file: data/en.wikipedia.org/wiki/Valence_(chemistry)-1.md
category: reference
title: Valence (chemistry)
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Valence_(chemistry)
date_saved: 2026-05-05T10:53:39.330202+00:00
instance: kb-cron
parent: valence-chemistry-20260505-0
summary:
valence = number of bonds + formal charge. In this convention, the nitrogen in an ammonium ion [NH4]+ bonds to four hydrogen atoms, but it is considered to be pentavalent because all five of nitrogen's valence electrons participate in the bonding. === Electrons and valence === The Rutherford model of the nuclear atom (1911) showed that the exterior of an atom is occupied by electrons, which suggests that electrons are responsible for the interaction of atoms and the formation of chemical bonds. In 1916, Gilbert N. Lewis explained valence and chemical bonding in terms of a tendency of (main-group) atoms to achieve a stable octet of 8 valence-shell electrons. According to Lewis, covalent bonding leads to octets by the sharing of electrons, and ionic bonding leads to octets by the transfer of electrons from one atom to the other. The term covalence is attributed to Irving Langmuir, who stated in 1919 that "the number of pairs of electrons which any given atom shares with the adjacent atoms is called the covalence of that atom". The prefix co- means "together", so that a co-valent bond means that the atoms share a valence. Subsequent to that, it is now more common to


id: valence-chemistry-20260505-2
file: data/en.wikipedia.org/wiki/Valence_(chemistry)-2.md
category: reference
title: Valence (chemistry)
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Valence_(chemistry)
date_saved: 2026-05-05T10:53:39.330202+00:00
instance: kb-cron
parent: valence-chemistry-20260505-0
summary:
* The perchlorate ion ClO−4 is monovalent, in other words, it has valence 1. ** Valences may also be different from absolute values of oxidation states due to different polarity of bonds. For example, in dichloromethane, CH2Cl2, carbon has valence 4 but oxidation state 0. *** Iron oxides appear in a crystal structure, so no typical molecule can be identified. In ferrous oxide, Fe has oxidation state +2; in ferric oxide, oxidation state +3. == "Maximum number of bonds" definition == Frankland took the view that the valence (he used the term "atomicity") of an element was a single value that corresponded to the maximum value observed. The number of unused valencies on atoms of what are now called the p-block elements is generally even, and Frankland suggested that the unused valencies saturated one another. For example, nitrogen has a maximum valence of 5, in forming ammonia two valencies are left unattached; sulfur has a maximum valence of 6, in forming hydrogen sulphide four valencies are left unattached. The International Union of Pure and Applied Chemistry (IUPAC) has made several attempts to arrive at an unambiguous definition of valence. The current version, adopted in 1994: The maximum number of univalent


id: van-der-waals-radius-20260505-0
file: data/en.wikipedia.org/wiki/Van_der_Waals_radius-0.md
category: reference
title: Van der Waals radius
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Van_der_Waals_radius
date_saved: 2026-05-05T10:53:40.589568+00:00
instance: kb-cron
parent: van-der-waals-radius-20260505-0
summary:
The van der Waals radius, rw, of an atom is the radius of an imaginary hard sphere representing the distance of closest approach for another atom. It is named after Johannes Diderik van der Waals, winner of the 1910 Nobel Prize in Physics, as he was the first to recognise that atoms were not simply points and to demonstrate the physical consequences of their size through the van der Waals equation of state. == van der Waals volume == The van der Waals volume, Vw, also called the atomic volume or molecular volume, is the atomic property most directly related to the van der Waals radius. It is the volume "occupied" by an individual atom (or molecule). The van der Waals volume may be calculated if the van der Waals radii (and, for molecules, the inter-atomic distances, and angles) are known. For a single atom, it is the volume of a sphere whose radius is the van der Waals radius of the atom: V w = 4 3 π r w 3 . {\displaystyle V_{\rm {w}}={4 \over 3}\pi r_{\rm {w}}^{3}.} For a molecule, it is the volume enclosed by the van der Waals surface. The van der Waals volume of


id: van-der-waals-radius-20260505-1
file: data/en.wikipedia.org/wiki/Van_der_Waals_radius-1.md
category: reference
title: Van der Waals radius
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Van_der_Waals_radius
date_saved: 2026-05-05T10:53:40.589568+00:00
instance: kb-cron
parent: van-der-waals-radius-20260505-0
summary:
V w = 4 3 π r w 3 + π r w 2 d {\displaystyle V_{\rm {w}}={4 \over 3}\pi r_{\rm {w}}^{3}+\pi r_{\rm {w}}^{2}d} can be solved by the normal methods for cubic functions. === Crystallographic measurements === The molecules in a molecular crystal are held together by van der Waals forces rather than chemical bonds. In principle, the closest that two atoms belonging to different molecules can approach one another is given by the sum of their van der Waals radii. By examining a large number of structures of molecular crystals, it is possible to find a minimum radius for each type of atom such that other non-bonded atoms do not encroach any closer. This approach was first used by Linus Pauling in his seminal work The Nature of the Chemical Bond. Arnold Bondi also conducted a study of this type, published in 1964, although he also considered other methods of determining the van der Waals radius in coming to his final estimates. Some of Bondi's figures are given in the table at the top of this article, and they remain the most widely used "consensus" values for the van der Waals radii of the elements. Scott Rowland and


id: vapor-20260505-0
file: data/en.wikipedia.org/wiki/Vapor-0.md
category: reference
title: Vapor
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Vapor
date_saved: 2026-05-05T10:53:41.866013+00:00
instance: kb-cron
parent: vapor-20260505-0
summary:
In physics, a vapor (American English) or vapour (Commonwealth English; see spelling differences) is a substance in the gas phase at a temperature lower than its critical temperature, which means that the vapor can be condensed to a liquid by increasing the pressure on it without reducing the temperature of the vapor. A vapor is different from an aerosol. An aerosol is a suspension of tiny particles of liquid, solid, or both within a gas. For example, water has a critical temperature of 647 K (374 °C; 705 °F), which is the highest temperature at which liquid water can exist at any pressure. In the atmosphere at ordinary temperatures gaseous water (known as water vapor) will condense into a liquid if its partial pressure is increased sufficiently. A vapor may coexist with a liquid (or a solid). When this is true, the two phases will be in equilibrium, and the gas-partial pressure will be equal to the equilibrium vapor pressure of the liquid (or solid). == Properties == Vapor refers to a gas phase at a temperature where the same substance can also exist in the liquid or solid state, below the critical temperature of the substance. (For example, water


id: volatility-chemistry-20260505-0
file: data/en.wikipedia.org/wiki/Volatility_(chemistry)-0.md
category: reference
title: Volatility (chemistry)
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Volatility_(chemistry)
date_saved: 2026-05-05T10:53:43.101348+00:00
instance: kb-cron
parent: volatility-chemistry-20260505-0
summary:
In chemistry, volatility is a material quality which describes how readily a substance vaporizes. At a given temperature and pressure, a substance with high volatility is more likely to exist as a vapour, while a substance with low volatility is more likely to be a liquid or solid. Volatility can also describe the tendency of a vapor to condense into a liquid or solid; less volatile substances will more readily condense from a vapor than highly volatile ones. Differences in volatility can be observed by comparing how fast substances within a group evaporate (or sublimate in the case of solids) when exposed to the atmosphere. A highly volatile substance such as rubbing alcohol (isopropyl alcohol) will quickly evaporate, while a substance with low volatility such as vegetable oil will remain condensed. In general, solids are much less volatile than liquids, but there are some exceptions. Solids that sublimate (change directly from solid to vapor) such as dry ice (solid carbon dioxide) or iodine can vaporize at a similar rate as some liquids under standard conditions. == Description == Volatility itself has no defined numerical value, but it is often described using vapor pressures or boiling points (for liquids). High vapor


id: adipocere-20260505-0
file: data/en.wikipedia.org/wiki/Adipocere-0.md
category: reference
title: Adipocere
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Adipocere
date_saved: 2026-05-05T10:53:44.708716+00:00
instance: kb-cron
parent: adipocere-20260505-0
summary:
Adipocere (), also known as corpse wax, grave wax or mortuary wax, is a wax-like organic substance formed by the anaerobic bacterial hydrolysis of fat in tissue, such as body fat in corpses. In its formation, putrefaction is replaced by a permanent firm cast of fatty tissues, internal organs, and the face. == History == Adipocere was first described by Sir Thomas Browne in his discourse Hydriotaphia, Urn Burial (1658): In a Hydropicall body ten years buried in a Church-yard, we met with a fat concretion, where the nitre of the Earth, and the salt and lixivious liquor of the body, had coagulated large lumps of fat, into the consistence of the hardest castile-soap: wherof part remaineth with us. The chemical process of adipocere formation, saponification, came to be understood in the 17th century when microscopes became widely available. In 1825, physician and lecturer Augustus Granville is believed to have (somewhat unwittingly) made candles from the adipocere of a mummy and used them to light the public lecture he gave to report on the mummy's dissection. Granville apparently thought that the waxy material from which he made the candles had been used to preserve the mummy, rather than its being


id: coffin-birth-20260505-0
file: data/en.wikipedia.org/wiki/Coffin_birth-0.md
category: reference
title: Coffin birth
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Coffin_birth
date_saved: 2026-05-05T10:53:46.045324+00:00
instance: kb-cron
parent: coffin-birth-20260505-0
summary:
Coffin birth, also known as postmortem fetal extrusion, is the expulsion of a nonviable fetus through the vaginal opening of the decomposing body of a deceased pregnant woman due to increasing pressure from intra-abdominal gases. This kind of postmortem delivery occurs very rarely during the decomposition of a body. The practice of chemical preservation, whereby chemical preservatives and disinfectant solutions are pumped into a body to replace natural body fluids (and the bacteria that reside therein), have made the occurrence of "coffin birth" so rare that the topic is rarely mentioned in international medical discourse. Typically during the decomposition of a human body, naturally occurring bacteria in the organs of the abdominal cavity (such as the stomach and intestines) generate gases as by-products of metabolism, which causes the body to swell. In some cases, the confined pressure of the gases can squeeze the uterus (the womb), even forcing it downward, and it may turn inside-out and be forced out of the body through the vaginal opening (a process called prolapse). If a fetus is contained within the uterus, it could therefore be expelled from the mother's body through the vaginal opening when the uterus turns inside-out, in a process that,


id: coffin-birth-20260505-1
file: data/en.wikipedia.org/wiki/Coffin_birth-1.md
category: reference
title: Coffin birth
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Coffin_birth
date_saved: 2026-05-05T10:53:46.045324+00:00
instance: kb-cron
parent: coffin-birth-20260505-0
summary:
== History == Numerous documented cases of postmortem fetal extrusion were described in the medical compendium Anomalies and Curiosities of Medicine, first published in 1896. The earliest presented case occurred in 1551 when a pregnant woman was tried and hanged by the courts of the Spanish Inquisition. Four hours after her death, and while the body still hung by the neck, two dead infants were seen to fall free of the body. This is unusual for the short amount of time elapsed between death and the postmortem delivery. As no information is given regarding other ambient circumstances, it is unclear whether the onset of putrefaction was accelerated, or if other causal factors were at work. In the city of Brussels, in 1633, a woman died in convulsions and three days later a fetus was spontaneously expelled. In Weissenfels, in 1861, postmortem fetal extrusion was observed sixty hours after the death of a pregnant woman. Other cases are described, though only a few describe the unexpected discovery of fetal remains following exhumation. Most cases occurred before burial; in some of these, the body was in the casket while in other cases the body was still on its deathbed or on a


id: coffin-birth-20260505-2
file: data/en.wikipedia.org/wiki/Coffin_birth-2.md
category: reference
title: Coffin birth
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Coffin_birth
date_saved: 2026-05-05T10:53:46.045324+00:00
instance: kb-cron
parent: coffin-birth-20260505-0
summary:
If the fetal remains are found in a fetal position and are wholly within the pelvic cavity of the adult, the fetus died and was interred before delivery. The pregnant woman may therefore have died due to labor complications. If the infant is found alongside the adult, with the head oriented in the same direction as the adult, then the infant was delivered, whether naturally or by caesarian incision, around the time of death, and thereafter interred. Delivered infants have also been interred between or alongside the tibiae (shins), but the infant is still oriented in the same direction as the adult. If the majority of the fetal remains are in the pelvic cavity of the adult, yet the legs are extended and/or the cranium lies among the ribs, then the infant may have been delivered and then placed on top of the mother's torso before burial. As both bodies skeletonized, the infant's bones would have settled among the mother's ribs and vertebrae. If the fetal remains are complete and in a position inferior to and in-line with the pelvic outlet, with the head oriented opposite to that of the mother (toward the foot of the coffin or grave), then


id: coffin-birth-20260505-3
file: data/en.wikipedia.org/wiki/Coffin_birth-3.md
category: reference
title: Coffin birth
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Coffin_birth
date_saved: 2026-05-05T10:53:46.045324+00:00
instance: kb-cron
parent: coffin-birth-20260505-0
summary:
== Comparable phenomena == There are also many cases where the remains of the fetus are found separate from the body of the mother, but expulsion was not through the birth canal, and separation of the two bodies may have been influenced by external environmental factors. The process of separation is so unusual that a specific term for the phenomenon may not have been proposed to the scientific community. These cases may have comparable results, but they are not cases of postmortem fetal extrusion. In April 2003, the body of Laci Peterson washed up on a shore near San Francisco Bay; she had been pregnant when she disappeared four months earlier, and the fetus she had been carrying was discovered on a separate beach. When questioned by the media, medical authorities initially speculated that a "coffin birth" might have occurred. However, at autopsy the cervix was found to be in a prepartum condition. Medical examiners later concluded that while Peterson's body was in the bay, the skin over the abdominal cavity had ruptured due to natural decompositional processes. Seawater entered the abdominal cavity and washed out most of the internal organs, along with the fetus. In 2007, a 23-year-old woman


id: ozone-cracking-20260505-0
file: data/en.wikipedia.org/wiki/Ozone_cracking-0.md
category: reference
title: Ozone cracking
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Ozone_cracking
date_saved: 2026-05-05T10:53:47.331836+00:00
instance: kb-cron
parent: ozone-cracking-20260505-0
summary:
Cracks can be formed in many different elastomers by ozone attack, and the characteristic form of attack of vulnerable rubbers is known as ozone cracking. The problem was formerly very common, especially in tires, but is now rarely seen in those products owing to preventive measures. However, it does occur in many other safety-critical items such as fuel lines and rubber seals, such as gaskets and O-rings, where ozone attack is considered unlikely. Only a trace amount of the gas is needed to initiate cracking, and so these items can also succumb to the problem. == Susceptible elastomers == Tiny traces of ozone in the air will attack double bonds in rubber chains, with natural rubber, polybutadiene, styrene-butadiene rubber and nitrile rubber being most sensitive to degradation. Every repeat unit in the first three materials has a double bond, so every unit can be degraded by ozone. Nitrile rubber is a copolymer of butadiene and acrylonitrile units, but the proportion of acrylonitrile is usually lower than butadiene, so attack occurs. Butyl rubber is more resistant but still has a small number of double bonds in its chains, so attack is possible. Exposed surfaces are attacked first, the density of cracks


id: polymer-degradation-20260505-0
file: data/en.wikipedia.org/wiki/Polymer_degradation-0.md
category: reference
title: Polymer degradation
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Polymer_degradation
date_saved: 2026-05-05T10:53:48.491975+00:00
instance: kb-cron
parent: polymer-degradation-20260505-0
summary:
Polymer degradation is the lowering of a polymer, such as strength, caused by changes in its chemical composition. Polymers and particularly plastics are subject to degradation at all stages of their product life cycle, including during their initial processing, use, disposal into the environment and recycling. The rate of this degradation varies significantly; biodegradation can take decades, whereas some industrial processes can completely decompose a polymer in hours. Technologies have been developed to both inhibit or promote degradation. For instance, polymer stabilizers ensure plastic items are produced with the desired properties, extend their useful lifespans, and facilitate their recycling. Conversely, biodegradable additives accelerate the degradation of plastic waste by improving its biodegradability. Some forms of plastic recycling can involve the complete degradation of a polymer back into monomers or other chemicals. In general, the effects of heat, light, air and water are the most significant factors in the degradation of plastic polymers. The major chemical changes are oxidation and chain scission, leading to a reduction in the molecular weight and degree of polymerization of the polymer. These changes affect physical properties like strength, malleability, melt flow index, appearance and colour. The changes in properties are often termed "aging". == Susceptibility


id: polymer-degradation-20260505-1
file: data/en.wikipedia.org/wiki/Polymer_degradation-1.md
category: reference
title: Polymer degradation
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Polymer_degradation
date_saved: 2026-05-05T10:53:48.491975+00:00
instance: kb-cron
parent: polymer-degradation-20260505-0
summary:
Drinking water which has been chlorinated to kill microbes may contain trace levels of chlorine. The World Health Organization recommends an upper limit of 5 ppm. Although low, 5 ppm is enough to slowly attack certain types of plastic, particularly when the water is heated, as it is for washing. Polyethylene, polybutylene and acetal resin (polyoxymethylene) pipework and fittings are all susceptible. Attack leads to hardening of pipework, which can leave it brittle and more susceptible to mechanical failure. === Electronics === Plastics are used extensively in the manufacture of electrical items, such as circuit boards and electrical cables. These applications can be harsh, exposing the plastic to a mixture of thermal, chemical and electrochemical attack. Many electric items like transformers, microprocessors or high-voltage cables operate at elevated temperatures for years, or even decades, resulting in low-level but continuous thermal oxidation. This can be exacerbated by direct contact with metals, which can promote the formation of free-radicals, for instance, by the action of Fenton reactions on hydroperoxides. High voltage loads can also damage insulating materials such as dielectrics, which degrade via electrical treeing caused by prolonged electrical field stress. === Galvanic action === Polymer degradation by galvanic action was first


id: polymer-degradation-20260505-2
file: data/en.wikipedia.org/wiki/Polymer_degradation-2.md
category: reference
title: Polymer degradation
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Polymer_degradation
date_saved: 2026-05-05T10:53:48.491975+00:00
instance: kb-cron
parent: polymer-degradation-20260505-0
summary:
The act of recycling plastic degrades its polymer chains, usually as a result of thermal damage similar to that seen during initial processing. In some cases, this is turned into an advantage by intentionally and completely depolymerising the plastic back into its starting monomers, which can then be used to generate fresh, un-degraded plastic. In theory, this chemical (or feedstock) recycling offers infinite recyclability, but it is also more expensive and can have a higher carbon footprint because of its energy costs. Mechanical recycling, where the plastic is simply remelted and reformed, is more common, although this usually results in a lower-quality product. Alternatively, plastic may simply be burnt as a fuel in a waste-to-energy process. === Remelting === Thermoplastic polymers like polyolefins can be remelted and reformed into new items. This approach is referred to as mechanical recycling and is usually the simplest and most economical form of recovery. Post-consumer plastic will usually already bear a degree of degradation. Another round of melt-processing will exacerbate this, with the result being that mechanically recycled plastic will usually have poorer mechanical properties than virgin plastic. Degradation can be enhanced by high concentrations of hydroperoxides, cross-contamination between different types of plastic and


id: thermal-degradation-of-polymers-20260505-0
file: data/en.wikipedia.org/wiki/Thermal_degradation_of_polymers-0.md
category: reference
title: Thermal degradation of polymers
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Thermal_degradation_of_polymers
date_saved: 2026-05-05T10:53:49.711264+00:00
instance: kb-cron
parent: thermal-degradation-of-polymers-20260505-0
summary:
In polymers, such as plastics, thermal degradation refers to a type of polymer degradation where damaging chemical changes take place at elevated temperatures, without the simultaneous involvement of other compounds such as oxygen. Simply put, even in the absence of air, polymers will begin to degrade if heated high enough. It is distinct from thermal-oxidation, which can usually take place at less elevated temperatures. The onset of thermal degradation dictates the maximum temperature at which a polymer can be used. It is an important limitation in how the polymer is manufactured and processed. For instance, polymers become less viscous at higher temperatures which makes injection moulding easier and faster, but thermal degradation places a ceiling temperature on this. Polymer devolatilization is similarly effected. At high temperatures, the components of the long chain backbone of the polymer can break (chain scission) and react with one another (cross-link) to change the properties of the polymer. These reactions result in changes to the molecular weight (and molecular weight distribution) of the polymer and can affect its properties by causing reduced ductility and increased embrittlement, chalking, scorch, colour changes, cracking and general reduction in most other desirable physical properties. == Reaction pathways == ===


id: weapon-focus-20260505-0
file: data/en.wikipedia.org/wiki/Weapon_focus-0.md
category: reference
title: Weapon focus
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Weapon_focus
date_saved: 2026-05-05T10:53:50.972604+00:00
instance: kb-cron
parent: weapon-focus-20260505-0
summary:
Weapon focus is the concentration on a weapon by a witness of a crime and the subsequent inability to accurately remember other details of the crime. Weapon focus is a factor that heavily affects the reliability of eyewitness testimony. This effect involves a witness to a crime diverting their attention to the weapon the perpetrator is holding, thus causing memory impairments and leaving less attention for other details in the scene, such as the attacker's face, clothing or vehicle. Several studies support the notion of weapon focus, particularly in terms of greater attention paid to the weapon and its effects on recognition and recall. Elizabeth Loftus, Yuille and Burns have all been associated with studies showing the existence of a weapon focus effect. == Background information == In the field of forensic psychology, researchers have validated the weapon focus effect and shown that a witness will remember less about a crime, or the perpetrator of a crime, when a weapon is present, as opposed to if the weapon is not present at an identical crime. As for the reason of the phenomenon, the two leading explanations attribute it to the cognitive arousal of the witness, or to the overall unusualness


id: weapon-focus-20260505-1
file: data/en.wikipedia.org/wiki/Weapon_focus-1.md
category: reference
title: Weapon focus
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Weapon_focus
date_saved: 2026-05-05T10:53:50.972604+00:00
instance: kb-cron
parent: weapon-focus-20260505-0
summary:
== Causes of weapon focus == A leading explanation for a cause of weapon focus is the unusualness of the situation, being a witness to a crime. Since the initial research conducted by Johnson and Scott (1976) and Loftus et al. (1987), others have demonstrated a similar effect using unusual objects rather than weapons. These findings are described as the "Unusual Item Hypothesis." This effect is seen when an object does not fit with the schema of the situation. Attention is then drawn to the out-of-place object and less attention is paid to other objects in the scene making recalling these other objects more difficult. This effect was observed in Toronto in 1997 when a robber entered a coffee shop and demanded money while threatening to strangle a goose he was holding if he did not receive the money. Meanwhile, the customers were too focused on the strangeness of a goose in the coffee shop to observe any details about the perpetrator himself. This effect has also been observed in the lab. For example, Pickel (1998) demonstrated an effect comparable to weapon focus using a video in which a man approached a cashier and presented a whole raw chicken or


id: weapon-focus-20260505-2
file: data/en.wikipedia.org/wiki/Weapon_focus-2.md
category: reference
title: Weapon focus
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Weapon_focus
date_saved: 2026-05-05T10:53:50.972604+00:00
instance: kb-cron
parent: weapon-focus-20260505-0
summary:
== Differences in weapon focus effects == Differences in eyewitness memory can be attributed to whether the perpetrator holding a weapon is consistent with the eyewitness's schema. People use schemas to organize knowledge and provide a basis for future understanding. Researcher Kerri Pickel posits that if the object held by the target falls in line with the witness's schema, they will not divert as much attention to it and the characteristics of the perpetrator will be encoded normally - it is only when the combination of the perpetrator and the object is not consistent with the schema that the weapon focus effect will be significant in distorting memory. Pickel looked into some differences in the memory of the eyewitnesses, when the physical traits of the perpetrator change. Firstly, the difference in memory was examined between eyewitnesses that saw a white perpetrator versus a black perpetrator. Her research, conducted in 2009, showed that the weapon focus effect weakens with 'black' perpetrators in comparison to 'white' perpetrators, and that the weapon focus effect is not significant when a "Black perpetrator wore a style of clothing that is strongly associated with Black men". It is suggested that individuals who observe a black perpetrator


id: weapon-focus-20260505-3
file: data/en.wikipedia.org/wiki/Weapon_focus-3.md
category: reference
title: Weapon focus
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Weapon_focus
date_saved: 2026-05-05T10:53:50.972604+00:00
instance: kb-cron
parent: weapon-focus-20260505-0
summary:
== Reducing weapon focus and its impact on criminal cases == Pickel, Ross, and Truelove (2006) decided to take a more in depth look at these ideas and apply them specifically to reducing weapon focus. If weapon focus is an automatic process, then the capture of attention may be out of an eyewitness' control. However, if there is no automatic capture of the witness' attention, then weapon focus effect may be able to be overcome. Specific training can be developed to teach a person who may be at risk of an armed robbery, such as a bank teller or cashier, to perform an identification that is comparable to if there was no weapon present. The data indicate that weapons do not capture attention automatically and involuntarily. If a witness was given a lecture about weapon focus and the problems that can arise in memory formation in an incident when a weapon is present at the scene, they can more accurately identify a perpetrator of a crime. This shows that with proper training weapon focus effect can be overcome and an eyewitness' testimony becomes more accurate. These findings, however, are theoretical and need to be replicated in real world situations to


id: welt-bruise-20260505-0
file: data/en.wikipedia.org/wiki/Welt_(bruise)-0.md
category: reference
title: Welt (bruise)
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Welt_(bruise)
date_saved: 2026-05-05T10:53:52.188315+00:00
instance: kb-cron
parent: welt-bruise-20260505-0
summary:
A welt is a bloodshot stripe on the skin. They are hematoma, a special form of a bruise. Welts occur when blunt force is applied to the body with elongated objects without sharp edges. Like other haematomas, welts change their colors as they heal, which usually takes two to four weeks. The colors include purplish black, reddish blue, brown or yellowish green. That makes it possible to determine their approximate age. A special form of welts—when the impact tool is not flat—are double welts. They occur after blows with stick-like, long objects, such as a cane for caning, a riding crop or a garden hose. The energy of the striking tool hitting the skin forces the tissue fluid or tissue matrix to the outside of the instrument. This leaves a rather anemic, brighter area of skin, so the area of impact of the striking tool on the skin is usually free of injury. However, capillary vessels located under the skin at the edges of the impacting object rupture, resulting in two parallel, striated skin lacerations. Such double straps in a child indicate abuse and require clarification. == References ==


id: bamboo-blossom-20260505-0
file: data/en.wikipedia.org/wiki/Bamboo_blossom-0.md
category: reference
title: Bamboo blossom
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Bamboo_blossom
date_saved: 2026-05-05T10:53:53.961137+00:00
instance: kb-cron
parent: bamboo-blossom-20260505-0
summary:
Bamboo blossoming is a natural phenomenon in which the bamboos in a location flower or "blossom" and become hung with bamboo seeds. Most flowering plant species blossom annually, or more than once a year, but bamboo is unusual in that the plants only flower at intervals of several years, and after flowering, the plant dies. Typically, all specimens of a species flower and then die the same year, then new plants grow from their seeds. == Phenomenon == Bamboos usually have a life cycle around 40 to 80 years, varying among species. Normally, new bamboos grow up from bamboo shoots at the roots. At infrequent intervals for most species, they will start to blossom. After blossom, flowers produce fruit (called "bamboo rice" in parts of India and China). Following this, the bamboo forest dies out. Since a bamboo forest is typically composed of a single species, the death of bamboos occurs in a large area. Many bamboo species flower at extremely long intervals such as 65 or even 120 years. These taxa exhibit mass flowering (or gregarious flowering), with all plants in a particular cohort flowering over a several-year period. Any plant derived through clonal propagation from this cohort will


id: bamboo-blossom-20260505-1
file: data/en.wikipedia.org/wiki/Bamboo_blossom-1.md
category: reference
title: Bamboo blossom
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Bamboo_blossom
date_saved: 2026-05-05T10:53:53.961137+00:00
instance: kb-cron
parent: bamboo-blossom-20260505-0
summary:
=== Ecology and animals === The mass fruiting also has direct economic and ecological consequences, however. For example, devastating consequences occur when the Melocanna bambusoides population flowers and fruits once every 30–35 years around the Bay of Bengal. The death of the bamboo plants following their fruiting means the local people lose their building material, and the large increase in bamboo fruit leads to a rapid increase in rodent populations. As the number of rodents increases, they consume all available food, including grain fields and stored food, sometimes leading to famine. These rats can also carry dangerous diseases, such as typhus, typhoid, and bubonic plague, which can reach epidemic proportions as the rodents increase in number. The relationship between rat populations and bamboo flowering was examined in a 2009 Nova documentary Rat Attack. The sudden death of large areas of bamboo puts pressure on animals that depend on bamboo as a food source, such as the endangered giant panda. It has also been hypothesized that ancestors of the domesticated chicken adapted to this sporadic burst of food supply by aggressively laying eggs when the blossom and seed dispersal occurs. === Bamboo genetics === Flowering produces large quantities of seeds, typically


id: br-ckner-egeson-lockyer-cycle-20260505-0
file: data/en.wikipedia.org/wiki/Brückner-Egeson-Lockyer_cycle-0.md
category: reference
title: Brückner-Egeson-Lockyer cycle
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Brückner-Egeson-Lockyer_cycle
date_saved: 2026-05-05T10:53:55.170244+00:00
instance: kb-cron
parent: br-ckner-egeson-lockyer-cycle-20260505-0
summary:
The Brückner-Egeson-Lockyer cycle is a climatic cycle of approximately 30-40 years. Originally described by Charles Egeson on the basis of periodic flooding events in southern Australia that correlated with sunspot activity. It was supported by William J.S. Lockyer and later by Eduard Brückner. The initials "BEL" or paratridecadal cycles have since been suggested to explain various other long-term cycles including those affecting economics, human physiology, and historical events. Charles Egeson (who possibly also went by the name of John Joseph George) first suggested 33 year cycles in Australian weather in 1889. Egeson was an assistant to Henry Chamberlain Russell in the Sydney Observatory. When Russell was away in Europe, he began to publish forecasts based on his cycles in the newspapers. The forecast of a drought caused considerable public alarm and led to his dismissal from the Sydney Observatory in 1890. In 1893, a warrant was issued for the arrest of Charles Egeson in a case of cheating. A news report in 1903 recorded his death in a lunatic asylum. Egeson's 1889 publication was shortly followed in 1890 by Eduard Brückner's who suggested 33 to 35 year cycles. William J. S. Lockyer, son of Sir Norman Lockyer, associated these cycles


id: chandler-wobble-20260505-0
file: data/en.wikipedia.org/wiki/Chandler_wobble-0.md
category: reference
title: Chandler wobble
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Chandler_wobble
date_saved: 2026-05-05T10:53:56.390473+00:00
instance: kb-cron
parent: chandler-wobble-20260505-0
summary:
The Chandler wobble or Chandler variation of latitude is a small deviation in the Earth's axis of rotation relative to the solid earth, which was discovered by and named after American astronomer Seth Carlo Chandler in 1891. It amounts to change of about 9 metres (30 ft) in the point at which the axis intersects the Earth's surface and has a period of 433 days. This wobble, which is an astronomical nutation, combines with another wobble with a period of six years, so that the total polar motion varies with a period of about 7 years. The Chandler wobble is an example of the kind of motion that can occur for a freely rotating object that is not a sphere; this is called a free nutation. Somewhat confusingly, the direction of the Earth's rotation axis relative to the stars also varies with different periods, and these motions—caused by the tidal forces of the Moon and Sun—are also called nutations, except for the slowest, which are precessions of the equinoxes. == Predictions == The existence of Earth's free nutation was predicted by Isaac Newton in Corollaries 20 to 22 of Proposition 66, Book 1 of the Philosophiæ Naturalis Principia Mathematica, and


id: list-of-cycles-20260505-0
file: data/en.wikipedia.org/wiki/List_of_cycles-0.md
category: reference
title: List of cycles
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/List_of_cycles
date_saved: 2026-05-05T10:53:57.688381+00:00
instance: kb-cron
parent: list-of-cycles-20260505-0
summary:
This is a list of recurring cycles. See also Index of wave articles, Time, and Pattern. == Planetary cycles == === Astronomical cycles === Astronomy – Axial precession – CNO cycle – Eclipse cycle – Eclipse – Full moon cycle – Galactic year – Great Year – Lunar phase – Mesoamerican calendars – Metonic cycle – Milankovitch cycles – Mira – Moon – Nutation – Orbit – Orbital period – Saros cycle – Sothic cycle – Secularity – Sidereal year – Sunspot – Tide – Tropical year – Year === Climate and weather cycles === Animal migration – Avalanche – Carbon cycle – Climate change – Climate change and agriculture – Climate model – Climate oscillation – Clock of the Long Now – Ecology – El Niño/La Niña – Environmental geography – Global cooling – Global warming – Historical temperature record – Hydrogen cycle – Ice age – Transhumance – Milankovitch cycles – Monsoon – Pleistocene – Season – Sulfur cycle – Sunspot – Tide – Timeline of meteorology – 1500-year climate cycle === Geological cycles === Age of the Earth – Aluminum cycle – Arsenic cycle – Boron cycle – Bromine cycle – Cadmium cycle – Calcium cycle –


id: decade-20260505-0
file: data/en.wikipedia.org/wiki/Decade-0.md
category: reference
title: Decade
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Decade
date_saved: 2026-05-05T10:53:58.977393+00:00
instance: kb-cron
parent: decade-20260505-0
summary:
A decade (from Ancient Greek: δεκάς, dekas, lit. 'group of 10') is a period of ten years. It may also be called a decennium (from Latin: decennium, lit. 'ten-year'). Decades may describe any ten-year period, such as those of a person's life, or refer to specific groupings of calendar years. == Usage == Any period of ten years is a 'decade' or 'decennium'. For example, the statement that "during his last decade, Mozart explored chromatic harmony to a degree rare at the time" refers to the last 10 years of Wolfgang Amadeus Mozart's life without regard to which calendar years are encompassed. Also, 'the first decade' of a person's life begins on the day of their birth and ends at the end of their 10th year of life when they have their 10th birthday; the second decade of life starts with their 11th year of life (during which one is typically still referred to as being "10") and ends at the end of their 20th year of life, on their 20th birthday; similarly, the third decade of life, when one is in one's twenties or 20s, starts with the 21st year of life, and so on, with subsequent decades of


id: deciduous-20260505-0
file: data/en.wikipedia.org/wiki/Deciduous-0.md
category: reference
title: Deciduous
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Deciduous
date_saved: 2026-05-05T10:54:00.179035+00:00
instance: kb-cron
parent: deciduous-20260505-0
summary:
In the fields of horticulture and botany, the term deciduous () means "falling off at maturity" and "tending to fall off", in reference to trees and shrubs that seasonally shed leaves, usually in the autumn; to the shedding of petals, after flowering; and to the shedding of ripe fruit. The antonym of deciduous in the botanical sense is evergreen. Generally, the term "deciduous" means "the dropping of a part that is no longer needed or useful" and the "falling away after its purpose is finished". In plants, it is the result of natural processes. "Deciduous" has a similar meaning when referring to animal parts, such as deciduous antlers in deer, deciduous teeth (baby teeth) in some mammals (including humans); or decidua, the uterine lining that sheds off after birth. == Botany == In botany and horticulture, deciduous plants, including trees, shrubs and herbaceous perennials, are those that lose all of their leaves for part of the year. This process is called abscission. In some cases leaf loss coincides with winter—namely in temperate or polar climates. In other parts of the world, including tropical, subtropical, and arid regions, plants lose their leaves during the dry season or other seasons, depending on


id: deciduous-20260505-1
file: data/en.wikipedia.org/wiki/Deciduous-1.md
category: reference
title: Deciduous
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Deciduous
date_saved: 2026-05-05T10:54:00.179035+00:00
instance: kb-cron
parent: deciduous-20260505-0
summary:
Many deciduous plants flower during the period when they are leafless, as this increases the effectiveness of pollination. The absence of leaves improves wind transmission of pollen for wind-pollinated plants and increases the visibility of the flowers to insects in insect-pollinated plants. This strategy is not without risks, as the flowers can be damaged by frost or, in dry season regions, result in water stress on the plant. Spring leafout and fall leaf drop are triggered by a combination of daylight and air temperatures. The exact conditions required will vary with the species, but generally more cold-tolerant genera such as Salix will leaf-out earlier and lose their leaves later, while genera such as Fraxinus and Juglans can only grow in warm, frost-free conditions so they need at least 13 hours of daylight and air temperatures of around 70 °F (21 °C) to leaf out. They will be among the earliest trees to lose their leaves in the fall. In sub-Arctic climates such as Alaska, leaves begin turning colors as early as August. However, for most temperate regions it takes place in late September through early November and in subtropical climates such as the southern United States, it may be November


id: deciduous-20260505-2
file: data/en.wikipedia.org/wiki/Deciduous-2.md
category: reference
title: Deciduous
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Deciduous
date_saved: 2026-05-05T10:54:00.179035+00:00
instance: kb-cron
parent: deciduous-20260505-0
summary:
Plants with deciduous foliage have advantages and disadvantages compared to plants with evergreen foliage. Since deciduous plants lose their leaves to conserve water or to better survive winter weather conditions, they must regrow new foliage during the next suitable growing season; this uses resources which evergreens do not need to expend. Evergreens suffer greater water loss during the winter and they also can experience greater predation pressure, especially when small. Deciduous trees experience much less branch and trunk breakage from glaze ice storms when leafless, and plants can reduce water loss due to the reduction in availability of liquid water during cold winter days. Losing leaves in winter may reduce damage from insects; repairing leaves and keeping them functional may be more costly than just losing and regrowing them. Removing leaves also reduces cavitation which can damage xylem vessels in plants. This then allows deciduous plants to have xylem vessels with larger diameters and therefore a greater rate of transpiration (and hence CO2 uptake as this occurs when stomata are open) during the summer growth period. ==== Deciduous woody plants ==== The deciduous characteristic has developed repeatedly among woody plants. Trees include maple, many oaks and nothofagus, elm, beech, aspen,


id: eternal-return-20260505-0
file: data/en.wikipedia.org/wiki/Eternal_return-0.md
category: reference
title: Eternal return
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Eternal_return
date_saved: 2026-05-05T10:54:01.384776+00:00
instance: kb-cron
parent: eternal-return-20260505-0
summary:
Eternal return (or eternal recurrence) is a philosophical concept which states that time repeats itself in an infinite loop, and that exactly the same events will continue to occur in exactly the same way, over and over again, for eternity. In ancient Greece, the concept of eternal return was most prominently associated with Empedocles and with Stoicism, the school of philosophy founded by Zeno of Citium. The Stoics believed that the universe is periodically destroyed and reborn, and that each universe is exactly the same as the one before. This doctrine was fiercely criticised by Christian authors such as Augustine, who saw in it a fundamental denial of free will and of the possibility of salvation. The spread of Christianity therefore diminished classical theories of eternal return. The concept was revived in the 19th century by German philosopher Friedrich Nietzsche. Having briefly presented the idea as a thought experiment in The Gay Science, he explored it more thoroughly in his novel Thus Spoke Zarathustra, in which the protagonist learns to overcome his horror of the thought of eternal return. It is not known whether Nietzsche believed in the literal truth of eternal return, or, if he did not, what he


id: eternal-return-20260505-1
file: data/en.wikipedia.org/wiki/Eternal_return-1.md
category: reference
title: Eternal return
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Eternal_return
date_saved: 2026-05-05T10:54:01.384776+00:00
instance: kb-cron
parent: eternal-return-20260505-0
summary:
=== Christian response === Christian authors attacked the doctrine of eternal recurrence on various grounds. Origen argued that the theory was incompatible with free will (although he did allow the possibility of diverse and non-identical cycles). Augustine of Hippo (354–430 AD) objected to the fact that salvation was not possible in the Stoic scheme, arguing that even if a temporary happiness was attained, a soul could not be truly blessed if it was doomed to return again to misery. Augustine also mentions "certain philosophers" who cite Ecclesiastes 1:9–10 as evidence of eternal return: "What is that which hath been? It is that which shall be. And what is that which is done? It is that which shall be done: and there is no new thing under the sun. Who can speak and say, See, this is new? It hath been already of old time, which was before us." Augustine denies that this has reference to the recurrence of specific people, objects, and events, instead interpreting the passage in a more general sense. In support of his argument, he appeals to scriptural passages such as Romans 6:9, which affirms that Christ "being raised from the dead dieth no more". == Friedrich


id: eternal-return-20260505-2
file: data/en.wikipedia.org/wiki/Eternal_return-2.md
category: reference
title: Eternal return
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Eternal_return
date_saved: 2026-05-05T10:54:01.384776+00:00
instance: kb-cron
parent: eternal-return-20260505-0
summary:
=== Interpretation === Martin Heidegger points out that Nietzsche's first mention of eternal recurrence in The Gay Science presents this concept as a hypothetical question rather than postulating it as a fact. Many readings argue that Nietzsche was not attempting to make a cosmological or theoretical claim i.e. saying that eternal recurrence is a true statement about how the world works. Instead, the emotional reaction to the thought experiment serves to reveal whether one is living life to the best. According to Heidegger, the significant point is the burden imposed by the question of eternal recurrence, regardless of whether or not such a thing could possibly be true. The idea is similar to Nietzsche's concept of amor fati, which he describes in Ecce Homo: "My formula for greatness in a human being is amor fati: that one wants nothing to be different, not forward, not backward, not in all eternity. Not merely to bear what is necessary, still less conceal it ... but love it." On the other hand, Nietzsche's posthumously published notebooks contain an attempt at a logical proof of eternal return, which is often adduced in support of the claim that Nietzsche believed in the theory as a


id: flower-20260505-0
file: data/en.wikipedia.org/wiki/Flower-0.md
category: reference
title: Flower
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Flower
date_saved: 2026-05-05T10:54:02.594490+00:00
instance: kb-cron
parent: flower-20260505-0
summary:
Flowers, also known as blossoms and blooms, are the reproductive structures of flowering plants. Typically, they are structured in four circular levels around the end of a stalk. These include: sepals, which are modified leaves that support the flower; petals, often designed to attract pollinators; male stamens, where pollen is presented; and female gynoecia, where pollen is received and its movement is facilitated to the egg. When flowers are arranged in a group, they are known collectively as an inflorescence. The development of flowers is a complex and important part in the life cycles of flowering plants. In most plants, flowers are able to produce sex cells of both sexes. Pollen, which can produce the male sex cells, is transported between the male and female parts of flowers in pollination. Pollination can occur between different plants, as in cross-pollination, or between flowers on the same plant or even the same flower, as in self-pollination. Pollen movement may be caused by animals, such as birds and insects, or non-living things like wind and water. The colour and structure of flowers assist in the pollination process. After pollination, the sex cells are fused together in the process of fertilisation, which is a


id: flower-20260505-1
file: data/en.wikipedia.org/wiki/Flower-1.md
category: reference
title: Flower
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Flower
date_saved: 2026-05-05T10:54:02.594490+00:00
instance: kb-cron
parent: flower-20260505-0
summary:
==== Petals ==== The petals, collectively called the corolla, are almost or completely fibreless leaf-like structures that form the innermost whorl of the perianth. They are often delicate and thin and are usually coloured, shaped, or scented, to encourage and facilitate pollination. The petals may be fused together. Petals often feature UV patterns invisible to humans but visible to pollinators. In some flowers, petals and sepals are indistinguishable from one another. === Reproductive === All flowering plants are heterosporous, that is, every individual plant produces two types of spores. Spores are formed from mature plants, which contain two sets of chromosomes, and are divided into microspores and megaspores—the precursors to pollen and embryo sacs respectively. Pollen and embryo sacs are the male and female gametophytes, sex cell-producing structures, and contain just one set of chromosomes. Microspores are produced by meiosis inside anthers, the male part of flowers, and megaspores are produced inside ovules contained within the ovary. As with all heterosporous plants, the gametophytes also develop inside the spores. ==== Male ==== The androecium is the whorl of male parts called stamens, which produce pollen. Stamens consist typically of an anther, made up of four pollen sacs arranged in two


id: flower-20260505-2
file: data/en.wikipedia.org/wiki/Flower-2.md
category: reference
title: Flower
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Flower
date_saved: 2026-05-05T10:54:02.594490+00:00
instance: kb-cron
parent: flower-20260505-0
summary:
=== Colour === In contrast to the mostly green vegetative parts of plants, flowers are often colourful. This includes the petals and, in some plants, the stamens, anthers, stigmas, ovaries, pollen, styles, and even nectar. These colours are produced mainly by biological pigments, which are molecules that can absorb and retain energy from light. Specific pigments, and so colours, provide different benefits to the plant. These benefits include protecting the plant against degradation and guiding pollinators—both general and specific—to the plant. Colour, or colour effects, may also be produced by structural coloration, in which colour is produced by tiny surface structures interfering with waves of light. This includes iridescence (as in some tulips) and photonic crystals (as in edelweiss), which diffract light using tiny grooves. The colour of flowers can also change; sometimes this acts as a signal to pollinators (as in Viola cornuta). Change may also occur as a result of temperature; pH, as in the anthoxanthins found in Hydrangea; metals; sugars; and cell shape. == Development == Floral development begins with the transformation of vegetative growth into floral growth. This is regulated by both genetic and environmental factors. The eventual formation of a flower starts with a shoot


id: flower-20260505-3
file: data/en.wikipedia.org/wiki/Flower-3.md
category: reference
title: Flower
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Flower
date_saved: 2026-05-05T10:54:02.594490+00:00
instance: kb-cron
parent: flower-20260505-0
summary:
Flowers that use biotic vectors attract and use animals to transfer pollen from one flower to the next. Often they are shaped and designed to both attract pollinators and ensure pollen is transferred effectively. Flowers most commonly employ insects, but also: birds, bats, lizards, other mammals, snails and slugs, and, in rare cases, crustaceans and worms. Rewards given to pollinators by flowers to encourage pollination include: food (such as pollen, starch, or nectar), mates, shelter, a place to raise their young, and pseudocopulation (sexual deception). In the latter, the flower is scented or shaped so as to encourage sexual arousal and pollination from the subsequent intercourse. They may also be attracted by various stimuli such as size and scent (as in carrion flowers). Colour is also a factor, and includes nectar guides, which show pollinators where to look for nectar; they may be visible only under ultraviolet light. Many flowers have close relationships with just one or a few specific pollinators. They may be structured to allow or encourage pollination from these organisms. This increases efficiency, because there is a higher chance pollination comes from pollen of the same species of plant. This close relationship is an example of coevolution,


id: flower-20260505-4
file: data/en.wikipedia.org/wiki/Flower-4.md
category: reference
title: Flower
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Flower
date_saved: 2026-05-05T10:54:02.594490+00:00
instance: kb-cron
parent: flower-20260505-0
summary:
Flowers originated between 150 and 190 million years ago, during the Jurassic. Although molecular analyses indicate this early appearance of angiosperms—flowering plants, the earliest definitive evidence from the fossil record comes from between 125 and 130 million years ago, during the Early Cretaceous. The exact time at which angiosperms diverged from other seed plants is a classic open question in evolutionary biology. Prior to the advent of flowers, plants reproduced using cones (as in gymnosperms), and spores (as in pteridophytes). The transformation of spore-producing leaves into structures like stamens and carpels, is the most clear milestone in the complex evolution of flowers. There is debate both over whether these and other changes happened gradually or as sudden shifts like homeotic mutations, and which aspect of flower morphology came first. The flower was the angiosperms' most significant evolutionary innovation, granting the ability to effectively take advantage of animal pollinators. Other evolutionary advantages included: being able to have both male and female parts on the same axis; and on this axis have carpels, to protect the ovules; stamens, to present the pollen; and the perianth, to provide protection. In addition, they pioneered double fertilisation, which allows energy investment (into endosperm) to be


id: flower-20260505-5
file: data/en.wikipedia.org/wiki/Flower-5.md
category: reference
title: Flower
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Flower
date_saved: 2026-05-05T10:54:02.594490+00:00
instance: kb-cron
parent: flower-20260505-0
summary:
Flowers are the subject of much symbolism, and feature often in art, ritual, religious practices, and festivals. Plants have been cultivated in gardens for their flowers for around ten thousand years. Flowers are associated with burial in many cultures, and are often placed by headstones to pay respect. They are also placed by statues or temples of religious or other figures—sometimes formed into floral wreaths. In some places, the dead are buried covered in flowers or on a bed of flowers. They are also associated with love and celebration, and given to others in many places for this reason. Economic demand has led to the cultivation of flowers that are longer-lasting, more colourful, and visually appealing. Flowers feature extensively in art across a variety of mediums, and different flowers are ascribed symbolic meanings. For example, violets may represent modesty, virtue, or affection. In addition to hidden meanings, flowers are used in flags, emblems, and seals. In this way, they represent countries or places. Some countries have national flowers; for example, Hibiscus × rosa-sinensis is the national flower of Malaysia. In literature, flowers feature in imagery of places and as metaphors for pleasure, beauty, and life. == Notes == == References


id: free-flowering-20260505-0
file: data/en.wikipedia.org/wiki/Free-flowering-0.md
category: reference
title: Free-flowering
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Free-flowering
date_saved: 2026-05-05T10:54:03.900218+00:00
instance: kb-cron
parent: free-flowering-20260505-0
summary:
In gardening, the term free-flowering is used to describe flowering plants that have a long bloom time and may often lack a defined blooming season, whereby producing flowers profusely over an extended period of time, at times all-year round. The terms long-flowering and long-blooming are also used for perennial plants that bloom for much of the year. == Examples == Examples of free-flowering or long flowering plants include salvias, thunbergias, loniceras, roses, lavenders, periwinkles, gaillardias, oleanders, begonias, bougainvilleas, morning glories, geraniums/pelargoniums, hibiscuses, and lantanas. == List == This list includes plant species that are free-flowering, particularly in warmer climates: == References ==


id: leaf-flushing-20260505-0
file: data/en.wikipedia.org/wiki/Leaf_flushing-0.md
category: reference
title: Leaf flushing
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Leaf_flushing
date_saved: 2026-05-05T10:54:05.171046+00:00
instance: kb-cron
parent: leaf-flushing-20260505-0
summary:
Leaf flushing or leaf out is the production of a flush of new leaves typically produced simultaneously on all branches of a bare plant or tree. Young leaves often have less chlorophyll and the leaf flush may be white or red, the latter due to presence of pigments, particularly anthocyanins. Leaf flushing succeeds leaf fall, and is delayed by winter in the temperate zone or by extreme dryness in the tropics. Leaf fall and leaf flushing in tropical deciduous forests can overlap in some species, called leaf-exchanging species, producing new leaves during the same period when old leaves are shed or almost immediately after. Leaf-flushing may be synchronized among trees of a single species or even across species in an area. In the seasonal tropics, leaf flushing phenology may be influenced by herbivory and water stress. == Red leaf flush == In tropical regions, leaves often flush red when young and in the phase of expansion to mature size. Red flushing is frequent among woody species, reported from 20 to 40% of the woody species in a site in Costa Rica, in 36% of species in Barro Colorado Island, Panama, about 49% of species in Kibale National Park, Uganda, and


id: list-of-solar-cycles-20260505-0
file: data/en.wikipedia.org/wiki/List_of_solar_cycles-0.md
category: reference
title: List of solar cycles
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/List_of_solar_cycles
date_saved: 2026-05-05T10:54:06.386047+00:00
instance: kb-cron
parent: list-of-solar-cycles-20260505-0
summary:
Solar cycles are nearly periodic 11-year changes in the Sun's activity that are based on the number of sunspots present on the Sun's surface. The first solar cycle conventionally is said to have started in 1755. The source data are the revised International Sunspot Numbers (ISN v2.0), as available at SILSO. Sunspot counts exist since 1610 but the cycle numbering is not well defined during the Maunder minimum. It was proposed that one cycle might have been lost in the late 18th century, but this remains not fully confirmed. Solar cycles can be reconstructed indirectly, using the radiocarbon 14C proxy, for the last millennium. The smoothing is done using the traditional SIDC smoothing algorithm. Using this algorithm, if the month in question is notated month 0, a weighted average is formed of months −6 to 6, where months −5 to 5 are given weightings of 1, and months −6 and 6 are given weightings of 0.5. Other smoothing formulas exist, and they usually give slightly different values for the amplitude and timings of the solar cycles. An example is the Meeus smoothing formula, with related solar cycles characteristics available in this STCE news item. The start of solar cycle 25


id: lunar-effect-20260505-0
file: data/en.wikipedia.org/wiki/Lunar_effect-0.md
category: reference
title: Lunar effect
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Lunar_effect
date_saved: 2026-05-05T10:54:07.720486+00:00
instance: kb-cron
parent: lunar-effect-20260505-0
summary:
The lunar effect is a link between different stages of the 29.5-day lunar cycle and the behavior and physiology of various species, purportedly including humans. The changing phase and position of the Moon in its orbit impacts night lighting and ocean tides on Earth. Various organisms have adapted to this repeating cycle. A considerable number of studies have examined the effect on humans. By the late 1980s, there were at least 40 published studies on the purported lunar-lunacy connection, and at least 20 published studies on the purported lunar-birthrate connection. Literature reviews and metanalyses have found no correlation between the lunar cycle and human biology or behavior. In cases such as the approximately monthly cycle of menstruation in humans (but not other mammals), the coincidence in timing reflects no known lunar influence. The widespread and persistent beliefs about the influence of the Moon may depend on illusory correlation – the perception of an association that does not in fact exist. In a number of marine animals, there is strong evidence for the effects of lunar cycles. Observed effects relating to reproductive synchrony may depend on external cues relating to the presence or amount of moonlight. Corals contain light-sensitive cryptochromes, proteins


id: lunar-effect-20260505-1
file: data/en.wikipedia.org/wiki/Lunar_effect-1.md
category: reference
title: Lunar effect
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Lunar_effect
date_saved: 2026-05-05T10:54:07.720486+00:00
instance: kb-cron
parent: lunar-effect-20260505-0
summary:
==== Law and order ==== Senior police officers in Brighton, UK, announced in June 2007 that they were planning to deploy more officers over the summer to counter trouble they believe is linked to the lunar cycle. This followed research by the Sussex Police force that concluded there was a rise in violent crime when the Moon was full. A spokeswoman for the police force said "research carried out by us has shown a correlation between violent incidents and full moons". A police officer responsible for the research told the BBC that "From my experience of 19 years of being a police officer, undoubtedly on full moons we do seem to get people with sort of strange behavior – more fractious, argumentative." Police in Ohio and Kentucky have blamed temporary rises in crime on the full Moon. In January 2008, New Zealand's Justice Minister Annette King suggested that a spate of stabbings in the country could have been caused by the lunar cycle. A reported correlation between Moon phase and the number of homicides in Miami-Dade County was found, through later analysis, not to be supported by the data and to have been the result of inappropriate and misleading statistical


id: lunar-effect-20260505-2
file: data/en.wikipedia.org/wiki/Lunar_effect-2.md
category: reference
title: Lunar effect
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Lunar_effect
date_saved: 2026-05-05T10:54:07.720486+00:00
instance: kb-cron
parent: lunar-effect-20260505-0
summary:
=== In animals === Many animals use moonlight for navigation and to time behavior. As an example of lunar navigation, numerous insect species, such as moths, use moonlight to stabilize their flight paths and maintain consistent trajectories. Corals contain light-sensitive cryptochromes, proteins that are sensitive to different levels of light. Corals at the Great Barrier Reef release their eggs and sperm simultaneously, always after full moon between October and December. Also, the Barau's petrel is known to time their mating season with the full moon. Spawning of coral Platygyra lamellina occurs at night during the summer on a date determined by the phase of the Moon; in the Red Sea, this is the three- to five-day period around the new Moon in July and the similar period in August. Acropora coral time their simultaneous release of sperm and eggs to just one or two days a year, after sundown with a full moon. Dipsastraea speciosa tends to synchronize spawning in the evening or night, around the last quarter moon of the lunar cycle. Another marine animal, the bristle worm Platynereis dumerilii, also spawns a few days after a full moon. It is used as a model for studying cryptochromes and


id: lunar-effect-20260505-3
file: data/en.wikipedia.org/wiki/Lunar_effect-3.md
category: reference
title: Lunar effect
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Lunar_effect
date_saved: 2026-05-05T10:54:07.720486+00:00
instance: kb-cron
parent: lunar-effect-20260505-0
summary:
== Bibliography == Abell, George (1979). Review of the book The Alleged Lunar Effect by Arnold Lieber, Skeptical Inquirer, Spring 1979, 68–73. Reprinted in Science Confronts the Paranormal, edited by Kendrick Frazier, Prometheus Books, ISBN 0-87975-314-5. Abell, George and Barry Singer (1981). Science and the Paranormal – probing the existence of the supernatural, Charles Scribner's Sons, chapter 5, ISBN 0-684-17820-6. Berman, Bob (2003). Fooled by the Full Moon – Scientists search for the sober truth behind some loony ideas, Discover, September 2003, page 30. Caton, Dan (2001). Natality and the Moon Revisited: Do Birth Rates Depend on the Phase of the Moon?, Bulletin of the American Astronomical Society, Vol 33, No. 4, 2001, p. 1371. A summary of the results of the paper. Diefendorf, David (2007), Amazing... But false! Hundreds of "facts" you thought were true, but aren't, Sterling Publishing, ISBN 978-1-4027-3791-6 Foster, Russell G.; Roenneberg, Till (2008). "Human Responses to the Geophysical Daily, Annual and Lunar Cycles". Current Biology. 18 (17): R784–R794. Bibcode:2008CBio...18.R784F. doi:10.1016/j.cub.2008.07.003. PMID 18786384. S2CID 15429616. Lunar cycles had, and continue to have, an influence upon human culture, though despite a persistent belief that our mental health and other behaviours are modulated by the phase of the


id: mautam-20260505-0
file: data/en.wikipedia.org/wiki/Mautam-0.md
category: reference
title: Mautam
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Mautam
date_saved: 2026-05-05T10:54:08.966871+00:00
instance: kb-cron
parent: mautam-20260505-0
summary:
Mautâm (lit. 'The Finish of Mau Bamboo') is a cyclic ecological phenomenon that occurs every 48–50 years in the northeastern Indian states of Tripura, Mizoram and Manipur, as well as in many places of Assam which are 30% covered by wild bamboo forests, and Chin State in Myanmar, particularly Hakha, Thantlang, Falam, Paletwa and Matupi Townships. It begins with a rat population boom, which in turn creates a widespread famine in those areas. During mautâm, Melocanna baccifera, a species of bamboo, flowers at one time across a wide area. This event is followed invariably by a plague of black rats in what is called a rat flood. The bamboo flowering brings a temporary windfall of seeds, and rats multiply, exhaust the bamboo seeds, leave the forests, forage on stored grain, and cause devastating famine. Regular rodent outbreaks associated with bamboo flowering (and subsequent fruiting and seeding) also occur in the nearby Indian states of Arunachal Pradesh, Assam, Manipur, and Nagaland, as well as in Laos, Japan, Madagascar, and South America. Thingtâm, a similar famine, occurs with the flowering of another species of bamboo, Bambusa tulda. == Characteristics == There are two versions of these famines. Both concern bamboo plants, one


id: mautam-20260505-1
file: data/en.wikipedia.org/wiki/Mautam-1.md
category: reference
title: Mautam
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Mautam
date_saved: 2026-05-05T10:54:08.966871+00:00
instance: kb-cron
parent: mautam-20260505-0
summary:
=== 1881-1882 thingtam famine === The onset of the mautam famine in 1881 led to the end of the East-West War. An immigration of 80 families and around 400 people immigrated out of Khalkam's settlement into the plains. The settlers under Khalkam who belonged to the Rangte subclan arrived in Cachar and settled near the Dharmakal tea estate. Their arrival was followed by another mass migration of men from the chiefs Poiboi, Lalhai and Chunglena. Countless areas such as Tipaimukh, Jalnacherra, Akai settlement, and Hailakandi Valley were settled by Lushai refugees, with some even arriving in rafts from the Tlawng River. The arrival of the refugees alarmed the British administration, whose tea planters considered them synonymous with raiders. The immigrants were investigated and revealed their intention to trade all their products in return for food and rice. Several chiefs sold out their ivory jewellery, guns and rubber deposits from the trees after their jhums had been exhausted. The immigrants themselves traded all their products, such as bamboo, cane and rubber, in return for food. The Western chiefs under Khalkam met with the Eastern chiefs under Poiboi and Lalhai to end the conflict and jointly request British aid for the famine.


id: mautam-20260505-2
file: data/en.wikipedia.org/wiki/Mautam-2.md
category: reference
title: Mautam
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Mautam
date_saved: 2026-05-05T10:54:08.966871+00:00
instance: kb-cron
parent: mautam-20260505-0
summary:
=== 1910-1911 mautam famine === The 1910-1911 mautam famine was preceded by a locust swarm (thangnang) which engulfed trees and created a noise described as a monsoon. The arrival of Halley's Comet at the same time also induced a bad omen at the time of many chiefs and elders. However, the excessive proliferation of locusts did not result in any consumption of food or trees. Individuals would scoop up the locusts in buckets and use the fatty oils or the bugs themselves as delicacies. Following the progression of mautam, the rat population did not boom equally across the Lushai Hills; the inhabitants between the Tuirini and Khawchhak Tuipui rivers were affected first by the mautam famine. However, the successful harvest of the year saw an excess of crops and grains that were stored away and used to offset the effects of the booming rat population. These excess storages, however, were localised to a few prominent settlements. The remaining rice paddies of the autumn season were devoured by rats. Since the Lushai Hills were under the sovereignty of the British Raj, the government took initiative in combating the phenomenon. They rewarded the killing of rats, proven by the production of their


id: mautam-20260505-3
file: data/en.wikipedia.org/wiki/Mautam-3.md
category: reference
title: Mautam
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Mautam
date_saved: 2026-05-05T10:54:08.966871+00:00
instance: kb-cron
parent: mautam-20260505-0
summary:
=== 1959 mautam famine === In 1951, the Anti-Famine Campaign Organization was formed under C. Rokhuma in anticipation of the upcoming mautam. The organisation would be organised with a chairman headed by Lalmawia, a vice chairman headed by H. Khuma, the organising secretary under C. Rokhuma, Pacchunga as treasurer, Lalbiaka as secretary and Vanthuama as a member. As an apolitical organisation, it attracted members from the Mizo Union and United Mizo Freedom Organization. In 1954, the AFCO met with the Supply Minister of Assam to warn about the upcoming Mautam. The minister, who did not believe that a flowering bamboo plant could lead to a rat population surge and induce a famine, instead passed the warnings off as a superstition, though assurances were granted that if a famine did occur, aid would be provided. In 1957, the rat population in Mizoram had been observed to grow. The district council had also counterintuitively banned pesticides, which was overturned with pressure from the AFCO. The mautam famine occurred earlier than expected in the cyclical phenomenon predictions. The flowers had been observed since 1957 and reports of rat swarms in search of bamboo fruits came from every area of Mizoram along with damages


id: mautam-20260505-4
file: data/en.wikipedia.org/wiki/Mautam-4.md
category: reference
title: Mautam
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Mautam
date_saved: 2026-05-05T10:54:08.966871+00:00
instance: kb-cron
parent: mautam-20260505-0
summary:
== See also == Bamboo blossom Population cycle == References == == Sources == Chatterjee, Suhas (1985). British rule in Mizoram. New Delhi: Mittal Publications. Kumawat, M.M.; Singh, K.M. (2014). "Rodent outbreak in relation to bamboo flowering in north-eastern region of India". Biological Agriculture & Horticulture. 30 (4): 243–252. Bibcode:2014BioAH..30..243K. doi:10.1080/01448765.2014.925828. S2CID 84596075. Retrieved 24 September 2021. Lalthangliana, B (2005). Culture and folklore of Mizoram. Publications Division Ministry of Information & Broadcasting. ISBN 978-81-230-2658-9. Lalthangliana, B (1989). Mizo Lal Ropui (in Mizo). Aizawl: R.T.M Press. Nag, Sajal (2008). Pied Pipers in North-East India: Bamboo-flowers, Rat-famine and the Politics of Philanthropy. (1881-2007)=New Delhi. Manohar Publishers & Distributors. ISBN 978-81-7304-311-6. Rokhuma, C. (1988). The secret of Famines found. Aizawl: Gosen Press. Retrieved 11 January 2025. == External links. == Indian army's new enemy is rats, UK: BBC, 3 June 2006, troops in the six north-eastern states of the country are used to dealing with insurgencies" but are now dealing with "an infestation of rats. Bamboo the life blood of the people: Alarm to Ecosystem, SOS arsenic. Mautaam in Manipur, archived from the original on 25 October 2011, retrieved 9 September 2013. "Rat Attack, Plant vs. Predator", NOVA (interview), PBS, 30 September 2008,


id: menstrual-cycle-20260505-0
file: data/en.wikipedia.org/wiki/Menstrual_cycle-0.md
category: reference
title: Menstrual cycle
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Menstrual_cycle
date_saved: 2026-05-05T10:54:10.214428+00:00
instance: kb-cron
parent: menstrual-cycle-20260505-0
summary:
The menstrual cycle is a series of natural changes in hormone production and the structures of the uterus and ovaries of the female reproductive system that makes pregnancy possible. The ovarian cycle controls the production and release of eggs and the cyclic release of estrogen and progesterone. The uterine cycle governs the preparation and maintenance of the lining of the uterus (womb) to receive an embryo. These cycles are concurrent and coordinated, normally last between 21 and 35 days, with a median length of 28 days. Menarche (the onset of the first period) usually occurs around the age of 12 years; menstrual cycles continue for about 30–45 years. Naturally occurring hormones drive the cycles; the cyclical rise and fall of the follicle stimulating hormone prompts the production and growth of oocytes (immature egg cells). The hormone estrogen stimulates the uterus lining (endometrium) to thicken to accommodate an embryo should fertilization occur. The blood supply of the thickened lining provides nutrients to a successfully implanted embryo. If implantation does not occur, the lining breaks down and blood is released. Triggered by falling progesterone levels, menstruation (commonly referred to as a "period") is the cyclical shedding of the lining, and is a


id: menstrual-cycle-20260505-1
file: data/en.wikipedia.org/wiki/Menstrual_cycle-1.md
category: reference
title: Menstrual cycle
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Menstrual_cycle
date_saved: 2026-05-05T10:54:10.214428+00:00
instance: kb-cron
parent: menstrual-cycle-20260505-0
summary:
The ovaries contain a finite number of egg stem cells, granulosa cells and theca cells, which together form primordial follicles. At around 20 weeks into gestation some 7 million immature eggs have already formed in an ovary. This decreases to around 2 million by the time a girl is born, and 300,000 by the time she has her first period. On average, one egg matures and is released during ovulation each month after menarche. Beginning at puberty, these mature to primary follicles independently of the menstrual cycle. The development of the egg is called oogenesis and only one cell survives the divisions to await fertilization. The other cells are discarded as polar bodies, which cannot be fertilized. The follicular phase is the first part of the ovarian cycle and it ends with the completion of the antral follicles. Meiosis (cell division) remains incomplete in the egg cells until the antral follicle is formed. During this phase usually only one ovarian follicle fully matures and gets ready to release an egg. The follicular phase shortens significantly with age, lasting around 14 days in women aged 18–24 compared with 10 days in women aged 40–44. Through the influence of a rise in


id: menstrual-cycle-20260505-2
file: data/en.wikipedia.org/wiki/Menstrual_cycle-2.md
category: reference
title: Menstrual cycle
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Menstrual_cycle
date_saved: 2026-05-05T10:54:10.214428+00:00
instance: kb-cron
parent: menstrual-cycle-20260505-0
summary:
Lasting about 14 days, the luteal phase is the final phase of the ovarian cycle and it corresponds to the secretory phase of the uterine cycle. During the luteal phase, the pituitary hormones FSH and LH cause the remaining parts of the dominant follicle to transform into the corpus luteum, which produces progesterone. The increased progesterone starts to induce the production of estrogen. The hormones produced by the corpus luteum also suppress production of the FSH and LH that the corpus luteum needs to maintain itself. The level of FSH and LH fall quickly, and the corpus luteum atrophies. Falling levels of progesterone trigger menstruation and the beginning of the next cycle. For an individual woman, the follicular phase often varies in length from cycle to cycle; by contrast, the length of her luteal phase will be fairly consistent from cycle to cycle at 10 to 16 days (average 14 days). === Uterine cycle === The uterine cycle has three phases: menses, proliferative and secretory. ==== Menstruation ==== Menstruation (also called menstrual bleeding, menses or a period) is the first and most evident phase of the uterine cycle and first occurs at puberty. Called menarche, the first period occurs at


id: menstrual-cycle-20260505-3
file: data/en.wikipedia.org/wiki/Menstrual_cycle-3.md
category: reference
title: Menstrual cycle
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Menstrual_cycle
date_saved: 2026-05-05T10:54:10.214428+00:00
instance: kb-cron
parent: menstrual-cycle-20260505-0
summary:
Although a normal and natural process, some women experience premenstrual syndrome with symptoms that may include acne, tender breasts, and tiredness. More severe symptoms that affect daily living are classed as premenstrual dysphoric disorder and are experienced by 3 to 8% of women. Dysmenorrhea (menstrual cramps or period pain) is felt as painful cramps in the abdomen that can spread to the back and upper thighs during the first few days of menstruation. Debilitating period pain is not normal and can be a sign of something severe such as endometriosis. These issues can significantly affect a woman's health and quality of life and timely interventions can improve the lives of these women. There are common culturally communicated misbeliefs that the menstrual cycle affects women's moods, causes depression or irritability, or that menstruation is a painful, shameful or unclean experience. Often a woman's normal mood variation is falsely attributed to the menstrual cycle. Much of the research is weak, but there appears to be a very small increase in mood fluctuations during the luteal and menstrual phases, and a corresponding decrease during the rest of the cycle. Changing levels of estrogen and progesterone across the menstrual cycle exert systemic effects on


id: menstrual-synchrony-20260505-0
file: data/en.wikipedia.org/wiki/Menstrual_synchrony-0.md
category: reference
title: Menstrual synchrony
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Menstrual_synchrony
date_saved: 2026-05-05T10:54:11.564618+00:00
instance: kb-cron
parent: menstrual-synchrony-20260505-0
summary:
Menstrual synchrony, also called the McClintock effect, or the Wellesley effect, is a hypothetical process whereby women who begin living together in close proximity would experience their menstrual cycle onsets (the onset of menstruation or menses) becoming more synchronized together in time than when previously living apart. "For example, the distribution of onsets of seven female lifeguards was scattered at the beginning of the summer, but after 3 months spent together, the onset of all seven cycles fell within a 4-day period." Martha McClintock's 1971 paper, published in Nature, says that menstrual cycle synchronization happens when the menstrual cycle onsets of two or more women become closer together in time than they were several months earlier. After the initial studies, several papers were published reporting methodological flaws in studies reporting menstrual synchrony, including McClintock's study. In addition, other studies were published that failed to find synchrony. The proposed mechanisms have also received scientific criticism. Reviews in 2006 and 2013 concluded that menstrual synchrony likely does not exist. == Overview == === Original study by Martha McClintock === Martha McClintock published the first study on menstrual synchrony among women living together in dormitories at Wellesley College, a women's liberal arts college


id: menstrual-synchrony-20260505-1
file: data/en.wikipedia.org/wiki/Menstrual_synchrony-1.md
category: reference
title: Menstrual synchrony
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Menstrual_synchrony
date_saved: 2026-05-05T10:54:11.564618+00:00
instance: kb-cron
parent: menstrual-synchrony-20260505-0
summary:
She said that she had been instructed in the menstrual laws by her maternal aunts and grandmother, who in their times were well-known, conservative Yurok women.... the young woman said that in old-time village life all of a household's fertile women who were not pregnant menstruated at the same time, a time dictated by the moon; that these women practised bathing rituals together at this time... If a woman got out of synchronization with the moon and with the other women of the household, she could 'get back in by sitting in the moonlight and talking to the moon, asking it to balance her'. == Scientific details == The phenomenon of menstrual synchrony is the closeness in time of the menstrual cycle onsets of two or more women. The phenomenon is not synchronization in the strict sense of concordance of menstrual cycle onsets but the term menstrual synchrony is still used perhaps misleadingly. As an undergraduate, Martha McClintock published the first study on menstrual synchrony; her report detailed the menstrual synchrony of undergraduate women living in a dormitory in Wellesley College. Since then, there have been attempts to replicate her findings and to determine the conditions under which synchrony occurs,


id: menstrual-synchrony-20260505-2
file: data/en.wikipedia.org/wiki/Menstrual_synchrony-2.md
category: reference
title: Menstrual synchrony
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Menstrual_synchrony
date_saved: 2026-05-05T10:54:11.564618+00:00
instance: kb-cron
parent: menstrual-synchrony-20260505-0
summary:
==== 1990s ==== Wilson, Kiefhabe, and Gravel conducted two studies with college women. The first study consisted of 132 women who were members of a sorority or roommates of members at the University of Missouri. The women were 18 to 22 years of age and the study aimed to replicate McClintock's original study. However, instead of asking women to recall when their last and next to last menstrual onsets occurred, one of the researchers visited the sorority daily to record the occurrence of menstrual onsets and to collect other biographical data. The second study consisted of 24 women living in a cooperative house near the University of Missouri. The women were 18 to 31 years of age. One of the researchers visited the house three times a week recording menstrual onset and collecting more extensive biographical and psychological test data than in the first study. They found no menstrual synchrony in either study. They considered the possibility that women with irregular cycles may reduce the likelihood of detecting synchrony, so they re-analyzed their data after they removed women with irregular cycles, but again there was no statistically significant effect of synchrony. They concluded that "It is clear no meaningful process


id: menstrual-synchrony-20260505-3
file: data/en.wikipedia.org/wiki/Menstrual_synchrony-3.md
category: reference
title: Menstrual synchrony
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Menstrual_synchrony
date_saved: 2026-05-05T10:54:11.564618+00:00
instance: kb-cron
parent: menstrual-synchrony-20260505-0
summary:
==== 2000s ==== Menstrual synchrony research declined after the published critiques in the 1990s and around the turn of the century. The two studies published during this decade incorporated the methodological critiques into their designs and used more appropriate statistical methods. Yang and Schank conducted the largest study to date with 186 Chinese college women. Ninety three of the women lived in 13 dorm rooms, 5 to 8 women per room. The other ninety three women lived in 16 dorm rooms, 4 to 8 women per room for a total of 29 rooms. The women were given notebooks to record the onset of each of their cycles and they collected data for over a year for most of the women. Following the statistical critiques of Schank, they argued that circular statistics were required to analyze periodic data for the existence of synchrony. However, menstrual cycles are variable in frequency (e.g., 28 or 31 day cycles) and in length. They pointed out that there are no statistical methods for analyzing messy data like this, so they developed Monte Carlo methods for detecting synchrony. They found that in 9 of the 29 groups, women's cycles converged for one cycle closer than expected


id: menstrual-synchrony-20260505-4
file: data/en.wikipedia.org/wiki/Menstrual_synchrony-4.md
category: reference
title: Menstrual synchrony
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Menstrual_synchrony
date_saved: 2026-05-05T10:54:11.564618+00:00
instance: kb-cron
parent: menstrual-synchrony-20260505-0
summary:
==== Coupled oscillators ==== When McClintock published her study on menstrual synchrony, she speculated that pheromones may cause menstrual synchrony. In a study on Norway rats, McClintock proposed and tested a coupled oscillator hypothesis (see section on rats below). The coupled-oscillator hypothesis proposed estrous cycles in rats were cause by two, estrous phase dependent pheromones that mutually modulated the length of cycles in a group and thereby causing synchrony. This idea was extended to humans in a study by Stern and McClintock. They investigated whether a coupled-oscillator mechanism first reported for Norway rats (see section below on rats) could also exist in humans. The coupled-oscillator hypothesis in humans proposed that human females release and receive pheromones that regulate the length of their menstrual cycles. This was assumed to occur without consciously detecting any odor. The study was conducted by collecting compounds from axillae (underarms) of donor women at prescribed phases during their menstrual cycles (i.e., the follicular phase, ovulatory phase, and luteal phase), and applying the compounds daily under the noses of recipient women. In order to collect the axillary compounds, the donor women wore cotton pads under their arms for at least 8 hours, and then the pads were


id: menstrual-synchrony-20260505-5
file: data/en.wikipedia.org/wiki/Menstrual_synchrony-5.md
category: reference
title: Menstrual synchrony
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Menstrual_synchrony
date_saved: 2026-05-05T10:54:11.564618+00:00
instance: kb-cron
parent: menstrual-synchrony-20260505-0
summary:
In order to work out why menstrual synchrony might have evolved, it is necessary to investigate why individuals who synchronized their cycles might have had increased survival and reproduction in the evolutionary past. The relevant field in this case is behavioral ecology. In mammalian mating systems generally, and among primates in particular, female spatio-temporal distribution – how clumped females are in the environment and how much they overlap their fertile periods – affects the ability of any single male to monopolize matings. The basic principle is that the more females are fertile at any one time, the harder it is for any single male to monopolize access to them, impregnating all simultaneously at the expense of rival males. In the case of nonhuman primates, once the number of co-cycling females rises above a critical threshold, a harem-holder may be unable to prevent other males from invading and mating with his females. A dominant male can maintain his monopoly only if his females stagger their fertile periods, so that he can impregnate them one at a time (see figure a, right). Suppose a group of female baboons need between them just one dominant male, desirable in view of his high-quality genes.


id: menstrual-synchrony-20260505-6
file: data/en.wikipedia.org/wiki/Menstrual_synchrony-6.md
category: reference
title: Menstrual synchrony
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Menstrual_synchrony
date_saved: 2026-05-05T10:54:11.564618+00:00
instance: kb-cron
parent: menstrual-synchrony-20260505-0
summary:
=== Olfactory influences on menstrual synchrony === College students' menstrual periods can become synchronised when they live together as roommates, according to research by McClintock (McClintock, 1971). Since then, numerous investigations have supported the existence of menstrual synchronisation among women, including close friends, mothers and daughters, and coworkers [reviewed by Weller and Weller in 1993]. Women who spent the most time together were more likely to exhibit menstrual synchrony in each of these investigations. The axillary region's scents have been shown to be capable of mediating these effects (Preti et al., 1987; Russell et al., 1980; Stern and McClintock, 1998), but their active ingredients have not yet been discovered. The main olfactory system, which receives sensory inputs from the olfactory mucosa and connects to the rest of the central nervous system via the main olfactory bulbs, and the accessory system, which receives inputs from the vomeronasal organ and connects to other brain centres via the accessory olfactory bulbs, are the two olfactory systems that are present in the majority of mammals (Scalia and Winans, 1976). There are connections from the olfactory bulbs to the hypothalamus, the brain region in charge of regulating the release of luteinizing hormone, in both systems.


id: menstrual-synchrony-20260505-7
file: data/en.wikipedia.org/wiki/Menstrual_synchrony-7.md
category: reference
title: Menstrual synchrony
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Menstrual_synchrony
date_saved: 2026-05-05T10:54:11.564618+00:00
instance: kb-cron
parent: menstrual-synchrony-20260505-0
summary:
==== Hamsters ==== In 1980, estrous synchrony was reported in female hamsters. In their study, hamsters were housed in four colony phase of the estrous cycle. They monitored and females in each room and removed the females that did not stay in phase. They placed a wire metal cage (i.e., condo consisting of four equally sized rectangular compartments) in the corner of each room. For each room, three animals were randomly selected and placed in three of the condo compartments. A fourth female was randomly selected from another room and placed in the remaining condo compartment. In the control condition, all four females placed in the condos came from the same room. Females were kept in the condos until all four animals exhibited 4 consecutive days of synchrony. They were then removed and a new group was formed until all combinations were tested. They found that the fourth female in the experimental condition always synchronized with the remaining three Their study was criticized as methodologically flawed because females were left together until the fourth female synchronized with the others. When female hamsters are subjected to the stress of stranger hamsters, their cycles become irregular. If only the female from another


id: metonic-cycle-20260505-0
file: data/en.wikipedia.org/wiki/Metonic_cycle-0.md
category: reference
title: Metonic cycle
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Metonic_cycle
date_saved: 2026-05-05T10:54:12.842537+00:00
instance: kb-cron
parent: metonic-cycle-20260505-0
summary:
The Metonic cycle or enneadecaeteris (from Ancient Greek: ἐννεακαιδεκαετηρίς, from ἐννεακαίδεκα, "nineteen") is a period of almost exactly 19 years after which the lunar phases recur at the same time of the year. The recurrence is not perfect, and by precise observation the Metonic cycle defined as 235 synodic months is just 2 hours, 4 minutes and 58 seconds longer than 19 tropical years. Meton of Athens, in the 5th century BC, judged the cycle to be a whole number of days, 6,940. Using these whole numbers facilitates the construction of a lunisolar calendar. A tropical year (about 365.24 days) is longer than 12 lunar months (about 354.36 days) and shorter than 13 of them (about 383.90 days). In a Metonic calendar (a type of lunisolar calendar), there are twelve years of 12 lunar months and seven years of 13 lunar months. == Application in traditional calendars == In the Babylonian and Hebrew lunisolar calendars, the years 3, 6, 8, 11, 14, 17, and 19 are the long (13-month) years of the Metonic cycle. This cycle forms the basis of the Greek and Hebrew calendars. A 19-year cycle is used for the computation of the date of Easter each year.


id: milankovitch-cycles-20260505-0
file: data/en.wikipedia.org/wiki/Milankovitch_cycles-0.md
category: reference
title: Milankovitch cycles
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Milankovitch_cycles
date_saved: 2026-05-05T10:54:14.094355+00:00
instance: kb-cron
parent: milankovitch-cycles-20260505-0
summary:
Milankovitch cycles describe the collective effects of changes in the Earth's movements on its climate over thousands of years. The phenomenon is named after the Serbian geophysicist and astronomer Milutin Milanković. In the 1920s, he provided a more definitive and quantitative analysis than James Croll's earlier hypothesis that variations in eccentricity, axial tilt, and precession combined to result in cyclical variations in the intra-annual and latitudinal distribution of solar radiation at the Earth's surface, and that this orbital forcing strongly influenced the Earth's climatic patterns. == Earth movements == The Earth's rotation around its axis, and revolution around the Sun, evolve over time due to gravitational interactions with other bodies in the Solar System. The variations are complex, but a few cycles are dominant. The Earth's orbit varies between nearly circular and mildly elliptical (its eccentricity varies). When the orbit is more elongated, there is more variation in the distance between the Earth and the Sun, and in the amount of solar radiation, at different times in the year. In addition, the rotational tilt of the Earth (its obliquity) changes slightly. A greater tilt makes the seasons more extreme. Finally, the direction in the fixed stars pointed to by the


id: milankovitch-cycles-20260505-1
file: data/en.wikipedia.org/wiki/Milankovitch_cycles-1.md
category: reference
title: Milankovitch cycles
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Milankovitch_cycles
date_saved: 2026-05-05T10:54:14.094355+00:00
instance: kb-cron
parent: milankovitch-cycles-20260505-0
summary:
The angle of the Earth's axial tilt with respect to the orbital plane (the obliquity of the ecliptic) varies between 22.1° and 24.5°, over a cycle of about 41,000 years. The current tilt is 23.446°, roughly halfway between its extreme values. The tilt last reached its maximum in 8,700 BCE, which correlates with the beginning of the Holocene, the current geological epoch. It is now in the decreasing phase of its cycle, and will reach its minimum around the year 11,800 CE. Increased tilt increases the amplitude of the seasonal cycle in insolation, providing more solar radiation in each hemisphere's summer and less in winter. However, these effects are not uniform everywhere on the Earth's surface. Increased tilt increases the total annual solar radiation at higher latitudes, and decreases the total closer to the equator. The current trend of decreasing tilt, by itself, will promote milder seasons (warmer winters and colder summers), as well as an overall cooling trend. Because most of the planet's snow and ice lies at high latitude, decreasing tilt may encourage the termination of an interglacial period (and lead to an overall cooler climate) and the onset of a glacial period for two reasons: 1) there


id: milankovitch-cycles-20260505-2
file: data/en.wikipedia.org/wiki/Milankovitch_cycles-2.md
category: reference
title: Milankovitch cycles
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Milankovitch_cycles
date_saved: 2026-05-05T10:54:14.094355+00:00
instance: kb-cron
parent: milankovitch-cycles-20260505-0
summary:
Of all the orbital cycles, Milankovitch believed that obliquity had the greatest effect on climate, and that it did so by varying the summer insolation in northern high latitudes. Therefore, he deduced a 41,000-year period for ice ages. However, subsequent research has shown that ice age cycles of the Quaternary glaciation over the last million years have been at a period of 100,000 years, which matches the eccentricity cycle. Various explanations for this discrepancy have been proposed, including frequency modulation or various feedbacks (from carbon dioxide, or ice sheet dynamics). Some models can reproduce the 100,000-year cycles as a result of non-linear interactions between small changes in the Earth's orbit and internal oscillations of the climate system. In particular, the mechanism of the stochastic resonance was originally proposed in order to describe this interaction. Jung-Eun Lee of Brown University proposes that precession changes the amount of energy that Earth absorbs, because the southern hemisphere's greater ability to grow sea ice reflects more energy away from Earth. Moreover, Lee says, "Precession only matters when eccentricity is large. That's why we see a stronger 100,000-year pace than a 21,000-year pace." Some others have argued that the length of the climate record is


id: orbit-20260505-0
file: data/en.wikipedia.org/wiki/Orbit-0.md
category: reference
title: Orbit
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Orbit
date_saved: 2026-05-05T10:54:16.558519+00:00
instance: kb-cron
parent: orbit-20260505-0
summary:
In celestial mechanics, an orbit is the curved trajectory of an object under the influence of an attracting force. Alternatively, it is known as an orbital revolution, because it is a rotation around an axis external to the moving body. Examples for orbits include the trajectory of a planet around a star, a natural satellite around a planet, or an artificial satellite around an object or position in space such as a planet, moon, asteroid, or Lagrange point. Normally, orbit refers to a regularly repeating trajectory, although it may also refer to a non-repeating trajectory. To a close approximation, planets, and satellites follow elliptic orbits, with the center of mass being orbited at a focal point of the ellipse, as described by Kepler's laws of planetary motion. Planets revolve around a star, a natural satellite around a planet, or an artificial satellite around an object or position in space such as a planet, moon, asteroid, or Lagrange point. For most situations, orbital motion is adequately approximated by Newtonian mechanics, which explains gravity as a force obeying an inverse-square law. However, Albert Einstein's general theory of relativity, which accounts for gravity as due to curvature of spacetime, with orbits following geodesics,


id: orbit-20260505-1
file: data/en.wikipedia.org/wiki/Orbit-1.md
category: reference
title: Orbit
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Orbit
date_saved: 2026-05-05T10:54:16.558519+00:00
instance: kb-cron
parent: orbit-20260505-0
summary:
Within a planetary system, various non-stellar objects follow elliptical orbits around the system's barycenter. These objects include planets, dwarf planets, asteroids and other minor planets, comets, meteoroids, and even space debris. A comet in a parabolic or hyperbolic orbit about a barycenter is not gravitationally bound to the star and therefore is not considered part of the star's planetary system. Bodies that are gravitationally bound to one of the planets in a planetary system, including natural satellites, artificial satellites, and the objects within ring systems, follow orbits about a barycenter near or within that planet. Owing to mutual gravitational perturbations, the eccentricities and inclinations of the planetary orbits vary over time. Mercury, the smallest planet in the Solar System, has the most eccentric orbit. At the present epoch, Mars has the next largest eccentricity while the smallest orbital eccentricities are seen with Venus and Neptune. As two objects orbit each other, the periapsis is that point at which the two objects are closest to each other. Less properly, "perifocus" or "pericentron" are used. The apoapsis is that point at which they are the farthest, or sometimes apifocus or apocentron. A line drawn from periapsis to apoapsis is the line-of-apsides. This


id: orbit-20260505-2
file: data/en.wikipedia.org/wiki/Orbit-2.md
category: reference
title: Orbit
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Orbit
date_saved: 2026-05-05T10:54:16.558519+00:00
instance: kb-cron
parent: orbit-20260505-0
summary:
As an illustration of an orbit around a planet, the Newton's cannonball model may prove useful (see image). This is a 'thought experiment', in which a cannon on top of a tall mountain is able to fire a cannonball horizontally at any chosen muzzle speed. The effects of air friction on the cannonball are ignored (or perhaps the mountain is high enough that the cannon is above the Earth's atmosphere, which is the same thing). If the cannon fires its ball with a low initial speed, the trajectory of the ball curves downward, and hits the ground (A). As the firing speed is increased, the cannonball hits the ground farther (B) away from the cannon, because while the ball is still falling towards the ground, the ground is increasingly curving away from it (see first point, above). All these motions are actually "orbits" in a technical sense—they are describing a portion of an elliptical path around the center of gravity—but the orbits are interrupted by striking the Earth. If the cannonball is fired with sufficient speed, the ground curves away from the ball at least as much as the ball falls—so the ball never strikes the ground. It is now


id: orbit-20260505-3
file: data/en.wikipedia.org/wiki/Orbit-3.md
category: reference
title: Orbit
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Orbit
date_saved: 2026-05-05T10:54:16.558519+00:00
instance: kb-cron
parent: orbit-20260505-0
summary:
An open orbit will have a parabolic shape if it has the velocity of exactly the escape velocity at that point in its trajectory, and it will have the shape of a hyperbola when its velocity is greater than the escape velocity. When two bodies approach each other with escape velocity or greater (relative to each other), they will briefly curve around each other at the time of their closest approach, and then separate, and fly apart. All closed orbits have the shape of an ellipse. A circular orbit is a special case, wherein the foci of the ellipse coincide. === Kepler's laws === Bodies following closed orbits repeat their paths with a certain time called the period. This motion is described by the empirical laws of Kepler, which can be mathematically derived from Newton's laws. These can be formulated as follows: The orbit of a planet around the Sun is an ellipse, with the Sun in one of the focal points of that ellipse. [This focal point is actually the barycenter of the Sun-planet system; for simplicity, this explanation assumes the Sun's mass is infinitely larger than that planet's.] The planet's orbit lies in a plane, called the orbital


id: orbit-20260505-4
file: data/en.wikipedia.org/wiki/Orbit-4.md
category: reference
title: Orbit
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Orbit
date_saved: 2026-05-05T10:54:16.558519+00:00
instance: kb-cron
parent: orbit-20260505-0
summary:
O = r cos ⁡ ( θ ) x ^ + r sin ⁡ ( θ ) y ^ = r r ^ {\displaystyle {\mathbf {O} }=r\cos(\theta ){\hat {\mathbf {x} }}+r\sin(\theta ){\hat {\mathbf {y} }}=r{\hat {\mathbf {r} }}} Newton's notation r ˙ {\displaystyle {\dot {r}}} and θ ˙ {\displaystyle {\dot {\theta }}} denotes the standard derivatives of how this distance and angle change over time. Taking the derivative of a vector to see how it changes over a tiny increment of time, δ t {\displaystyle \delta t} , subtracts its location at time t + δ t {\displaystyle t+\delta t} from that at time t {\displaystyle t} and divides by δ t {\displaystyle \delta t} . The result remains a vector. ==== Kepler's second law ==== Because the basis vector r ^ {\displaystyle {\hat {\mathbf {r} }}} moves as the object orbits, the first step is to differentiate it to determine the radial rate of change with time. From time t {\displaystyle t} to t + δ t {\displaystyle t+\delta t} , the vector r ^ {\displaystyle {\hat {\mathbf {r} }}} keeps its beginning at the origin and rotates from angle θ {\displaystyle \theta } to θ + θ ˙


id: orbit-20260505-5
file: data/en.wikipedia.org/wiki/Orbit-5.md
category: reference
title: Orbit
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Orbit
date_saved: 2026-05-05T10:54:16.558519+00:00
instance: kb-cron
parent: orbit-20260505-0
summary:
δ u δ θ = δ δ t ( 1 r ) δ t δ θ = − r ˙ r 2 θ ˙ = − r ˙ h δ 2 u δ θ 2 = − 1 h δ r ˙ δ t δ t δ θ = − r ¨ h θ ˙ = − r ¨ h 2 u 2 or r ¨ = − h 2 u 2 δ 2 u δ θ 2 {\displaystyle {\begin{aligned}{\frac {\delta u}{\delta \theta }}&={\frac {\delta }{\delta t}}\left({\frac {1}{r}}\right){\frac {\delta t}{\delta \theta }}=-{\frac {\dot {r}}{r^{2}{\dot {\theta }}}}=-{\frac {\dot {r}}{h}}\\{\frac {\delta ^{2}u}{\delta \theta ^{2}}}&=-{\frac {1}{h}}{\frac {\delta {\dot {r}}}{\delta t}}{\frac {\delta t}{\delta \theta }}=-{\frac {\ddot {r}}{h{\dot {\theta }}}}=-{\frac {\ddot {r}}{h^{2}u^{2}}}\ \ \ {\text{ or }}\ \ \ {\ddot {r}}=-h^{2}u^{2}{\frac {\delta ^{2}u}{\delta \theta ^{2}}}\end{aligned}}} Plugging these into (1) gives r ¨ − r θ ˙ 2 = − μ r 2 − h 2 u 2 δ 2 u δ θ 2 − 1 u ( h u 2 ) 2 = − μ u 2 {\displaystyle {\begin{aligned}{\ddot {r}}-r{\dot {\theta }}^{2}&=-{\frac {\mu }{r^{2}}}\\-h^{2}u^{2}{\frac {\delta ^{2}u}{\delta \theta ^{2}}}-{\frac {1}{u}}\left(hu^{2}\right)^{2}&=-\mu u^{2}\end{aligned}}} Hence: So for the gravitational force – or, more generally, for any inverse square force


id: orbit-20260505-6
file: data/en.wikipedia.org/wiki/Orbit-6.md
category: reference
title: Orbit
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Orbit
date_saved: 2026-05-05T10:54:16.558519+00:00
instance: kb-cron
parent: orbit-20260505-0
summary:
Six parameters are required to specify a Keplerian orbit about a body. For example, the three numbers that specify the body's initial position and the three values that specify its velocity will define a unique orbit that can be calculated forwards (or backwards) in time. An unperturbed orbit is two-dimensional in a plane fixed in space, known as the orbital plane. In three dimensions, the orientation of this plane relative to a plane of reference, such as the plane of the sky, can be determined by three angles. Extending the analysis to three dimensions requires simply rotating the two-dimensional plane to the required angles relative to the poles of the planetary body involved. By tradition, the standard set of orbital elements is called the set of Keplerian elements, after Johannes Kepler and his laws. These six Keplerian elements are as follows: Inclination (i) Longitude of the ascending node (Ω) Argument of periapsis (ω) Eccentricity (e) Semimajor axis (a) True anomaly at epoch (θ0). The orbital period is simply how long an orbiting body takes to complete one orbit, which can be derived from the semimajor axis, and the combined masses. In principle, once the orbital elements are known for a


id: orbit-20260505-7
file: data/en.wikipedia.org/wiki/Orbit-7.md
category: reference
title: Orbit
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Orbit
date_saved: 2026-05-05T10:54:16.558519+00:00
instance: kb-cron
parent: orbit-20260505-0
summary:
For an object in a sufficiently close orbit about a planetary body with a significant atmosphere, the orbit can decay because of drag. Particularly at each periapsis for an orbital with appreciable eccentricity, the object experiences atmospheric drag, losing energy. Each time, the orbit grows less eccentric (more circular) because the object loses kinetic energy precisely when that energy is at its maximum. This is similar to the effect of slowing a pendulum at its lowest point; the highest point of the pendulum's swing becomes lower. Eventually, the effect becomes so great that the maximum kinetic energy is not enough to return the orbit above the limits of the atmospheric drag effect. When this happens the body will rapidly spiral down and intersect the central body. The region for experiencing atmospheric drag varies by planet; a re-entry vehicle needs to draw much closer to Mars than to Earth, for example, and the drag is negligible for Mercury. The bounds of an atmosphere vary significantly due to solar forcing and space weather. During a solar maximum, the Earth's atmosphere causes drag up to a hundred kilometres higher than during a solar minimum. Orbits can be artificially influenced through the use of


id: orbit-20260505-8
file: data/en.wikipedia.org/wiki/Orbit-8.md
category: reference
title: Orbit
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Orbit
date_saved: 2026-05-05T10:54:16.558519+00:00
instance: kb-cron
parent: orbit-20260505-0
summary:
The effects of other gravitating bodies can be significant. For example, the orbit of the Moon cannot be accurately described without allowing for the action of the Sun's gravity as well as the Earth's. One approximate result is that bodies will usually have reasonably stable orbits around a heavier planet or moon, in spite of these perturbations, provided they are orbiting well within the heavier body's Hill sphere. A long-term impact of multi-body interactions can be apsidal precession, which is a gradual rotation of the line between the apsides. For an elliptical system, the result is a rosetta orbit. The ancient Greek astronomer Hipparchus noted just such an apsidal precession of the Moon's orbit, as the revolution of the Moon's apogee with a period of approximately 8.85 years. Apsidal precession can result from tidal perturbation, rotational perturbation, general relativity, or a combination of these effects. The detection of apsidal precession in a distant binary star system can be an indicator of the purturbative effect of an unseen third stellar companion. When there are more than two gravitating bodies it is referred to as an n-body problem. Most n-body problems have no closed form solution, although some special cases have been


id: orbit-20260505-9
file: data/en.wikipedia.org/wiki/Orbit-9.md
category: reference
title: Orbit
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Orbit
date_saved: 2026-05-05T10:54:16.558519+00:00
instance: kb-cron
parent: orbit-20260505-0
summary:
(6.6742 ± 0.001) × 10−11 (kg/m3)−1s−2. Thus the constant has dimension density−1 time−2. This corresponds to the following properties. Scaling of distances (including sizes of bodies, while keeping the densities the same) gives similar orbits without scaling the time: if for example distances are halved, masses are divided by 8, gravitational forces by 16 and gravitational accelerations by 2. Hence velocities are halved and orbital periods and other travel times related to gravity remain the same. For example, when an object is dropped from a tower, the time it takes to fall to the ground remains the same with a scale model of the tower on a scale model of the Earth. Scaling of distances while keeping the masses the same (in the case of point masses, or by adjusting the densities) gives similar orbits; if distances are multiplied by 4, gravitational forces and accelerations are divided by 16, velocities are halved and orbital periods are multiplied by 8. When all densities are multiplied by 4, orbits are the same; gravitational forces are multiplied by 16 and accelerations by 4, velocities are doubled and orbital periods are halved. When all densities are multiplied by 4, and all sizes are halved,


id: periodical-cicadas-20260505-0
file: data/en.wikipedia.org/wiki/Periodical_cicadas-0.md
category: reference
title: Periodical cicadas
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Periodical_cicadas
date_saved: 2026-05-05T10:54:17.789591+00:00
instance: kb-cron
parent: periodical-cicadas-20260505-0
summary:
The term periodical cicada is commonly used to refer to any of the seven species of the genus Magicicada of eastern North America, the 13- and 17-year cicadas. They are called periodical because nearly all individuals in a local population are developmentally synchronized and emerge in the same year. Although they are sometimes called "locusts", this is a misnomer, as cicadas belong to the taxonomic order Hemiptera (true bugs), suborder Auchenorrhyncha, while locusts are grasshoppers belonging to the order Orthoptera. Magicicada belongs to the cicada tribe Lamotialnini, a group of genera with representatives in Australia, Africa, and Asia, as well as the Americas. Magicicada species spend around 99.5% of their lives underground in an immature state called a nymph. While underground, the nymphs feed on xylem fluids from the roots of broadleaf forest trees in the eastern United States. In the spring of their 13th or 17th year, mature cicada nymphs emerge between late April and early June (depending on latitude), synchronously and in tremendous numbers. The adults are active for only about four to six weeks after the unusually prolonged developmental phase. The males aggregate in chorus centers and call there to attract mates. Mated females lay eggs in


id: periodical-cicadas-20260505-1
file: data/en.wikipedia.org/wiki/Periodical_cicadas-1.md
category: reference
title: Periodical cicadas
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Periodical_cicadas
date_saved: 2026-05-05T10:54:17.789591+00:00
instance: kb-cron
parent: periodical-cicadas-20260505-0
summary:
Nearly all cicadas spend years underground as juveniles, before emerging above ground for a short adult stage of several weeks to a few months. The seven periodical cicada species are so named because, in any one location, all members of the population are developmentally synchronized—they emerge as adults all at once in the same year. This periodicity is especially remarkable because their life cycles are so long—13 or 17 years. In contrast, for nonperiodical species, some adults mature each summer and emerge while the rest of the population continues to develop underground. Many people refer to these nonperiodical species as annual cicadas because some are seen every summer. This may lead some to conclude that the non-periodic cicadas have life cycles of 1 year. This is incorrect. The few known life cycles of "annual" species range from two to 10 years, although some could be longer. The nymphs of the periodical cicadas live underground, usually within 2 ft (61 cm) of the surface, feeding on the juices of plant roots. The nymphs of the periodical cicada undergo five instar stages in their development underground. The difference in the 13- and 17-year life cycle is said to be the time needed


id: periodical-cicadas-20260505-2
file: data/en.wikipedia.org/wiki/Periodical_cicadas-2.md
category: reference
title: Periodical cicadas
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Periodical_cicadas
date_saved: 2026-05-05T10:54:17.789591+00:00
instance: kb-cron
parent: periodical-cicadas-20260505-0
summary:
The nymphs emerge in very large numbers at nearly the same time, sometimes more than 1.5 million individuals per acre (370 individuals per m2). Their mass emergence is, among other things, an adaptation called predator satiation. Although periodical cicadas are easy prey for reptiles, birds, squirrels, cats, dogs and other small and large mammals, there are after synchronized emergence simply too many individuals for the predators to consume; many individuals thus remain behind to procreate. It has been hypothesized that the prime-number development times (13 and 17 years) improve avoidance of predators with shorter reproductive cycles and for this reason have been selected for. A predator with, for example, a three-year reproductive cycle, which happened to benefit from a brood emergence in a given year, will have gone through either four cycles plus one year (12 + 1) or five cycles plus two years (15 + 2) by the next time that the same brood emerges. In this way cicada generations always emerge when some portion of the predators they will confront are sexually immature and therefore incapable of taking maximum advantage of the momentarily limitless food supply. A second hypothesis posits that the prime-numbered developmental times are an adaptation


id: periodical-cicadas-20260505-3
file: data/en.wikipedia.org/wiki/Periodical_cicadas-3.md
category: reference
title: Periodical cicadas
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Periodical_cicadas
date_saved: 2026-05-05T10:54:17.789591+00:00
instance: kb-cron
parent: periodical-cicadas-20260505-0
summary:
=== Species === Seven recognized species are placed within Magicicada—three 17-year species and four 13-year species. These seven species are also sometimes grouped differently into three subgroups, the so-called Decim species group, Cassini species group, and Decula species group, reflecting strong similarities of each 17-year species with one or more species with a 13-year cycle. === Evolution and speciation === Not only are the periodical cicada life cycles curious for their use of the prime numbers 13 or 17, but their evolution is also intricately tied to one- and four-year changes in their life cycles. One-year changes are less common than four-year changes and are probably tied to variation in local climatic conditions. Four-year early and late emergences are common and involve a much larger proportion of the population than one-year changes. The different species are well-understood to have originated from a process of allochronic speciation, in which species subpopulations that are isolated from one another in time eventually become reproductively isolated as well. Research suggests that in extant periodical cicadas, the 13- and 17-year life cycles evolved at least eight different times in the last 4 million years and that different species with identical life cycles developed their overlapping


id: periodical-cicadas-20260505-4
file: data/en.wikipedia.org/wiki/Periodical_cicadas-4.md
category: reference
title: Periodical cicadas
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Periodical_cicadas
date_saved: 2026-05-05T10:54:17.789591+00:00
instance: kb-cron
parent: periodical-cicadas-20260505-0
summary:
== History == Marlatt wrote in his 1907 journal article that the earliest published account of the periodical cicada which had come under his observation appeared in a 1666 issue of the journal Philosophical Transactions of the Royal Society, which at the time had the name Philosophical Transactions. The account stated: A great Observer, who hath lived long in New England, did upon occasion, relate to a Friend of his in London, where he lately was, That some few Years since there was such a swarm of a certain sort of Insects in that English Colony, that for the space of 200 Miles they poyson'd and destroyed all the Trees of that Country; there being found innumerable little holes in the ground, out of which those Insects broke forth in the form of Maggots, which turned into Flyes that had a kind of taile or sting, which they struck into the Tree, and thereby envenomed and killed it. Marlatt also wrote that the next report of the cicada appeared in a work entitled New-Englands Memoriall, which was printed in Cambridge, Massachusetts in 1669. After describing a "pestilent fever" that had swept through the Plymouth Colony and neighboring Indians in 1633,


id: periodical-cicadas-20260505-5
file: data/en.wikipedia.org/wiki/Periodical_cicadas-5.md
category: reference
title: Periodical cicadas
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Periodical_cicadas
date_saved: 2026-05-05T10:54:17.789591+00:00
instance: kb-cron
parent: periodical-cicadas-20260505-0
summary:
There are a kind of Locusts which about every seventeen years come hither in incredible numbers ... In the interval between the years when they are so numerous, they are only seen or heard single in the woods. Based on Kalm's account and a specimen that Kalm had provided, in 1758 Carl Linnaeus named the insect Cicada septendecim in the tenth edition of his Systema Naturae. Moses Bartram, a son of John Bartram, described the next appearance of the brood (Brood X) that Kalm had observed in 1749 in an article entitled Observations on the cicada, or locust of America, which appears periodically once in 16 or 17 years that he wrote in 1766. Bartram's article, which a London journal published in 1768, noted that upon hatching from eggs deposited in the twigs of trees, the young insects ran down to the earth and "entered the first opening that they could find". He reported that he had been able to discover them 10 feet (3 m) below the surface, but that others had reportedly found them 30 feet (9 m) deep. In 1775, Thomas Jefferson recorded in his "Garden Book" Brood II's 17-year periodicity, writing that an acquaintance remembered "great


id: periodical-cicadas-20260505-6
file: data/en.wikipedia.org/wiki/Periodical_cicadas-6.md
category: reference
title: Periodical cicadas
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Periodical_cicadas
date_saved: 2026-05-05T10:54:17.789591+00:00
instance: kb-cron
parent: periodical-cicadas-20260505-0
summary:
The use of the newly emerged and succulent cicadas as an article of human diet has merely a theoretical interest, because, if for no other reason, they occur too rarely to have any real value. There is also the much stronger objection in the instinctive repugnance which all insects seem to inspire as an article of food to most civilized nations. Theoretically, the Cicada, collected at the proper time and suitably dressed and served, should be a rather attractive food. The larvae have lived solely on vegetable matter of the cleanest and most whole-some sort, and supposedly, therefore, would be much more palatable and suitable for food than the oyster, with its scavenger habit of living in the muddy ooze of river bottoms, or many other animals which are highly prized and which have not half so clean a record as the periodical Cicada. == Notes == == References == Latrobe, John H. B. (1845). Memoir of Benjamin Banneker: Read before the Maryland Historical Society at the Monthly Meeting, May 1, 1845. Baltimore, Maryland: Printed by John D. Toy. LCCN rc01003345. OCLC 85791076. Retrieved February 29, 2020 – via Internet Archive. == Further reading == Wikipedia Cicada page. Kritsky, Gene


id: phenology-20260505-0
file: data/en.wikipedia.org/wiki/Phenology-0.md
category: reference
title: Phenology
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Phenology
date_saved: 2026-05-05T10:54:19.081362+00:00
instance: kb-cron
parent: phenology-20260505-0
summary:
Phenology is the study of periodic events in biological life cycles and how these are influenced by seasonal and interannual variations in climate, as well as habitat factors (such as elevation). Examples include the date of emergence of leaves and flowers, the first flight of butterflies, the first appearance of migratory birds, the date of leaf colouring and fall in deciduous trees, the dates of egg-laying of birds and amphibia, or the timing of the developmental cycles of temperate-zone honey bee colonies. In the scientific literature on ecology, the term is used more generally to indicate the time frame for any seasonal biological phenomena, including the dates of last appearance (e.g., the seasonal phenology of a species may be from April through September). Because many such phenomena are very sensitive to small variations in climate, especially to temperature, phenological records can be a useful proxy for temperature in historical climatology, especially in the study of climate change and global warming. For example, viticultural records of grape harvests in Europe have been used to reconstruct a record of summer growing season temperatures going back more than 500 years. In addition to providing a longer historical baseline than instrumental measurements, phenological observations


id: phenology-20260505-1
file: data/en.wikipedia.org/wiki/Phenology-1.md
category: reference
title: Phenology
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Phenology
date_saved: 2026-05-05T10:54:19.081362+00:00
instance: kb-cron
parent: phenology-20260505-0
summary:
Robert Marsham, the founding father of modern phenological recording, was a wealthy landowner who kept systematic records of "Indications of spring" on his estate at Stratton Strawless, Norfolk from 1736. These took the form of dates of the first occurrence of events, such as flowering, bud burst, emergence or flight of an insect. Generations of Marsham's family maintained consistent records of the same events or "phenophases" over unprecedentedly long periods of time, eventually ending with the death of Mary Marsham in 1958, so that trends can be observed and related to long-term climate records. The data show significant variation in dates which broadly correspond with warm and cold years. Between 1850 and 1950, a long-term trend of gradual climate warming is observable, and during this same period the Marsham record of oak-leafing dates tended to become earlier. After 1960, the rate of warming accelerated, and this is mirrored by increasing earliness of oak leafing, recorded in the data collected by Jean Combes in Surrey. Over the past 250 years, the first leafing date of oak appears to have advanced by about 8 days, corresponding to overall warming on the order of 1.5 °C in the same period. Towards the end


id: phenology-20260505-2
file: data/en.wikipedia.org/wiki/Phenology-2.md
category: reference
title: Phenology
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Phenology
date_saved: 2026-05-05T10:54:19.081362+00:00
instance: kb-cron
parent: phenology-20260505-0
summary:
Recent technological advances in studying the earth from space have resulted in a new field of phenological research that is concerned with observing the phenology of whole ecosystems and stands of vegetation on a global scale using proxy approaches. These methods complement the traditional phenological methods which recorded the first occurrences of individual species and phenophases. The most successful of these approaches is based on tracking the temporal change of a Vegetation Index (like Normalized Difference Vegetation Index(NDVI)). NDVI makes use of the vegetation's typical low reflection in the red (red energy is mostly absorbed by growing plants for Photosynthesis) and strong reflection in the Near Infrared (Infrared energy is mostly reflected by plants due to their cellular structure). Due to its robustness and simplicity, NDVI has become one of the most popular remote sensing based products. Typically, a vegetation index is constructed in such a way that the attenuated reflected sunlight energy (1% to 30% of incident sunlight) is amplified by ratio-ing red and NIR following this equation: N D V I = N I R − r e d N I R + r e d {\displaystyle \mathrm {NDVI} ={\mathrm {NIR} -\mathrm {red} \over \mathrm {NIR} +\mathrm {red}


id: reproductive-synchrony-20260505-0
file: data/en.wikipedia.org/wiki/Reproductive_synchrony-0.md
category: reference
title: Reproductive synchrony
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Reproductive_synchrony
date_saved: 2026-05-05T10:54:20.327712+00:00
instance: kb-cron
parent: reproductive-synchrony-20260505-0
summary:
Reproductive synchrony is a term used in evolutionary biology and behavioral ecology. Reproductive synchrony—sometimes termed "ovulatory synchrony"—may manifest itself as "breeding seasonality". Where females undergo regular menstruation, "menstrual synchrony" is another possible term. Reproduction is said to be synchronised when fertile matings across a population are temporarily clustered, resulting in multiple conceptions (and consequent births) within a restricted time window. In marine and other aquatic contexts, the phenomenon may be referred to as mass spawning. Mass spawning has been observed and recorded in a large number of phyla, including in coral communities within the Great Barrier Reef. In primates, reproductive synchrony usually takes the form of conception and birth seasonality. The regulatory "clock", in this case, is the sun's position in relation to the tilt of the earth. In nocturnal or partly nocturnal primates—for example, owl monkeys—the periodicity of the moon may also come into play. Synchrony in general is for primates an important variable determining the extent of "paternity skew"—defined as the extent to which fertile matings can be monopolised by a fraction of the population of males. The greater the precision of female reproductive synchrony—the greater the number of ovulating females who must be guarded simultaneously—the harder it


id: rush-hour-20260505-0
file: data/en.wikipedia.org/wiki/Rush_hour-0.md
category: reference
title: Rush hour
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Rush_hour
date_saved: 2026-05-05T10:54:21.682519+00:00
instance: kb-cron
parent: rush-hour-20260505-0
summary:
A rush hour (American English, British English) or peak hour (Australian English, Indian English) is a part of the day during which traffic congestion on roads and crowding on public transport is at its highest. Normally, this happens twice every weekday: once in the morning and once in the afternoon or evening, the times during which most people commute. The term is often used for a period of peak congestion that may last for more than one hour. The term is very broad, but often refers specifically to private automobile transportation traffic, even when there is a large volume of cars on a road but not many people, or if the volume is normal but there is some disruption of speed. By analogy to vehicular traffic, the term Internet rush hour has been used to describe periods of peak data network usage, resulting in delays and slower delivery of data packets. == Definition == The name is sometimes a misnomer, as the peak period often lasts more than one hour and the "rush" refers to the volume of traffic, not the speed of its flow. Peak traffic periods may vary from country to country, city to city, from region to


id: rush-hour-20260505-1
file: data/en.wikipedia.org/wiki/Rush_hour-1.md
category: reference
title: Rush hour
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Rush_hour
date_saved: 2026-05-05T10:54:21.682519+00:00
instance: kb-cron
parent: rush-hour-20260505-0
summary:
Buses increase frequency from 4 per hour to 12 per hour on the Metrobus network, other routes increase limited and express services The Sydney Trains network runs double-decker electric multiple unit trains that allowed many more passengers to board the trains compared to the 1950s single-level 'Red Rattlers', and 'Silver Ghosts'. Time-of-day ticket prices allow train commuters to board trains before 6 am or after 7 pm at a cheaper rate on single or day return tickets Transit and/or HOV Lanes are installed on many major arterial roads, The Rail Clearways Program, which allows for broken-down trains on the Sydney Trains network to not affect the running of trains on separate lines due to building bypasses, and loop-backs alongside the existing track The Inner West Light Rail, which was the first operational light rail line in Sydney, increases headways during peak hour, providing services up to every eight minutes. Traffic congestion is managed through the Traffic Management Centre via a network of Closed Circuit TV's, with operators able to change the timing of traffic signals to reduce wait times Most major motorways have the ability for contraflow lane to allow continuing flow of traffic in case of a major accident


id: rush-hour-20260505-2
file: data/en.wikipedia.org/wiki/Rush_hour-2.md
category: reference
title: Rush hour
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Rush_hour
date_saved: 2026-05-05T10:54:21.682519+00:00
instance: kb-cron
parent: rush-hour-20260505-0
summary:
=== Philippines === Inside Metro Manila, the Unified Vehicular Volume Reduction Program, popularly known as the "number coding scheme", is implemented by the Metropolitan Manila Development Authority. The program stipulates that vehicles are prohibited from plying all roads within the metropolis, depending on the last digit of their license plates and on the day of the week. The vehicles are banned from 7 am to 7 pm. Unlike the public vehicles, the private vehicles have a five-hour window exception which runs from 10 am to 3 pm. However, the cities of Makati and San Juan do not implement the five-hour window. This table shows the license plates with numbers ending with its corresponding days: Exempted from the program are motorcycles, school buses, shuttle buses, ambulances, fire engines, police cars, military vehicles, those carrying a person needing immediate medical attention, and vehicles with diplomatic license plates. On the other hand, in other places, there are certain policies the municipal or city government are proposing or has implemented for the whole municipality or city. While most schools are open, peak hours in rapid transit trains on Manila Metro Rail Transit System and Manila Light Rail Transit System, and in commuter trains on


id: solar-cycle-20260505-0
file: data/en.wikipedia.org/wiki/Solar_cycle-0.md
category: reference
title: Solar cycle
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Solar_cycle
date_saved: 2026-05-05T10:54:22.923963+00:00
instance: kb-cron
parent: solar-cycle-20260505-0
summary:
The Solar cycle, also known as the solar magnetic activity cycle, sunspot cycle, or Schwabe cycle, is a periodic 11-year change in the Sun's activity measured in terms of variations in the number of observed sunspots on the Sun's surface. Over the period of a solar cycle, levels of solar radiation and ejection of solar material, the number and size of sunspots, solar flares, and coronal loops all exhibit a synchronized fluctuation from a period of minimum activity to a period of a maximum activity back to a period of minimum activity. The magnetic field of the Sun flips during each solar cycle, with the flip occurring when the solar cycle is near its maximum. After two solar cycles, the Sun's magnetic field returns to its original state, completing what is known as a Hale cycle. This cycle has been observed for centuries by changes in the Sun's appearance and by terrestrial phenomena such as aurora but was not clearly identified until 1843. Solar activity, driven by both the solar cycle and transient aperiodic processes, governs the environment of interplanetary space by creating space weather and impacting space- and ground-based technologies as well as the Earth's atmosphere and also possibly


id: solar-cycle-20260505-1
file: data/en.wikipedia.org/wiki/Solar_cycle-1.md
category: reference
title: Solar cycle
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Solar_cycle
date_saved: 2026-05-05T10:54:22.923963+00:00
instance: kb-cron
parent: solar-cycle-20260505-0
summary:
Until 2009, it was thought that 28 cycles had spanned the 309 years between 1699 and 2008, giving an average length of 11.04 years, but research then showed that the longest of these (1784–1799) may actually have been two cycles. If so then the average length would be only around 10.7 years. Since observations began cycles as short as 9 years and as long as 14 years have been observed, and if the cycle of 1784–1799 is double then one of the two component cycles had to be less than 8 years in length. Significant amplitude variations also occur. Several lists of proposed historical "grand minima" of solar activity exist. === Recent cycles === ==== Cycle 25 ==== Solar cycle 25 began in December 2019. Several predictions have been made for solar cycle 25 based on different methods, ranging from very weak to strong magnitude. A physics-based prediction relying on the data-driven solar dynamo and solar surface flux transport models seems to have predicted the strength of the solar polar field at the current minima correctly and forecasts a weak but not insignificant solar cycle 25 similar to or slightly stronger than cycle 24. Notably, they rule out the possibility


id: solar-cycle-20260505-2
file: data/en.wikipedia.org/wiki/Solar_cycle-2.md
category: reference
title: Solar cycle
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Solar_cycle
date_saved: 2026-05-05T10:54:22.923963+00:00
instance: kb-cron
parent: solar-cycle-20260505-0
summary:
The solar magnetic field structures the corona, giving it its characteristic shape visible at times of solar eclipses. Complex coronal magnetic field structures evolve in response to fluid motions at the solar surface, and emergence of magnetic flux produced by dynamo action in the solar interior. For reasons not yet understood in detail, sometimes these structures lose stability, leading to solar flares and coronal mass ejections (CME). Flares consist of an abrupt emission of energy (primarily at ultraviolet and X-ray wavelengths), which may or may not be accompanied by a coronal mass ejection, which consists of injection of energetic particles (primarily ionized hydrogen) into interplanetary space. Flares and CME are caused by sudden localized release of magnetic energy, which drives emission of ultraviolet and X-ray radiation as well as energetic particles. These eruptive phenomena can have a significant impact on Earth's upper atmosphere and space environment, and are the primary drivers of what is now called space weather. Consequently, the occurrence of both geomagnetic storms and solar energetic particle events shows a strong solar cycle variation, peaking close to sunspot maximum. The occurrence frequency of coronal mass ejections and flares is strongly modulated by the cycle. Flares of any given


id: solar-cycle-20260505-3
file: data/en.wikipedia.org/wiki/Solar_cycle-3.md
category: reference
title: Solar cycle
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Solar_cycle
date_saved: 2026-05-05T10:54:22.923963+00:00
instance: kb-cron
parent: solar-cycle-20260505-0
summary:
==== Spacecraft ==== CMEs (coronal mass ejections) produce a radiation flux of high-energy protons, sometimes known as solar cosmic rays. These can cause radiation damage to electronics and solar cells in satellites. Solar proton events also can cause single-event upset (SEU) events on electronics; at the same, the reduced flux of galactic cosmic radiation during solar maximum decreases the high-energy component of particle flux. CME radiation is dangerous to astronauts on a space mission who are outside the shielding produced by the Earth's magnetic field. Future mission designs (e.g., for a Mars Mission) therefore incorporate a radiation-shielded "storm shelter" for astronauts to retreat to during such an event. Gleißberg developed a CME forecasting method that relies on consecutive cycles. The increased irradiance during solar maximum expands the envelope of the Earth's atmosphere, causing low-orbiting space debris to re-enter more quickly. ==== Galactic cosmic ray flux ==== The outward expansion of solar ejecta into interplanetary space provides overdensities of plasma that are efficient at scattering high-energy cosmic rays entering the Solar System from elsewhere in the galaxy. The frequency of solar eruptive events is modulated by the cycle, changing the degree of cosmic ray scattering in the outer Solar System accordingly.


id: solar-cycle-20260505-4
file: data/en.wikipedia.org/wiki/Solar_cycle-4.md
category: reference
title: Solar cycle
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Solar_cycle
date_saved: 2026-05-05T10:54:22.923963+00:00
instance: kb-cron
parent: solar-cycle-20260505-0
summary:
==== Solar radio flux ==== Emission from the Sun at centimetric (radio) wavelength is due primarily to coronal plasma trapped in the magnetic fields overlying active regions. The F10.7 index is a measure of the solar radio flux per unit frequency at a wavelength of 10.7 cm, near the peak of the observed solar radio emission. F10.7 is often expressed in SFU or solar flux units (1 SFU = 10−22 W m−2 Hz−1). It represents a measure of diffuse, nonradiative coronal plasma heating. It is an excellent indicator of overall solar activity levels and correlates well with solar UV emissions. Sunspot activity has a major effect on long distance radio communications, particularly on the shortwave bands although medium wave and low VHF frequencies are also affected. High levels of sunspot activity lead to improved signal propagation on higher frequency bands, although they also increase the levels of solar noise and ionospheric disturbances. These effects are caused by impact of the increased level of solar radiation on the ionosphere. 10.7 cm solar flux could interfere with point-to-point terrestrial communications. ==== Clouds ==== Speculations about the effects of cosmic-ray changes over the cycle potentially include: Changes in ionization affect the aerosol abundance


id: solar-cycle-20260505-5
file: data/en.wikipedia.org/wiki/Solar_cycle-5.md
category: reference
title: Solar cycle
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Solar_cycle
date_saved: 2026-05-05T10:54:22.923963+00:00
instance: kb-cron
parent: solar-cycle-20260505-0
summary:
The 11-year solar cycle is thought to be one-half of a 22-year Babcock–Leighton solar dynamo cycle, which corresponds to an oscillatory exchange of energy between toroidal and poloidal solar magnetic fields which is mediated by solar plasma flows which also provides energy to the dynamo system at every step. At solar-cycle maximum, the external poloidal dipolar magnetic field is near its dynamo-cycle minimum strength, but an internal toroidal quadrupolar field, generated through differential rotation within the tachocline, is near its maximum strength. At this point in the dynamo cycle, buoyant upwelling within the Convection zone forces emergence of the toroidal magnetic field through the photosphere, giving rise to pairs of sunspots, roughly aligned east–west with opposite magnetic polarities. The magnetic polarity of sunspot pairs alternates every solar cycle, a phenomenon described by Hale's law. During the solar cycle's declining phase, energy shifts from the internal toroidal magnetic field to the external poloidal field, and sunspots diminish in number. At solar minimum, the toroidal field is, correspondingly, at minimum strength, sunspots are relatively rare and the poloidal field is at maximum strength. During the next cycle, differential rotation converts magnetic energy back from the poloidal to the toroidal field, with a


id: sunspottery-20260505-0
file: data/en.wikipedia.org/wiki/Sunspottery-0.md
category: reference
title: Sunspottery
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Sunspottery
date_saved: 2026-05-05T10:54:24.159453+00:00
instance: kb-cron
parent: sunspottery-20260505-0
summary:
Sunspottery was a pejorative term assigned to a late 19th century trend to ascribe sunspots and their cycles as causes for various cyclic phenomena, including disease outbreaks and economic crises. William Stanley Jevons read a paper On the influence of the sun-spot period upon the price of corn at a meeting of the British Association for the Advancement of Science in 1875. This was followed by a great deal of media interest and a period when it became fanciful to blame the sun for all kinds of events, with the term being used pejoratively in the newspapers. Jevons later wrote to the Times in April 1879 stating that despite the abuse of his idea "the activity of commerce in England ultimately depends upon the solar activity". A pamphlet dealing with the facts of solar cycles, use, and misuse published in 1883 was reviewed by The Globe, which noted that "Sun-spottery is not what it is represented to be, but is, for the most part, humbug." Sunspot cycles affecting weather and various other phenomena have however continued to be of interest and are still a serious subject of study. == See also == Brückner-Egeson-Lockyer cycle Sunspots (economics) == References ==


id: swarm-behaviour-20260505-0
file: data/en.wikipedia.org/wiki/Swarm_behaviour-0.md
category: reference
title: Swarm behaviour
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Swarm_behaviour
date_saved: 2026-05-05T10:54:25.436021+00:00
instance: kb-cron
parent: swarm-behaviour-20260505-0
summary:
Swarm behaviour, or swarming, is a collective behaviour exhibited by entities, particularly animals, of similar size which aggregate together, perhaps milling about the same spot or perhaps moving en masse or migrating in some direction. It is a highly interdisciplinary topic. As a term, swarming is applied particularly to insects, but can also be applied to any other entity or animal that exhibits swarm behaviour. The term flocking or murmuration can refer specifically to swarm behaviour in birds, herding to refer to swarm behaviour in tetrapods, and shoaling or schooling to refer to swarm behaviour in fish. Phytoplankton also gather in huge swarms called blooms, although these organisms are algae and are not self-propelled the way most animals are. By extension, the term "swarm" is applied also to inanimate entities which exhibit parallel behaviours, as in a robot swarm, an earthquake swarm or a star swarm. From a more abstract point of view, swarm behaviour is the collective motion of a large number of self-propelled entities. From the perspective of the mathematical modeller, it is an emergent behaviour arising from simple rules that are followed by individuals and does not involve any central coordination. Swarm behaviour is also studied by


id: swarm-behaviour-20260505-1
file: data/en.wikipedia.org/wiki/Swarm_behaviour-1.md
category: reference
title: Swarm behaviour
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Swarm_behaviour
date_saved: 2026-05-05T10:54:25.436021+00:00
instance: kb-cron
parent: swarm-behaviour-20260505-0
summary:
A further key concept in the field of swarm intelligence is stigmergy. Stigmergy is a mechanism of indirect coordination between agents or actions. The principle is that the trace left in the environment by an action stimulates the performance of a next action, by the same or a different agent. In that way, subsequent actions tend to reinforce and build on each other, leading to the spontaneous emergence of coherent, apparently systematic activity. Stigmergy is a form of self-organization. It produces complex, seemingly intelligent structures, without need for any planning, control, or even direct communication between the agents. As such it supports efficient collaboration between extremely simple agents, who lack any memory, intelligence or even awareness of each other. === Swarm intelligence === Swarm intelligence is the collective behaviour of decentralized, self-organized systems, natural or artificial. The concept is employed in work on artificial intelligence. The expression was introduced by Gerardo Beni and Jing Wang in 1989, in the context of cellular robotic systems. Swarm intelligence systems are typically made up of a population of simple agents such as boids interacting locally with one another and with their environment. The agents follow very simple rules, and although there is no


id: swarm-behaviour-20260505-2
file: data/en.wikipedia.org/wiki/Swarm_behaviour-2.md
category: reference
title: Swarm behaviour
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Swarm_behaviour
date_saved: 2026-05-05T10:54:25.436021+00:00
instance: kb-cron
parent: swarm-behaviour-20260505-0
summary:
Particle swarm optimization is another algorithm widely used to solve problems related to swarms. It was developed in 1995 by Kennedy and Eberhart and was first aimed at simulating the social behaviour and choreography of bird flocks and fish schools. The algorithm was simplified and it was observed to be performing optimization. The system initially seeds a population with random solutions. It then searches in the problem space through successive generations using stochastic optimization to find the best solutions. The solutions it finds are called particles. Each particle stores its position as well as the best solution it has achieved so far. The particle swarm optimizer tracks the best local value obtained so far by any particle in the local neighbourhood. The remaining particles then move through the problem space following the lead of the optimum particles. At each time iteration, the particle swarm optimiser accelerates each particle toward its optimum locations according to simple mathematical rules. In a related approach, Shvalb et al. (2024) introduced a statistical-physics-based framework for controlling large-scale multi-robot systems. By modeling robots as particles within a statistical ensemble, the study leverages macroscopic parameters—such as density and flow fields—to guide collective behavior without the need for


id: swarm-behaviour-20260505-3
file: data/en.wikipedia.org/wiki/Swarm_behaviour-3.md
category: reference
title: Swarm behaviour
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Swarm_behaviour
date_saved: 2026-05-05T10:54:25.436021+00:00
instance: kb-cron
parent: swarm-behaviour-20260505-0
summary:
In temperate climates, honey bees usually form swarms in late spring. A swarm typically contains about half the workers together with the old queen, while the new queen stays back with the remaining workers in the original hive. When honey bees emerge from a hive to form a swarm, they may gather on a branch of a tree or on a bush only a few meters from the hive. The bees cluster about the queen and send out 20–50 scouts to find suitable new nest locations. The scouts are the most experienced foragers in the cluster. If a scout finds a suitable location, she returns to the cluster and promotes it by dancing a version of the waggle dance. This dance conveys information about the quality, direction, and distance of the new site. The more excited she is about her findings, the more vigorously she dances. If she can convince others they may take off and check the site she found. If they approve they may promote it as well. In this decision-making process, scouts check several sites, often abandoning their own original site to promote the superior site of another scout. Several different sites may be promoted by different


id: swarm-behaviour-20260505-4
file: data/en.wikipedia.org/wiki/Swarm_behaviour-4.md
category: reference
title: Swarm behaviour
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Swarm_behaviour
date_saved: 2026-05-05T10:54:25.436021+00:00
instance: kb-cron
parent: swarm-behaviour-20260505-0
summary:
Locusts are the swarming phase of the short-horned grasshoppers of the family Acrididae. Some species can breed rapidly under suitable conditions and subsequently become gregarious and migratory. They form bands as nymphs and swarms as adults—both of which can travel great distances, rapidly stripping fields and greatly damaging crops. The largest swarms can cover hundreds of square miles and contain billions of locusts. A locust can eat its own weight (about 2 grams) in plants every day. That means one million locusts can eat more than one tonne of food each day, and the largest swarms can consume over 100,000 tonnes each day. Swarming in locusts has been found to be associated with increased levels of serotonin which causes the locust to change colour, eat much more, become mutually attracted, and breed much more easily. Researchers propose that swarming behaviour is a response to overcrowding and studies have shown that increased tactile stimulation of the hind legs or, in some species, simply encountering other individuals causes an increase in levels of serotonin. The transformation of the locust to the swarming variety can be induced by several contacts per minute over a four-hour period. Notably, an innate predisposition to aggregate has


id: swarm-behaviour-20260505-5
file: data/en.wikipedia.org/wiki/Swarm_behaviour-5.md
category: reference
title: Swarm behaviour
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Swarm_behaviour
date_saved: 2026-05-05T10:54:25.436021+00:00
instance: kb-cron
parent: swarm-behaviour-20260505-0
summary:
Other animals may use similar drafting techniques when migrating. Lobsters, for example, migrate in close single-file formation "lobster trains", sometimes for hundreds of miles. The Mediterranean and other seas present a major obstacle to soaring birds, which must cross at the narrowest points. Massive numbers of large raptors and storks pass through areas such as Gibraltar, Falsterbo, and the Bosphorus at migration times. More common species, such as the European honey buzzard, can be counted in hundreds of thousands in autumn. Other barriers, such as mountain ranges, can also cause funnelling, particularly of large diurnal migrants. This is a notable factor in the Central American migratory bottleneck. This concentration of birds during migration can put species at risk. Some spectacular migrants have already gone extinct, the most notable being the passenger pigeon. During migration the flocks were a mile (1.6 km) wide and 300 miles (500 km) long, taking several days to pass and containing up to a billion birds. === Marine life === ==== Fish ==== The term "shoal" can be used to describe any group of fish, including mixed-species groups, while "school" is used for more closely knit groups of the same species swimming in a highly synchronised


id: swarm-behaviour-20260505-6
file: data/en.wikipedia.org/wiki/Swarm_behaviour-6.md
category: reference
title: Swarm behaviour
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Swarm_behaviour
date_saved: 2026-05-05T10:54:25.436021+00:00
instance: kb-cron
parent: swarm-behaviour-20260505-0
summary:
Copepods are a group of tiny crustaceans found in the sea and lakes. Many species are planktonic (drifting in sea waters), and others are benthic (living on the ocean floor). Copepods are typically 1 to 2 millimetres (0.04 to 0.08 in) long, with a teardrop shaped body and large antennae. Although like other crustaceans they have an armoured exoskeleton, they are so small that in most species this thin armour, and the entire body, is almost totally transparent. Copepods have a compound, median single eye, usually bright red, in the centre of the transparent head. Copepods also swarm. For example, monospecific swarms have been observed regularly around coral reefs and sea grass, and in lakes. Swarms densities were about one million copepods per cubic metre. Typical swarms were one or two metres in diameter, but some exceeded 30 cubic metres. Copepods need visual contact to keep together, and they disperse at night. Spring produces blooms of swarming phytoplankton which provide food for copepods. Planktonic copepods are usually the dominant members of the zooplankton, and are in turn major food organisms for many other marine animals. In particular, copepods are prey to forage fish and jellyfish, both of which can assemble


id: swarm-behaviour-20260505-7
file: data/en.wikipedia.org/wiki/Swarm_behaviour-7.md
category: reference
title: Swarm behaviour
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Swarm_behaviour
date_saved: 2026-05-05T10:54:25.436021+00:00
instance: kb-cron
parent: swarm-behaviour-20260505-0
summary:
A collection of people can also exhibit swarm behaviour, such as pedestrians or soldiers swarming the parapets. In Cologne, Germany, two biologists from the University of Leeds demonstrated flock like behaviour in humans. The group of people exhibited similar behavioural pattern to a flock, where if five percent of the flock changed direction the others would follow. If one person was designated as a predator and everyone else was to avoid him, the flock behaved very much like a school of fish. Understanding how humans interact in crowds is important if crowd management is to effectively avoid casualties at football grounds, music concerts and subway stations. The mathematical modelling of flocking behaviour is a common technology, and has found uses in animation. Flocking simulations have been used in many films to generate crowds which move realistically. Tim Burton's Batman Returns was the first movie to make use of swarm technology for rendering, realistically depicting the movements of a group of bats using the boids system. The Lord of the Rings film trilogy made use of similar technology, known as Massive, during battle scenes. Swarm technology is particularly attractive because it is cheap, robust, and simple. An ant-based computer simulation using


id: swarm-behaviour-20260505-8
file: data/en.wikipedia.org/wiki/Swarm_behaviour-8.md
category: reference
title: Swarm behaviour
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Swarm_behaviour
date_saved: 2026-05-05T10:54:25.436021+00:00
instance: kb-cron
parent: swarm-behaviour-20260505-0
summary:
Military swarming is a behaviour where autonomous or partially autonomous units of action attack an enemy from several different directions and then regroup. Pulsing, where the units shift the point of attack, is also a part of military swarming. Military swarming involves the use of a decentralized force against an opponent, in a manner that emphasizes mobility, communication, unit autonomy and coordination or synchronization. Historically military forces used principles of swarming without really examining them explicitly, but now active research consciously examines military doctrines that draw ideas from swarming. Merely because multiple units converge on a target, they are not necessarily swarming. Siege operations do not involve swarming, because there is no manoeuvre; there is convergence but on the besieged fortification. Nor do guerrilla ambushes constitute swarms, because they are "hit-and-run". Even though the ambush may have several points of attack on the enemy, the guerillas withdraw when they either have inflicted adequate damage, or when they are endangered. In 2014 the U. S. Office of Naval Research released a video showing tests of a swarm of small autonomous drone attack boats that can steer and take coordinated offensive action as a group. == Gallery == == Myths == There


id: time-loop-20260505-0
file: data/en.wikipedia.org/wiki/Time_loop-0.md
category: reference
title: Time loop
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Time_loop
date_saved: 2026-05-05T10:54:26.798374+00:00
instance: kb-cron
parent: time-loop-20260505-0
summary:
The time loop or temporal loop is a plot device in fiction whereby characters re-experience a span of time which is repeated, sometimes more than once, with some hope of breaking out of the cycle of repetition. Time loops are constantly resetting; when a certain condition is met, such as a death of a character or a certain point in time, the loop starts again, possibly with one or more characters retaining the memories from the previous loop. A time loop is also sometimes used to describe a scenario involving time travel where events form a circular chain of causality. In this context, actions in the past lead to future events, which then trigger the original journey back in time, creating a self-contained loop without a clear starting point. This concept challenges the conventional linear view of time and is often explored in science fiction and theories of temporal physics, such as those involving closed timelike curves. == History == An early example of a time loop is the 1915 Russian novel Strange Life of Ivan Osokin, where the main character gets to live his life over again but struggles to change it the second time around. The episode "The


id: acoustic-attenuation-20260505-0
file: data/en.wikipedia.org/wiki/Acoustic_attenuation-0.md
category: reference
title: Acoustic attenuation
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Acoustic_attenuation
date_saved: 2026-05-05T10:54:29.602856+00:00
instance: kb-cron
parent: acoustic-attenuation-20260505-0
summary:
In acoustics, acoustic attenuation is a measure of the energy loss of sound propagation through an acoustic transmission medium. Most media have viscosity and are therefore not ideal media. When sound propagates in such media, there is always thermal consumption of energy caused by viscosity. This effect can be quantified through the Stokes's law of sound attenuation. Sound attenuation may also be a result of heat conductivity in the media as has been shown by G. Kirchhoff in 1868. The Stokes-Kirchhoff attenuation formula takes into account both viscosity and thermal conductivity effects. For heterogeneous media, besides media viscosity, acoustic scattering is another main reason for removal of acoustic energy. Acoustic attenuation in a lossy medium plays an important role in many scientific researches and engineering fields, such as medical ultrasonography, vibration and noise reduction. == Power-law frequency-dependent acoustic attenuation == Many experimental and field measurements show that the acoustic attenuation coefficient of a wide range of viscoelastic materials, such as soft tissue, polymers, soil, and porous rock, can be expressed as the following power law with respect to frequency: P ( x + Δ x ) = P ( x ) e − α ( ω ) Δ x ,


id: aharonov-bohm-effect-20260505-0
file: data/en.wikipedia.org/wiki/Aharonov–Bohm_effect-0.md
category: reference
title: Aharonov–Bohm effect
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Aharonov–Bohm_effect
date_saved: 2026-05-05T10:54:30.845762+00:00
instance: kb-cron
parent: aharonov-bohm-effect-20260505-0
summary:
The Aharonov–Bohm effect, sometimes called the Ehrenberg–Siday–Aharonov–Bohm effect, is a quantum-mechanical phenomenon in which an electrically charged particle is affected by an electromagnetic potential ( φ {\displaystyle \varphi } , A {\displaystyle \mathbf {A} } ), despite being confined to a region in which both the magnetic field B {\displaystyle \mathbf {B} } and electric field E {\displaystyle \mathbf {E} } are zero. The underlying mechanism is the coupling of the electromagnetic potential with the complex phase of a charged particle's wave function, and the Aharonov–Bohm effect is accordingly illustrated by interference experiments. The most commonly described case, sometimes called the Aharonov–Bohm solenoid effect, takes place when the wave function of a charged particle passing around a long solenoid experiences a phase shift as a result of the enclosed magnetic field, despite the magnetic field being negligible in the region through which the particle passes and the particle's wavefunction being negligible inside the solenoid. This phase shift has been observed experimentally. There are also magnetic Aharonov–Bohm effects on bound energies and scattering cross sections, but these cases have not been experimentally tested. An electric Aharonov–Bohm phenomenon was also predicted, in which a charged particle is affected by regions with different


id: aharonov-bohm-effect-20260505-1
file: data/en.wikipedia.org/wiki/Aharonov–Bohm_effect-1.md
category: reference
title: Aharonov–Bohm effect
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Aharonov–Bohm_effect
date_saved: 2026-05-05T10:54:30.845762+00:00
instance: kb-cron
parent: aharonov-bohm-effect-20260505-0
summary:
=== Global action vs. local forces === Similarly, the Aharonov–Bohm effect illustrates that the Lagrangian approach to dynamics, based on energies, is not just a computational aid to the Newtonian approach, based on forces. Thus the Aharonov–Bohm effect validates the view that forces are an incomplete way to formulate physics, and potential energies must be used instead. In fact Richard Feynman complained that he had been taught electromagnetism from the perspective of electromagnetic fields, and he wished later in life he had been taught to think in terms of the electromagnetic potential instead, as this would be more fundamental. In Feynman's path-integral view of dynamics, the potential field directly changes the phase of an electron wave function, and it is these changes in phase that lead to measurable quantities. === Locality of electromagnetic effects === The Aharonov–Bohm effect shows that the local E and B fields do not contain full information about the electromagnetic field, and the electromagnetic four-potential, (Φ, A), must be used instead. By Stokes' theorem, the magnitude of the Aharonov–Bohm effect can be calculated using the electromagnetic fields alone, or using the four-potential alone. But when using just the electromagnetic fields, the effect depends on the field


id: aharonov-bohm-effect-20260505-2
file: data/en.wikipedia.org/wiki/Aharonov–Bohm_effect-2.md
category: reference
title: Aharonov–Bohm effect
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Aharonov–Bohm_effect
date_saved: 2026-05-05T10:54:30.845762+00:00
instance: kb-cron
parent: aharonov-bohm-effect-20260505-0
summary:
In quantum mechanics the same particle can travel between two points by a variety of paths. Therefore, this phase difference can be observed by placing a solenoid between the slits of a double-slit experiment (or equivalent). An ideal solenoid (i.e. infinitely long and with a perfectly uniform current distribution) encloses a magnetic field B {\displaystyle \mathbf {B} } , but does not produce any magnetic field outside of its cylinder, and thus the charged particle (e.g. an electron) passing outside experiences no magnetic field B {\displaystyle \mathbf {B} } . (This idealization simplifies the analysis but it's important to realize that the Aharonov–Bohm effect does not rely on it, provided the magnetic flux returns outside the electron paths, for example if one path goes through a toroidal solenoid and the other around it, and the solenoid is shielded so that it produces no external magnetic field.) However, there is a (curl-free) vector potential A {\displaystyle \mathbf {A} } outside the solenoid with an enclosed flux, and so the relative phase of particles passing through one slit or the other is altered by whether the solenoid current is turned on or off. This corresponds to an observable shift of the interference


id: aharonov-bohm-effect-20260505-3
file: data/en.wikipedia.org/wiki/Aharonov–Bohm_effect-3.md
category: reference
title: Aharonov–Bohm effect
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Aharonov–Bohm_effect
date_saved: 2026-05-05T10:54:30.845762+00:00
instance: kb-cron
parent: aharonov-bohm-effect-20260505-0
summary:
== Electric effect == Just as the phase of the wave function depends upon the magnetic vector potential, it also depends upon the scalar electric potential. By constructing a situation in which the electrostatic potential varies for two paths of a particle, through regions of zero electric field, an observable Aharonov–Bohm interference phenomenon from the phase shift has been predicted; again, the absence of an electric field means that, classically, there would be no effect. From the Schrödinger equation, the phase of an eigenfunction with energy E {\displaystyle E} goes as e − i E t / ℏ {\displaystyle e^{-iEt/\hbar }} . The energy, however, will depend upon the electrostatic potential V {\displaystyle V} for a particle with charge q {\displaystyle q} . In particular, for a region with constant potential V {\displaystyle V} (zero field), the electric potential energy q V {\displaystyle qV} is simply added to E {\displaystyle E} , resulting in a phase shift: Δ φ = − q V t ℏ , {\displaystyle \Delta \varphi =-{\frac {qVt}{\hbar }},} where t is the time spent in the potential. For example, we may have a pair of large flat conductors, connected to a battery of voltage Δ V


id: aharonov-bohm-effect-20260505-4
file: data/en.wikipedia.org/wiki/Aharonov–Bohm_effect-4.md
category: reference
title: Aharonov–Bohm effect
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Aharonov–Bohm_effect
date_saved: 2026-05-05T10:54:30.845762+00:00
instance: kb-cron
parent: aharonov-bohm-effect-20260505-0
summary:
The Aharonov–Bohm effect can be understood from the fact that one can only measure absolute values of the wave function. While this allows for measurement of phase differences through quantum interference experiments, there is no way to specify a wavefunction with constant absolute phase. In the absence of an electromagnetic field one can come close by declaring the eigenfunction of the momentum operator with zero momentum to be the function "1" (ignoring normalization problems) and specifying wave functions relative to this eigenfunction "1". In this representation the i-momentum operator is (up to a factor ℏ / i {\displaystyle \hbar /i} ) the differential operator ∂ i = ∂ ∂ x i {\displaystyle \partial _{i}={\frac {\partial }{\partial x^{i}}}} . However, by gauge invariance, it is equally valid to declare the zero momentum eigenfunction to be e − i ϕ ( x ) {\displaystyle e^{-i\phi (x)}} at the cost of representing the i-momentum operator (up to a factor) as ∇ i = ∂ i + i ( ∂ i ϕ ) {\displaystyle \nabla _{i}=\partial _{i}+i(\partial _{i}\phi )} i.e. with a pure gauge vector potential A = d ϕ {\displaystyle A=d\phi } . There is no real asymmetry because representing the former in


id: aharonov-casher-effect-20260505-0
file: data/en.wikipedia.org/wiki/Aharonov–Casher_effect-0.md
category: reference
title: Aharonov–Casher effect
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Aharonov–Casher_effect
date_saved: 2026-05-05T10:54:32.083006+00:00
instance: kb-cron
parent: aharonov-casher-effect-20260505-0
summary:
The Aharonov–Casher effect is a quantum mechanical phenomenon predicted in 1984 by Yakir Aharonov and Aharon Casher, in which a traveling magnetic dipole is affected by an electric field. It is dual to the Aharonov–Bohm effect, in which the quantum phase of a charged particle depends upon which side of a magnetic flux tube it comes through. In the Aharonov–Casher effect, the particle has a magnetic moment and the tubes are charged instead. It was observed in a gravitational neutron interferometer in 1989 and later by fluxon interference of magnetic vortices in Josephson junctions. It has also been seen with electrons and atoms. In both effects the particle acquires a phase shift ( φ {\displaystyle \varphi } ) while traveling along some path P. In the Aharonov–Bohm effect it is φ A B = q ℏ ∫ P A ⋅ d x {\displaystyle \varphi _{\rm {AB}}={\frac {q}{\hbar }}\int _{P}\mathbf {A} \cdot d\mathbf {x} } While for the Aharonov–Casher effect it is φ A C = 1 ℏ c 2 ∫ P ( E × μ ) ⋅ d x {\displaystyle \varphi _{\rm {AC}}={\frac {1}{\hbar c^{2}}}\int _{P}(\mathbf {E} \times {\boldsymbol {\mu }})\cdot d\mathbf {x} } where q {\displaystyle q} is its


id: andreev-reflection-20260505-0
file: data/en.wikipedia.org/wiki/Andreev_reflection-0.md
category: reference
title: Andreev reflection
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Andreev_reflection
date_saved: 2026-05-05T10:54:33.330992+00:00
instance: kb-cron
parent: andreev-reflection-20260505-0
summary:
Andreev reflection, named after the Russian physicist Alexander F. Andreev, is a type of particle scattering which occurs at interfaces between a superconductor (S) and a normal state material (N). It is a charge-transfer process by which normal current in N is converted to supercurrent in S. Each Andreev reflection transfers a charge 2e across the interface, avoiding the forbidden single-particle transmission within the superconducting energy gap. This effect is generally called Andreev reflection but is sometimes called Andreev–Saint-James reflection, as it was predicted independently by Saint-James and de Gennes and by Andreev in the early sixties. == Overview == The process involves an electron incident on the interface from the normal state material at energies less than the superconducting energy gap. The incident electron forms a Cooper pair in the superconductor with the retroreflection of a hole of opposite spin and velocity but equal momentum to the incident electron, as seen in the figure. The barrier transparency is assumed to be high, with no oxide or tunnel layer which reduces instances of normal electron-electron or hole-hole scattering at the interface. Since the pair consists of an up and down spin electron, a second electron of opposite spin to the


id: antiferromagnetism-20260505-0
file: data/en.wikipedia.org/wiki/Antiferromagnetism-0.md
category: reference
title: Antiferromagnetism
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Antiferromagnetism
date_saved: 2026-05-05T10:54:34.556757+00:00
instance: kb-cron
parent: antiferromagnetism-20260505-0
summary:
Antiferromagnetism is a type of magnetic ordering, just like ferromagnetism and ferrimagnetism. In antiferromagnets the individual magnetic dipoles are oriented in a way such that the bulk of the material has a vanishing total magnetization. In the simplest model, as shown in the picture on the right, neighboring moments in an ordered lattice are oriented antiparallel to one another. Since there are multiple ways of arranging magnetic moments on a lattice (especially in three dimensions), there exist various types of antiferromagnetic ordering. The phenomenon of antiferromagnetism was first introduced by Lev Landau in 1933. Generally, antiferromagnetic order may exist at sufficiently low temperatures, but vanishes at and above the Néel temperature – named after Louis Néel, who had first identified this type of magnetic ordering. Above the Néel temperature, the material is typically paramagnetic. == Measurement == When no external field is applied, the antiferromagnetic structure corresponds to a vanishing total magnetization. In an external magnetic field, a kind of ferrimagnetic behavior may be displayed in the antiferromagnetic phase, with the absolute value of one of the sublattice magnetizations differing from that of the other sublattice, resulting in a nonzero net magnetization. Although the net magnetization should be zero at


id: arago-s-rotations-20260505-0
file: data/en.wikipedia.org/wiki/Arago's_rotations-0.md
category: reference
title: Arago's rotations
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Arago's_rotations
date_saved: 2026-05-05T10:54:35.884320+00:00
instance: kb-cron
parent: arago-s-rotations-20260505-0
summary:
Arago's rotations is an observable magnetic phenomenon that involves the interactions between a magnetized needle and a moving metal disk. The effect was discovered by François Arago in 1824. At the time of their discovery, Arago's rotations were surprising effects that were difficult to explain. In 1831, Michael Faraday introduced the theory of electromagnetic induction, which explained how the effects happen in detail. == History == === Early observations and publications === As has so often occurred in other branches of science, the discovery of the magnetic rotations was made nearly simultaneously by several persons, for all of whom priority has been claimed. About 1824, Gambey the celebrated instrument-maker of Paris, had made the casual observation that a compass-needle, when disturbed and set oscillating around its pivot, comes to rest sooner if the bottom of the compass-box is of copper than if it is of wood or other material. Barlow and Marsh, at Woolwich, had at the same time been observing the effect on a magnetic needle of rotating in its neighborhood a sphere of iron. Arago the astronomer, who is said to have learned of the phenomenon from Gambey, but who is also said to have independently discovered it


id: arago-s-rotations-20260505-1
file: data/en.wikipedia.org/wiki/Arago's_rotations-1.md
category: reference
title: Arago's rotations
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Arago's_rotations
date_saved: 2026-05-05T10:54:35.884320+00:00
instance: kb-cron
parent: arago-s-rotations-20260505-0
summary:
=== Investigations of the phenomena by other scientists === At this stage the phenomenon was investigated by several English experimenters, by Babbage and Herschel, by Christie, and, later, by Sturgeon and by Faraday. Babbage and Herschel measured the amount of retarding force exerted on the needle by different materials, and found the most powerful to be silver and copper (which are the two best conductors of electricity), after them gold and zinc, whilst lead, mercury and bismuth were very inferior in power. In 1825 they announced the successful reversal of Arago's experiment; for by spinning the magnet underneath a pivoted copper disk (Fig. 2) they had caused the latter to rotate briskly. They also made the notable observation that slits cut radially in a copper disk (Fig. 3) diminished its tendency to be rotated by the spinning magnet. If the rotatory force of the unslit disk be reckoned as 100, one radial slit reduced it to 88, two radial slits to 77, four to 48, and eight to 24. Amperè, in 1826, showed that a rotating disk of copper also exercises a turning moment upon a neighbouring copper wire through which a current is flowing. Seebeck in Germany, Prévost and


id: arago-s-rotations-20260505-2
file: data/en.wikipedia.org/wiki/Arago's_rotations-2.md
category: reference
title: Arago's rotations
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Arago's_rotations
date_saved: 2026-05-05T10:54:35.884320+00:00
instance: kb-cron
parent: arago-s-rotations-20260505-0
summary:
With the explanation given by Faraday of the Arago rotations, as being merely due to induced eddy-currents, the peculiar interest which they excited whilst their cause was unknown, seems almost to have died out. True, a few years later some interest was revived when Foucault showed that they were capable of heating the metal disk, if in spite of the drag the rotation was forcibly continued in the magnetic field. Why this observation should have caused the eddy-currents discovered by Faraday as the explanation of Arago's phenomenon to be dubbed Foucault's currents is not clear. If anyone is entitled to the honour of having the eddy-currents named after him, it is obviously Faraday or Arago, not Foucault. A little later, Le Roux produced the paradox that a copper disk rotated between concentric magnet poles is not heated thereby, and does not suffer any drag. The explanation of this is as follows. If there is an annular north pole in front of one face of the disk, and an annular south pole in front of the other face, though there is a magnetic field produced right through the disk, there are no eddy-currents. For if all round the disk there are


id: askaryan-radiation-20260505-0
file: data/en.wikipedia.org/wiki/Askaryan_radiation-0.md
category: reference
title: Askaryan radiation
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Askaryan_radiation
date_saved: 2026-05-05T10:54:37.138257+00:00
instance: kb-cron
parent: askaryan-radiation-20260505-0
summary:
The Askaryan radiation also known as Askaryan effect is the phenomenon whereby a particle traveling faster than the phase velocity of light in a dense dielectric (such as salt, ice or the lunar regolith) produces a shower of secondary charged particles which contains a charge anisotropy and emits a cone of coherent radiation in the radio or microwave part of the electromagnetic spectrum. The signal is a result of the Cherenkov radiation from individual particles in the shower. Wavelengths greater than the extent of the shower interfere constructively and thus create a radio or microwave signal which is strongest at the Cherenkov angle. The effect is named after Gurgen Askaryan, a Soviet-Armenian physicist who postulated it in 1962. The radiation was first observed experimentally in 2000, 38 years after its theoretical prediction. So far the effect has been observed in silica sand, rock salt, ice, and Earth's atmosphere. The effect is of primary interest in using bulk matter to detect ultra-high energy neutrinos. The Antarctic Impulse Transient Antenna (ANITA) experiment uses antennas attached to a balloon flying over Antarctica to detect the Askaryan radiation produced by showers of particles when cosmic neutrinos interact in the ice. Several experiments have also


id: attenuation-20260505-0
file: data/en.wikipedia.org/wiki/Attenuation-0.md
category: reference
title: Attenuation
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Attenuation
date_saved: 2026-05-05T10:54:38.322362+00:00
instance: kb-cron
parent: attenuation-20260505-0
summary:
In physics, attenuation is the gradual loss of flux intensity through a medium. For instance, dark glasses attenuate sunlight, lead attenuates X-rays, and water and air attenuate both light and sound at variable attenuation rates. Hearing protectors help reduce acoustic flux from flowing into the ears. This phenomenon is called acoustic attenuation and is measured in decibels (dBs). In electrical engineering and telecommunications, attenuation affects the propagation of waves and signals in electrical circuits, in optical fibers, and in air. Electrical attenuators and optical attenuators are commonly manufactured components in this field. == Background == In many cases, attenuation is an exponential function of the path length through the medium. In optics and in chemical spectroscopy, this is known as the Beer–Lambert law. In engineering, attenuation is usually measured in units of decibels per unit length of medium (dB/cm, dB/km, etc.) and is represented by the attenuation coefficient of the medium in question. Attenuation also occurs in earthquakes; when the seismic waves move farther away from the hypocenter, they grow smaller as they are attenuated by the ground. == Ultrasound == One area of research in which attenuation plays a prominent role is in ultrasound physics. Attenuation in ultrasound is


id: attenuation-20260505-1
file: data/en.wikipedia.org/wiki/Attenuation-1.md
category: reference
title: Attenuation
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Attenuation
date_saved: 2026-05-05T10:54:38.322362+00:00
instance: kb-cron
parent: attenuation-20260505-0
summary:
where P 1 {\displaystyle P_{1}} is the input power into a 100 m long cable terminated with the nominal value of its characteristic impedance, and P 2 {\displaystyle P_{2}} is the output power at the far end of this cable. Attenuation in a coaxial cable is a function of the materials and the construction. === Radiography === The beam of X-ray is attenuated when photons are absorbed when the x-ray beam passes through the tissue. Interaction with matter varies between high energy photons and low energy photons. Photons travelling at higher energy are more capable of travelling through a tissue specimen as they have less chances of interacting with matter. This is mainly due to the photoelectric effect which states that "the probability of photoelectric absorption is approximately proportional to (Z/E)3, where Z is the atomic number of the tissue atom and E is the photon energy. In context of this, an increase in photon energy (E) will result in a rapid decrease in the interaction with matter. In CT imaging, attenuation describes the density or darkness of the image. === Optics === Attenuation in fiber optics, also known as transmission loss, is the reduction in intensity of the light


id: attenuation-20260505-2
file: data/en.wikipedia.org/wiki/Attenuation-2.md
category: reference
title: Attenuation
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Attenuation
date_saved: 2026-05-05T10:54:38.322362+00:00
instance: kb-cron
parent: attenuation-20260505-0
summary:
=== Applications === In optical fibers, attenuation is the rate at which the signal light decreases in intensity. For this reason, glass fiber (which has a low attenuation) is used for long-distance fiber optic cables; plastic fiber has a higher attenuation and, hence, shorter range. There also exist optical attenuators that decrease the signal in a fiber optic cable intentionally. Attenuation of light is also important in physical oceanography. This same effect is an important consideration in weather radar, as raindrops absorb a part of the emitted beam that is more or less significant, depending on the wavelength used. Due to the damaging effects of high-energy photons, it is necessary to know how much energy is deposited in tissue during diagnostic treatments involving such radiation. In addition, gamma radiation is used in cancer treatments where it is important to know how much energy will be deposited in healthy and in tumorous tissue. In computer graphics attenuation defines the local or global influence of light sources and force fields. === Radio === Attenuation is an important consideration in the modern world of wireless telecommunications. Attenuation limits the range of radio signals and is affected by the materials a signal must travel


id: biefeld-brown-effect-20260505-0
file: data/en.wikipedia.org/wiki/Biefeld–Brown_effect-0.md
category: reference
title: Biefeld–Brown effect
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Biefeld–Brown_effect
date_saved: 2026-05-05T10:54:39.546162+00:00
instance: kb-cron
parent: biefeld-brown-effect-20260505-0
summary:
The Biefeld–Brown effect () is an electrical phenomenon, first noticed by inventor Thomas Townsend Brown in the 1920s, where high voltage applied to the electrodes of an asymmetric capacitor causes a net propulsive force toward the smaller electrode. Brown believed this effect was an anti-gravity force, and referred to it as electrogravitics based on it being an electricity/gravity phenomenon. Detailed studies in vacuum chambers failed to replicate Brown's observations and follow-up studies attribute the force measure to corona wind from electrical discharge. == Overview == It is generally assumed that the Biefeld–Brown effect produces an ionic wind that transfers its momentum to surrounding neutral particles. It describes a force observed on an asymmetric capacitor when high voltage is applied to the capacitor's electrodes. Once suitably charged up to high DC potentials, a thrust at the negative terminal, pushing it away from the positive terminal, is generated. The use of an asymmetric capacitor, with the negative electrode being larger than the positive electrode, allowed for more thrust to be produced in the direction from the low-flux to the high-flux region compared to a conventional capacitor. These asymmetric capacitors became known as Asymmetrical Capacitor Thrusters (ACT). These devices can be observed in


id: biefeld-brown-effect-20260505-1
file: data/en.wikipedia.org/wiki/Biefeld–Brown_effect-1.md
category: reference
title: Biefeld–Brown effect
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Biefeld–Brown_effect
date_saved: 2026-05-05T10:54:39.546162+00:00
instance: kb-cron
parent: biefeld-brown-effect-20260505-0
summary:
The effect is generally believed to rely on corona discharge, which allows air molecules to become ionized near sharp points and edges. Usually, two electrodes are used with a high voltage between them, ranging from a few kilovolts and up to megavolt levels, where one electrode is small or sharp, and the other larger and smoother. The most effective distance between electrodes occurs at an electric potential gradient of about 10 kV/cm, which is just below the nominal breakdown voltage of air between two sharp points, at a current density level usually referred to as the saturated corona current condition. This creates a high field gradient around the smaller, positively charged electrode. Around this electrode, ionization occurs, that is, electrons are stripped from the atoms in the surrounding medium; they are literally pulled right off by the electrode's charge. This leaves a cloud of positively charged ions in the medium, which are attracted to the negative smooth electrode by Coulomb's law, where they are neutralized again. This produces an equally scaled opposing force in the lower electrode. This effect can be used for propulsion (see EHD thruster), fluid pumps and recently also in EHD cooling systems. The velocity achievable by


id: breath-figure-self-assembly-20260505-0
file: data/en.wikipedia.org/wiki/Breath-figure_self-assembly-0.md
category: reference
title: Breath-figure self-assembly
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Breath-figure_self-assembly
date_saved: 2026-05-05T10:54:40.829980+00:00
instance: kb-cron
parent: breath-figure-self-assembly-20260505-0
summary:
Breath-figure self-assembly is the self-assembly process of the formation of honeycomb micro-scaled polymer patterns by the condensation of water droplets. "Breath-figure" refers to the fog that forms when water vapor contacts a cold surface. In the modern era systematic study of the process of breath-figures water condensation was carried out by Aitken and Rayleigh, among others. Half a century later the interest in the breath-figure formation was revived in a view of study of atmospheric processes, and in particular the extended study of a dew formation which turned out to be a complicated physical process. The experimental and theoretical study of dew formation has been carried out by Beysens. Thermodynamic and kinetic aspects of dew formation, which are crucial for understanding of formation of breath-figures inspired polymer patterns will be addressed further in detail. Breakthrough in the application of the breath-figures patterns was achieved in 1994–1995 when Widawski, François and Pitois reported manufacturing of polymer films with a self-organized, micro-scaled, honeycomb morphology using the breath-figures condensation process. The reported process was based on the rapidly evaporated polymer solutions exerted to humidity. The introduction to experimental techniques involved in manufacturing of micropatterned surfaces is supplied in reference 1; image representing typical


id: buttered-toast-phenomenon-20260505-0
file: data/en.wikipedia.org/wiki/Buttered_toast_phenomenon-0.md
category: reference
title: Buttered toast phenomenon
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Buttered_toast_phenomenon
date_saved: 2026-05-05T10:54:42.080554+00:00
instance: kb-cron
parent: buttered-toast-phenomenon-20260505-0
summary:
The buttered toast phenomenon is an observation that buttered toast tends to land butter-side down after it falls. It is used as an idiom representing pessimistic outlooks. Various people have attempted to determine whether there is an actual tendency for bread to fall in this fashion, with varying results. == Origins == Written accounts can be traced to the mid-19th century. The phenomenon is often attributed to a parodic poem of James Payn from 1884: I never had a slice of bread, Particularly large and wide, That did not fall upon the floor, And always on the buttered side! In the past, this has often been considered just a pessimistic belief. A 1991 study by the BBC's television series Q.E.D. found that when toast is tossed into the air, it lands butter-side down just one-half of the time (as would be predicted by chance). However, several scientific studies have found that when toast is dropped from a table (as opposed to being thrown in the air), it more often falls butter-side down. A study on this subject by Robert Matthews won the Ig Nobel Prize for physics in 1996. == Explanation == The problem has been studied modelling the toast


id: butterfly-effect-20260505-0
file: data/en.wikipedia.org/wiki/Butterfly_effect-0.md
category: reference
title: Butterfly effect
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Butterfly_effect
date_saved: 2026-05-05T10:54:43.276736+00:00
instance: kb-cron
parent: butterfly-effect-20260505-0
summary:
In chaos theory, the butterfly effect is the sensitive dependence on initial conditions in which a small change in one state of a deterministic nonlinear system can result in large differences in a later state. The term is closely associated with the work of the mathematician and meteorologist Edward Norton Lorenz. He noted that the butterfly effect is derived from the example of the details of a tornado (the exact time of formation, the exact path taken) being influenced by minor perturbations such as a distant butterfly flapping its wings several weeks earlier. Lorenz originally used a seagull causing a storm but was persuaded to make it more poetic with the use of a butterfly and tornado by 1972. He discovered the effect when he observed runs of his weather model with initial condition data that were rounded in a seemingly inconsequential manner. He noted that the weather model would fail to reproduce the results of runs with the unrounded initial condition data. A very small change in initial conditions had created a significantly different outcome. The idea that small causes may have large effects in weather was earlier acknowledged by the French mathematician and physicist Henri Poincaré. The American


id: butterfly-effect-20260505-1
file: data/en.wikipedia.org/wiki/Butterfly_effect-1.md
category: reference
title: Butterfly effect
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Butterfly_effect
date_saved: 2026-05-05T10:54:43.276736+00:00
instance: kb-cron
parent: butterfly-effect-20260505-0
summary:
Following proposals from colleagues, in later speeches and papers, Lorenz used the more poetic butterfly. According to Lorenz, when he failed to provide a title for a talk he was to present at the 139th meeting of the American Association for the Advancement of Science in 1972, Philip Merilees concocted Does the flap of a butterfly's wings in Brazil set off a tornado in Texas? as a title. Although a butterfly flapping its wings has remained constant in the expression of this concept, the location of the butterfly, the consequences, and the location of the consequences have varied widely. The phrase refers to the effect of a butterfly's wings creating tiny changes in the atmosphere that may ultimately alter the path of a tornado or delay, accelerate, or even prevent the occurrence of a tornado in another location. The butterfly does not power or directly create the tornado, but the term is intended to imply that the flap of the butterfly's wings can cause the tornado: in the sense that the flap of the wings is a part of the initial conditions of an interconnected complex web; one set of conditions leads to a tornado, while the other set of


id: butterfly-effect-20260505-2
file: data/en.wikipedia.org/wiki/Butterfly_effect-2.md
category: reference
title: Butterfly effect
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Butterfly_effect
date_saved: 2026-05-05T10:54:43.276736+00:00
instance: kb-cron
parent: butterfly-effect-20260505-0
summary:
which, unlike most chaotic maps, has a closed-form solution: x n = sin 2 ⁡ ( 2 n θ π ) {\displaystyle x_{n}=\sin ^{2}(2^{n}\theta \pi )} where the initial condition parameter θ {\displaystyle \theta } is given by θ = 1 π sin − 1 ⁡ ( x 0 1 / 2 ) {\displaystyle \theta ={\tfrac {1}{\pi }}\sin ^{-1}(x_{0}^{1/2})} . For rational θ {\displaystyle \theta } , after a finite number of iterations x n {\displaystyle x_{n}} maps into a periodic sequence. But almost all θ {\displaystyle \theta } are irrational, and, for irrational θ {\displaystyle \theta } , x n {\displaystyle x_{n}} never repeats itself – it is non-periodic. This solution equation clearly demonstrates the two key features of chaos – stretching and folding: the factor 2n shows the exponential growth of stretching, which results in sensitive dependence on initial conditions (the butterfly effect), while the squared sine function keeps x n {\displaystyle x_{n}} folded within the range [0, 1]. == In physical systems == === In weather === ==== Overview ==== The butterfly effect is most familiar in terms of weather; it can easily be demonstrated in standard weather prediction models, for example. The climate scientists James Annan


id: butterfly-effect-20260505-3
file: data/en.wikipedia.org/wiki/Butterfly_effect-3.md
category: reference
title: Butterfly effect
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Butterfly_effect
date_saved: 2026-05-05T10:54:43.276736+00:00
instance: kb-cron
parent: butterfly-effect-20260505-0
summary:
=== In quantum mechanics === The potential for sensitive dependence on initial conditions (the butterfly effect) has been studied in a number of cases in semiclassical and quantum physics, including atoms in strong fields and the anisotropic Kepler problem. Some authors have argued that extreme (exponential) dependence on initial conditions is not expected in pure quantum treatments; however, the sensitive dependence on initial conditions demonstrated in classical motion is included in the semiclassical treatments developed by Martin Gutzwiller and John B. Delos and co-workers. The random matrix theory and simulations with quantum computers prove that some versions of the butterfly effect in quantum mechanics do not exist. Other authors suggest that the butterfly effect can be observed in quantum systems. Zbyszek P. Karkuszewski et al. consider the time evolution of quantum systems which have slightly different Hamiltonians. They investigate the level of sensitivity of quantum systems to small changes in their given Hamiltonians. David Poulin et al. presented a quantum algorithm to measure fidelity decay, which "measures the rate at which identical initial states diverge when subjected to slightly different dynamics". They consider fidelity decay to be "the closest quantum analog to the (purely classical) butterfly effect". Whereas the classical


id: casimir-effect-20260505-0
file: data/en.wikipedia.org/wiki/Casimir_effect-0.md
category: reference
title: Casimir effect
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Casimir_effect
date_saved: 2026-05-05T10:54:44.616020+00:00
instance: kb-cron
parent: casimir-effect-20260505-0
summary:
In quantum field theory, the Casimir effect (or Casimir force) is a physical force acting on the macroscopic boundaries of a confined space which arises from the quantum fluctuations of a field. The term Casimir pressure is sometimes used when it is described in units of force per unit area. It is named after the Dutch physicist Hendrik Casimir, who predicted the effect for electromagnetic systems in 1948. In the same year Casimir, together with Dirk Polder, described a similar effect experienced by a neutral atom in the vicinity of a macroscopic interface which is called the Casimir–Polder force. Their result is a generalization of the London–van der Waals force and includes retardation due to the finite speed of light. The fundamental principles leading to the London–van der Waals force, the Casimir force, and the Casimir–Polder force can be formulated on the same footing. In 1997, a direct experiment by Steven K. Lamoreaux quantitatively measured the Casimir force to be within 5% of the value predicted by the theory. The Casimir effect can be understood by the idea that the presence of macroscopic material interfaces, such as electrical conductors and dielectrics, alters the vacuum expectation value of the energy of


id: casimir-effect-20260505-1
file: data/en.wikipedia.org/wiki/Casimir_effect-1.md
category: reference
title: Casimir effect
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Casimir_effect
date_saved: 2026-05-05T10:54:44.616020+00:00
instance: kb-cron
parent: casimir-effect-20260505-0
summary:
The causes of the Casimir effect are described by quantum field theory, which states that all of the various fundamental fields, such as the electromagnetic field, must be quantized at each and every point in space. In a simplified view, a "field" in physics may be envisioned as if space were filled with interconnected vibrating balls and springs, and the strength of the field can be visualized as the displacement of a ball from its rest position. Vibrations in this field propagate and are governed by the appropriate wave equation for the particular field in question. The second quantization of quantum field theory requires that each such ball-spring combination be quantized, that is, that the strength of the field be quantized at each point in space. At the most basic level, the field at each point in space is a simple harmonic oscillator, and its quantization places a quantum harmonic oscillator at each point. Excitations of the field correspond to the elementary particles of particle physics. However, even the vacuum has a vastly complex structure, so all calculations of quantum field theory must be made in relation to this model of the vacuum. The vacuum has, implicitly, all of the


id: casimir-effect-20260505-2
file: data/en.wikipedia.org/wiki/Casimir_effect-2.md
category: reference
title: Casimir effect
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Casimir_effect
date_saved: 2026-05-05T10:54:44.616020+00:00
instance: kb-cron
parent: casimir-effect-20260505-0
summary:
F ( p ) = − δ ⟨ E ( s ) ⟩ δ s | p . {\displaystyle F(p)=-\left.{\frac {\delta \langle E(s)\rangle }{\delta s}}\right\vert _{p}\,.} This value is finite in many practical calculations. Attraction between the plates can be easily understood by focusing on the one-dimensional situation. Suppose that a moveable conductive plate is positioned at a short distance a from one of two widely separated plates (distance l apart). With a ≪ l, the states within the slot of width a are highly constrained so that the energy E of any one mode is widely separated from that of the next. This is not the case in the large region l where there is a large number of states (about ⁠l/a⁠) with energy evenly spaced between E and the next mode in the narrow slot, or in other words, all slightly larger than E. Now on shortening a by an amount da (which is negative), the mode in the narrow slot shrinks in wavelength and therefore increases in energy proportional to −⁠da/a⁠, whereas all the ⁠l/a⁠ states that lie in the large region lengthen and correspondingly decrease their energy by an amount proportional to −⁠da/l⁠ (note the different


id: casimir-effect-20260505-3
file: data/en.wikipedia.org/wiki/Casimir_effect-3.md
category: reference
title: Casimir effect
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Casimir_effect
date_saved: 2026-05-05T10:54:44.616020+00:00
instance: kb-cron
parent: casimir-effect-20260505-0
summary:
⟨ E ⟩ A = lim s → 0 ⟨ E ( s ) ⟩ A = − ℏ c π 2 6 a 3 ζ ( − 3 ) . {\displaystyle {\frac {\langle E\rangle }{A}}=\lim _{s\to 0}{\frac {\langle E(s)\rangle }{A}}=-{\frac {\hbar c\pi ^{2}}{6a^{3}}}\zeta (-3)\,.} But ζ(−3) = ⁠1/120⁠ and so one obtains ⟨ E ⟩ A = − ℏ c π 2 720 a 3 . {\displaystyle {\frac {\langle E\rangle }{A}}=-{\frac {\hbar c\pi ^{2}}{720a^{3}}}\,.} The analytic continuation has evidently lost an additive positive infinity, somehow exactly accounting for the zero-point energy (not included above) outside the slot between the plates, but which changes upon plate movement within a closed system. The Casimir force per unit area ⁠Fc/A⁠ for idealized, perfectly conducting plates with vacuum between them is F c A = − d d a ⟨ E ⟩ A = − ℏ c π 2 240 a 4 {\displaystyle {\frac {F_{\mathrm {c} }}{A}}=-{\frac {d}{da}}{\frac {\langle E\rangle }{A}}=-{\frac {\hbar c\pi ^{2}}{240a^{4}}}} where ħ is the reduced Planck constant, c is the speed of light, a is the distance between the two plates The force is negative, indicating that the force is attractive: by moving the two plates closer together, the


id: casimir-effect-20260505-4
file: data/en.wikipedia.org/wiki/Casimir_effect-4.md
category: reference
title: Casimir effect
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Casimir_effect
date_saved: 2026-05-05T10:54:44.616020+00:00
instance: kb-cron
parent: casimir-effect-20260505-0
summary:
⟨ E ( t ) ⟩ = 1 2 ∑ n ℏ | ω n | exp ⁡ ( − t | ω n | ) , {\displaystyle \langle E(t)\rangle ={\frac {1}{2}}\sum _{n}\hbar |\omega _{n}|\exp {\bigl (}-t|\omega _{n}|{\bigr )}\,,} where the limit t → 0+ is taken in the end. The divergence of the sum is typically manifested as ⟨ E ( t ) ⟩ = C t 3 + finite {\displaystyle \langle E(t)\rangle ={\frac {C}{t^{3}}}+{\textrm {finite}}\,} for three-dimensional cavities. The infinite part of the sum is associated with the bulk constant C which does not depend on the shape of the cavity. The interesting part of the sum is the finite part, which is shape-dependent. The Gaussian regulator ⟨ E ( t ) ⟩ = 1 2 ∑ n ℏ | ω n | exp ⁡ ( − t 2 | ω n | 2 ) {\displaystyle \langle E(t)\rangle ={\frac {1}{2}}\sum _{n}\hbar |\omega _{n}|\exp \left(-t^{2}|\omega _{n}|^{2}\right)} is better suited to numerical calculations because of its superior convergence properties, but is more difficult to use in theoretical calculations. Other, suitably smooth, regulators may be used as well. The zeta function regulator ⟨ E ( s ) ⟩ = 1 2


id: casimir-effect-20260505-5
file: data/en.wikipedia.org/wiki/Casimir_effect-5.md
category: reference
title: Casimir effect
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Casimir_effect
date_saved: 2026-05-05T10:54:44.616020+00:00
instance: kb-cron
parent: casimir-effect-20260505-0
summary:
== Repulsive forces == There are a few instances where the Casimir effect can give rise to repulsive forces between uncharged objects. Evgeny Lifshitz showed (theoretically) that in certain circumstances (most commonly involving liquids), repulsive forces can arise. This has sparked interest in applications of the Casimir effect toward the development of levitating devices. An experimental demonstration of the Casimir-based repulsion predicted by Lifshitz was carried out by Munday et al. who described it as "quantum levitation". Other scientists have also suggested the use of gain media to achieve a similar levitation effect, though this is controversial because these materials seem to violate fundamental causality constraints and the requirement of thermodynamic equilibrium (Kramers–Kronig relations). Casimir and Casimir–Polder repulsion can in fact occur for sufficiently anisotropic electrical bodies; for a review of the issues involved with repulsion see Milton et al. A notable recent development on repulsive Casimir forces relies on using chiral materials. Q.-D. Jiang at Stockholm University and Nobel Laureate Frank Wilczek at MIT show that chiral "lubricant" can generate repulsive, enhanced, and tunable Casimir interactions. Timothy Boyer showed in his work published in 1968 that a conductor with spherical symmetry will also show this repulsive force, and the


id: cavitation-20260505-0
file: data/en.wikipedia.org/wiki/Cavitation-0.md
category: reference
title: Cavitation
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Cavitation
date_saved: 2026-05-05T10:54:45.984996+00:00
instance: kb-cron
parent: cavitation-20260505-0
summary:
Cavitation in fluid mechanics and engineering normally is the phenomenon in which the static pressure of a liquid reduces to below the liquid's vapor pressure, leading to the formation of small vapor-filled cavities in the liquid. When subjected to higher pressure, these cavities, called "bubbles" or "voids", collapse and can generate shock waves that may damage machinery. As a concrete propeller example: The pressure on the suction side of the propeller blades can be very low and when the pressure falls to that of the vapour pressure of the working liquid, cavities filled with gas vapour can form. The process of the formation of these cavities is referred to as cavitation. If the cavities move into the regions of higher pressure (lower velocity), they will implode or collapse. These shock waves are strong when they are very close to the imploded bubble, but rapidly weaken as they propagate away from the implosion. Cavitation collapse is therefore a significant cause of wear in some engineering contexts. Collapsing voids that implode near to a hard surface cause cyclic stress through repeated implosion. This results in surface fatigue of the material, causing a type of damage also called "cavitation damage" or "cavitation erosion".


id: cavitation-20260505-1
file: data/en.wikipedia.org/wiki/Cavitation-1.md
category: reference
title: Cavitation
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Cavitation
date_saved: 2026-05-05T10:54:45.984996+00:00
instance: kb-cron
parent: cavitation-20260505-0
summary:
=== Inertial cavitation === Inertial cavitation was first observed in the late 19th century, considering the collapse of a spherical void within a liquid. When a volume of liquid is subjected to a sufficiently low pressure, it may rupture and form a cavity. This phenomenon is coined cavitation inception and may occur behind the blade of a rapidly rotating propeller or on any surface vibrating in the liquid with sufficient amplitude and acceleration. A fast-flowing river can cause cavitation on rock surfaces, particularly when there is a drop-off, such as on a waterfall. Vapor gases evaporate into the cavity from the surrounding medium; thus, the cavity is not a vacuum at all, but rather a low-pressure vapor (gas) bubble. Once the conditions which caused the bubble to form are no longer present, such as when the bubble moves downstream, the surrounding liquid begins to implode due its higher pressure, building up momentum as it moves inward. As the bubble finally collapses, the inward momentum of the surrounding liquid causes a sharp increase of pressure and temperature of the vapor within. The bubble eventually collapses to a minute fraction of its original size, at which point the gas within dissipates into


id: cavitation-20260505-2
file: data/en.wikipedia.org/wiki/Cavitation-2.md
category: reference
title: Cavitation
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Cavitation
date_saved: 2026-05-05T10:54:45.984996+00:00
instance: kb-cron
parent: cavitation-20260505-0
summary:
=== Hydrodynamic cavitation === Hydrodynamic cavitation is the process of vaporisation, bubble generation and bubble implosion which occurs in a flowing liquid as a result of a decrease and subsequent increase in local pressure. Cavitation will only occur if the local pressure declines to some point below the saturated vapor pressure of the liquid and subsequent recovery above the vapor pressure. If the recovery pressure is not above the vapor pressure then flashing is said to have occurred. In pipe systems, cavitation typically occurs either as the result of an increase in the kinetic energy (through an area constriction) or an increase in the pipe elevation. Hydrodynamic cavitation can be produced by passing a liquid through a constricted channel at a specific flow velocity or by mechanical rotation of an object through a liquid. In the case of the constricted channel and based on the specific (or unique) geometry of the system, the combination of pressure and kinetic energy can create the hydrodynamic cavitation cavern downstream of the local constriction generating high energy cavitation bubbles. Based on the thermodynamic phase change diagram, an increase in temperature could initiate a known phase change mechanism known as boiling. However, a decrease in


id: cavitation-20260505-3
file: data/en.wikipedia.org/wiki/Cavitation-3.md
category: reference
title: Cavitation
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Cavitation
date_saved: 2026-05-05T10:54:45.984996+00:00
instance: kb-cron
parent: cavitation-20260505-0
summary:
=== Acoustic cavitation and ultrasonic cavitation === Inertial cavitation can also occur in the presence of an acoustic field. Microscopic gas bubbles that are generally present in a liquid will be forced to oscillate due to an applied acoustic field. If the acoustic intensity is sufficiently high, the bubbles will first grow in size and then rapidly collapse. Hence, inertial cavitation can occur even if the rarefaction in the liquid is insufficient for a Rayleigh-like void to occur. Ultrasonic cavitation inception will occur when the acceleration of the ultrasound source is enough to produce the needed pressure drop. This pressure drop depends on the value of the acceleration and the size of the affected volume by the pressure wave. The dimensionless number that predicts ultrasonic cavitation is the Garcia-Atance number. High power ultrasonic horns produce accelerations high enough to create a cavitating region that can be used for homogenization, dispersion, deagglomeration, erosion, cleaning, milling, emulsification, extraction, disintegration, and sonochemistry. == Applications == === Water transport === In supercavitation, cavitation is intentionally induced around high-speed submerged objects to reduce skin drag. === Chemical engineering === In industry, cavitation is often used to homogenize, or mix and break down, suspended particles in


id: cavitation-20260505-4
file: data/en.wikipedia.org/wiki/Cavitation-4.md
category: reference
title: Cavitation
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Cavitation
date_saved: 2026-05-05T10:54:45.984996+00:00
instance: kb-cron
parent: cavitation-20260505-0
summary:
Cavitation is usually an undesirable occurrence. In devices such as propellers and pumps, cavitation causes a great deal of noise, damage to components, vibrations, and a loss of efficiency. Noise caused by cavitation can be particularly undesirable in naval vessels where such noise may render them more easily detectable by passive sonar. Cavitation has also become a concern in the renewable energy sector as it may occur on the blade surface of tidal stream turbines. When the cavitation bubbles collapse, they force energetic liquid into very small volumes, thereby creating spots of high temperature and emitting shock waves, the latter of which are a source of noise. The noise created by cavitation is a particular problem for military submarines, as it increases the chances of being detected by passive sonar. Although the collapse of a small cavity is a relatively low-energy event, highly localized collapses can erode metals, such as steel, over time. The pitting caused by the collapse of cavities produces great wear on components and can dramatically shorten a propeller's or pump's lifetime. After a surface is initially affected by cavitation, it tends to erode at an accelerating pace. The cavitation pits increase the turbulence of the fluid


id: cavitation-20260505-5
file: data/en.wikipedia.org/wiki/Cavitation-5.md
category: reference
title: Cavitation
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Cavitation
date_saved: 2026-05-05T10:54:45.984996+00:00
instance: kb-cron
parent: cavitation-20260505-0
summary:
==== Cavitation solutions ==== Since all pumps require well-developed inlet flow to meet their potential, a pump may not perform or be as reliable as expected due to a faulty suction piping layout such as a close-coupled elbow on the inlet flange. When poorly developed flow enters the pump impeller, it strikes the vanes and is unable to follow the impeller passage. The liquid then separates from the vanes causing mechanical problems due to cavitation, vibration and performance problems due to turbulence and poor filling of the impeller. This results in premature seal, bearing and impeller failure, high maintenance costs, high power consumption, and less-than-specified head and/or flow. To have a well-developed flow pattern, pump manufacturer's manuals recommend about (10 diameters?) of straight pipe run upstream of the pump inlet flange. Unfortunately, piping designers and plant personnel must contend with space and equipment layout constraints and usually cannot comply with this recommendation. Instead, it is common to use an elbow close-coupled to the pump suction which creates a poorly developed flow pattern at the pump suction. With a double-suction pump tied to a close-coupled elbow, flow distribution to the impeller is poor and causes reliability and performance shortfalls. The elbow


id: cavitation-20260505-6
file: data/en.wikipedia.org/wiki/Cavitation-6.md
category: reference
title: Cavitation
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Cavitation
date_saved: 2026-05-05T10:54:45.984996+00:00
instance: kb-cron
parent: cavitation-20260505-0
summary:
=== Engines === Some larger diesel engines suffer from cavitation due to high compression and undersized cylinder walls. Vibrations of the cylinder wall induce alternating low and high pressure in the coolant against the cylinder wall. The result is pitting of the cylinder wall, which will eventually let cooling fluid leak into the cylinder and combustion gases to leak into the coolant. It is possible to prevent this from happening with the use of chemical additives in the cooling fluid that form a protective layer on the cylinder wall. This layer will be exposed to the same cavitation, but rebuilds itself. Additionally a regulated overpressure in the cooling system (regulated and maintained by the coolant filler cap spring pressure) prevents the forming of cavitation. From about the 1980s, new designs of smaller gasoline engines also displayed cavitation phenomena. One answer to the need for smaller and lighter engines was a smaller coolant volume and a correspondingly higher coolant flow velocity. This gave rise to rapid changes in flow velocity and therefore rapid changes of static pressure in areas of high heat transfer. Where resulting vapor bubbles collapsed against a surface, they had the effect of first disrupting protective oxide layers


id: cavitation-20260505-7
file: data/en.wikipedia.org/wiki/Cavitation-7.md
category: reference
title: Cavitation
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Cavitation
date_saved: 2026-05-05T10:54:45.984996+00:00
instance: kb-cron
parent: cavitation-20260505-0
summary:
== History == As early as 1754, the Swiss mathematician Leonhard Euler (1707–1783) speculated about the possibility of cavitation. In 1859, the English mathematician William Henry Besant (1828–1917) published a solution to the problem of the dynamics of the collapse of a spherical cavity in a fluid, which had been presented by the Anglo-Irish mathematician George Stokes (1819–1903) as one of the Cambridge University Senate-house problems and riders for the year 1847. In 1894, Irish fluid dynamicist Osborne Reynolds (1842–1912) studied the formation and collapse of vapor bubbles in boiling liquids and in constricted tubes. The term cavitation first appeared in 1895 in a paper by John Isaac Thornycroft (1843–1928) and Sydney Walker Barnaby (1855–1925)—son of Sir Nathaniel Barnaby (1829 – 1915), who had been Chief Constructor of the Royal Navy—to whom it had been suggested by the British engineer Robert Edmund Froude (1846–1924), third son of the English hydrodynamicist William Froude (1810–1879). Early experimental studies of cavitation were conducted in 1894–5 by Thornycroft and Barnaby and by the Anglo-Irish engineer Charles Algernon Parsons (1854–1931), who constructed a stroboscopic apparatus to study the phenomenon. Thornycroft and Barnaby were the first researchers to observe cavitation on the back sides of propeller


id: cherenkov-radiation-20260505-0
file: data/en.wikipedia.org/wiki/Cherenkov_radiation-0.md
category: reference
title: Cherenkov radiation
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Cherenkov_radiation
date_saved: 2026-05-05T10:54:47.273838+00:00
instance: kb-cron
parent: cherenkov-radiation-20260505-0
summary:
Cherenkov radiation () is an electromagnetic radiation emitted when a charged particle (such as an electron) passes through a dielectric medium (such as distilled water) at a speed greater than the phase velocity (speed of propagation of a wavefront in a medium) of light in that medium. A classic example of Cherenkov radiation is the characteristic blue glow of an underwater nuclear reactor. Its cause is similar to the cause of a sonic boom, the sharp sound heard when faster-than-sound movement occurs. The phenomenon is named after Soviet physicist Pavel Cherenkov. == History == The radiation is named after the Soviet scientist Pavel Cherenkov, the 1958 Nobel Prize winner, who was the first to detect it experimentally under the supervision of Sergey Vavilov at the Lebedev Institute in 1934. Therefore, it is also known as Vavilov–Cherenkov radiation. Cherenkov saw a faint bluish light around a radioactive preparation in water during experiments. His doctorate thesis was on luminescence of uranium salt solutions that were excited by gamma rays instead of less energetic visible light, as done commonly. He discovered the anisotropy of the radiation and came to the conclusion that the bluish glow was not a fluorescent phenomenon. A theory of


id: cherenkov-radiation-20260505-1
file: data/en.wikipedia.org/wiki/Cherenkov_radiation-1.md
category: reference
title: Cherenkov radiation
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Cherenkov_radiation
date_saved: 2026-05-05T10:54:47.273838+00:00
instance: kb-cron
parent: cherenkov-radiation-20260505-0
summary:
In their original work on the theoretical foundations of Cherenkov radiation, Tamm and Frank wrote, "This peculiar radiation can evidently not be explained by any common mechanism such as the interaction of the fast electron with individual atom or as radiative scattering of electrons on atomic nuclei. On the other hand, the phenomenon can be explained both qualitatively and quantitatively if one takes into account the fact that an electron moving in a medium does radiate light even if it is moving uniformly provided that its velocity is greater than the velocity of light in the medium." === Emission angle === In the figure on the geometry, the particle (red arrow) travels in a medium with speed v p {\displaystyle v_{\text{p}}} such that c n < v p < c , {\displaystyle {\frac {c}{n}}<v_{\text{p}}<c,} where c {\displaystyle c} is speed of light in vacuum, and n {\displaystyle n} is the refractive index of the medium. If the medium is water, the condition is 0.75 c < v p < c {\displaystyle 0.75c<v_{\text{p}}<c} , since n ≈ 1.33 {\displaystyle n\approx 1.33} for water at 20 °C. We define the ratio between the speed of the particle and the speed of light


id: cherenkov-radiation-20260505-2
file: data/en.wikipedia.org/wiki/Cherenkov_radiation-2.md
category: reference
title: Cherenkov radiation
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Cherenkov_radiation
date_saved: 2026-05-05T10:54:47.273838+00:00
instance: kb-cron
parent: cherenkov-radiation-20260505-0
summary:
The Frank–Tamm formula describes the amount of energy E {\displaystyle E} emitted from Cherenkov radiation, per unit length traveled x {\displaystyle x} and per frequency ω {\displaystyle \omega } . μ ( ω ) {\displaystyle \mu (\omega )} is the permeability and n ( ω ) {\displaystyle n(\omega )} is the index of refraction of the material the charged particle moves through. q {\displaystyle q} is the electric charge of the particle, v {\displaystyle v} is the speed of the particle, and c {\displaystyle c} is the speed of light in vacuum. Unlike fluorescence or emission spectra that have characteristic spectral peaks, Cherenkov radiation is continuous. Around the visible spectrum, the relative intensity per unit frequency is approximately proportional to the frequency, with higher frequencies (shorter wavelengths) being more intense in Cherenkov radiation. Visible Cherenkov radiation is observed to be brilliant blue because while most Cherenkov radiation is in the ultraviolet spectrum, it is only with sufficiently accelerated charges that it even becomes visible; the sensitivity of the human eye peaks at green, and is very low in the violet portion of the spectrum. There is a cut-off frequency above which the equation cos ⁡ θ = 1 / (


id: cherenkov-radiation-20260505-3
file: data/en.wikipedia.org/wiki/Cherenkov_radiation-3.md
category: reference
title: Cherenkov radiation
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Cherenkov_radiation
date_saved: 2026-05-05T10:54:47.273838+00:00
instance: kb-cron
parent: cherenkov-radiation-20260505-0
summary:
Cherenkov radiation is commonly used in experimental particle physics for particle identification. One could measure (or put limits on) the velocity of an electrically charged elementary particle by the properties of the Cherenkov light it emits in a certain medium. If the momentum of the particle is measured independently, one could compute the mass of the particle by its momentum and velocity (see four-momentum), and hence identify the particle. The simplest type of particle identification device based on a Cherenkov radiation technique is the threshold counter, which answers whether the velocity of a charged particle is lower or higher than a certain value ( v 0 = c / n {\displaystyle v_{0}=c/n} , where c {\displaystyle c} is the speed of light, and n {\displaystyle n} is the refractive index of the medium) by looking at whether this particle emits Cherenkov light in a certain medium. Knowing particle momentum, one can separate particles lighter than a certain threshold from those heavier than the threshold. The most advanced type of a detector is the RICH, or ring-imaging Cherenkov detector, developed in the 1980s. In a RICH detector, a cone of Cherenkov light is produced when a high-speed charged particle traverses a


id: coand-effect-20260505-0
file: data/en.wikipedia.org/wiki/Coandă_effect-0.md
category: reference
title: Coandă effect
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Coandă_effect
date_saved: 2026-05-05T10:54:48.580764+00:00
instance: kb-cron
parent: coand-effect-20260505-0
summary:
The Coandă effect ( or ) is the tendency of a fluid jet to stay attached to a surface of any form. Merriam-Webster describes it as "the tendency of a jet of fluid emerging from an orifice to follow an adjacent flat or curved surface and to entrain fluid from the surroundings so that a region of lower pressure develops." It is named after Romanian inventor Henri Coandă, who was the first to recognize the practical application of the phenomenon in aircraft design around 1910. It was first documented explicitly in two patents issued in 1936. == History == A description of this phenomenon was provided by Thomas Young in a lecture to The Royal Society in 1800: The lateral pressure which urges the flame of a candle towards the stream of air from a blowpipe is probably exactly similar to that pressure which eases the inflection of a current of air near an obstacle. Mark the dimple which a slender stream of air makes on the surface of water. Bring a convex body into contact with the side of the stream and the place of the dimple will immediately show the current is deflected towards the body; and if


id: coand-effect-20260505-1
file: data/en.wikipedia.org/wiki/Coandă_effect-1.md
category: reference
title: Coandă effect
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Coandă_effect
date_saved: 2026-05-05T10:54:48.580764+00:00
instance: kb-cron
parent: coand-effect-20260505-0
summary:
With a much smaller radius (12 centimeters in the image on the right) a transverse difference arises between external and wall surface pressures of the jet, creating a pressure gradient depending upon ⁠h/r⁠, the relative curvature. This pressure gradient can appear in a zone before and after the origin of the jet where it gradually arises, and disappear at the point where the jet boundary layer separates from the wall, where the wall pressure reaches atmospheric pressure (and the transverse gradient becomes zero). Experiments made in 1956 with turbulent air jets at a Reynolds number of 106 at various jet widths (h) show the pressures measured along a circularly curved wall radius (r) at a series of horizontal distance from the origin of the jet (see the diagram on the right). Above a critical ⁠h/r⁠ ratio of 0.5 only local effects at the origin of the jet are seen extending over a small angle of 18° along the curved wall. The jet then immediately separates from the curved wall. A Coandă effect is therefore not seen here but only a local attachment: a pressure smaller than atmospheric pressure appears on the wall along a distance corresponding to a small angle


id: coand-effect-20260505-2
file: data/en.wikipedia.org/wiki/Coandă_effect-2.md
category: reference
title: Coandă effect
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Coandă_effect
date_saved: 2026-05-05T10:54:48.580764+00:00
instance: kb-cron
parent: coand-effect-20260505-0
summary:
An image of the surface pressure distribution of the jet round the cylindrical surface using the same values of the relative curvature ⁠h/r⁠, and the same angle θ as those found for the wall jet reported in the image on the right side here has been established: it may be found in reference (15) p. 104 and both images are quite similar: the Coandă effect of a free jet is inertial, the same as Coandă effect of a wall jet. However, an experimental measurement of the corresponding surface pressure distribution is not known. Experiments in 1959 by Bourque and Newmann concerning the reattachment of a two-dimensional turbulent jet to an offset parallel plate after enclosing a separation bubble where a low pressure vortex is confined (as in the image 5 in the preceding section) and also for a two-dimensional jet followed by a single flat plate inclined at an angle instead of the circularly curved wall in the diagram on the right here describing the experience of a wall jet: the jet separates from the plate, then curves towards the plate when the surrounding fluid is entrained and pressure lowered, and eventually reattaches to it, enclosing a separation bubble. The


id: coand-effect-20260505-3
file: data/en.wikipedia.org/wiki/Coandă_effect-3.md
category: reference
title: Coandă effect
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Coandă_effect
date_saved: 2026-05-05T10:54:48.580764+00:00
instance: kb-cron
parent: coand-effect-20260505-0
summary:
The experimental McDonnell Douglas YC-15 and its production derivative, the Boeing C-17 Globemaster III, also employ the effect. The NOTAR helicopter replaces the conventional propeller tail rotor with a Coandă effect tail (diagram on the left). A better understanding of Coandă effect was provided by the scientific literature produced by ACHEON EU FP7 project. This project utilized a particular symmetric nozzle to produce an effective modeling of the Coandă effect, and determined innovative STOL aircraft configurations based on the effect. This activity has been expanded by Dragan in the turbomachinery sector, with the objective of better optimizing the shape of rotating blades by Romanian Comoti Research Centre's work on turbomachinery. A practical use of the Coandă effect is for inclined hydropower screens, which separate debris, fish, etc., otherwise in the input flow to the turbines. Due to the slope, the debris falls from the screens without mechanical clearing, and due to the wires of the screen optimizing the Coandă effect, the water flows through the screen to the penstocks leading the water to the turbines. The Coandă effect is used in dual-pattern fluid dispensers in automobile windshield washers. The operation principle of oscillatory flowmeters also relies on the Coandă phenomenon.


id: coand-effect-20260505-4
file: data/en.wikipedia.org/wiki/Coandă_effect-4.md
category: reference
title: Coandă effect
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Coandă_effect
date_saved: 2026-05-05T10:54:48.580764+00:00
instance: kb-cron
parent: coand-effect-20260505-0
summary:
== Demonstration == The Coandă effect can be demonstrated by directing a small jet of air upwards at an angle over a ping pong ball. The jet is drawn to and follows the upper surface of the ball curving around it, due to the (radial) acceleration (slowing and turning) of the air around the ball. With enough airflow, this change in momentum is balanced by the equal and opposite force on the ball supporting its weight. This demonstration can be performed using a hairdryer on the lowest setting or a vacuum cleaner if the outlet can be attached to the pipe and aimed upwards at an angle. A common misconception is that the Coandă effect is demonstrated when a stream of tap water flows over the back of a spoon held lightly in the stream and the spoon is pulled into the stream (for example, Massey 1979, Fig 3.12 uses the Coandă effect to explain the deflection of water around a cylinder). While the flow looks very similar to the air flow over the ping pong ball above (if one could see the air flow), the cause is not really the Coandă effect. Here, because it is a flow of


id: coffee-ring-effect-20260505-0
file: data/en.wikipedia.org/wiki/Coffee_ring_effect-0.md
category: reference
title: Coffee ring effect
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Coffee_ring_effect
date_saved: 2026-05-05T10:54:49.911756+00:00
instance: kb-cron
parent: coffee-ring-effect-20260505-0
summary:
In physics, a "coffee ring" is a pattern left by a puddle of particle-laden liquid after it evaporates. The phenomenon is named for the characteristic ring-like deposit along the perimeter of a spill of coffee. It is also commonly seen after spilling red wine. The mechanism behind the formation of these and similar rings is known as the coffee ring effect or in some instances, the coffee stain effect, or simply ring stain. == Flow mechanism == The coffee-ring pattern originates from the capillary flow induced by the evaporation of the drop: liquid evaporating from the edge is replenished by liquid from the interior. The resulting current can carry nearly all the dispersed material to the edge. As a function of time, this process exhibits a "rush-hour" effect, that is, a rapid acceleration of the flow towards the edge at the final stage of the drying process. Evaporation induces a Marangoni flow inside a droplet. The flow, if strong, redistributes particles back to the center of the droplet. Thus, for particles to accumulate at the edges, the liquid must have a weak Marangoni flow, or something must occur to disrupt the flow. For example, surfactants can be added to reduce


id: condensation-cloud-20260505-0
file: data/en.wikipedia.org/wiki/Condensation_cloud-0.md
category: reference
title: Condensation cloud
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Condensation_cloud
date_saved: 2026-05-05T10:54:51.182419+00:00
instance: kb-cron
parent: condensation-cloud-20260505-0
summary:
A transient condensation cloud, also called a Wilson cloud, is observable surrounding large explosions in humid air. When a nuclear weapon or high explosive is detonated in sufficiently humid air, the "negative phase" of the shock wave causes a rarefaction of the air surrounding the explosion but not of the air contained within it. The rarefied air is temporarily cooled, which causes condensation of some of the water vapor within the rarefied air. When the pressure and temperature return to normal, the Wilson cloud dissipates. == Mechanism == Since heat does not leave the affected air mass, the change of pressure following a detonation is adiabatic, with an associated change of temperature. In humid air, the drop in temperature in the most rarefied portion of the shock wave can bring the air temperature below its dew point, at which moisture condenses to form a visible cloud of microscopic water droplets. Since the pressure effect of the wave is reduced by its expansion (the same pressure effect is spread over a larger radius), the vapor effect also has a limited radius. Such vapor can also be seen in low pressure regions during high–g subsonic maneuvers of aircraft in humid conditions. ==


id: convection-20260505-0
file: data/en.wikipedia.org/wiki/Convection-0.md
category: reference
title: Convection
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Convection
date_saved: 2026-05-05T10:54:52.394392+00:00
instance: kb-cron
parent: convection-20260505-0
summary:
Convection is the transfer of heat through the physical movement of fluids (liquids or gases), where warmer, less-dense material rises and cooler, denser material sinks. It is a single-phase or multiphase fluid flow that occurs spontaneously through the combined effects of material property heterogeneity and body forces on a fluid. When the cause of the convection is unspecified, convection due to the effects of thermal expansion (change in density) and gravity/buoyancy can be assumed (see convection in heat transfer). Convective flow may be transient (such as when a multiphase mixture of oil and water separates) or steady state (see convection cell). The convection may be due to gravitational, electromagnetic or fictitious body forces. Heat transfer by natural convection plays a role in the structure of Earth's atmosphere, its oceans, and its mantle. Discrete convective cells in the atmosphere can be identified by clouds, with stronger convection resulting in thunderstorms. Natural convection also plays a role in stellar physics. Convection is often categorised or described by the main effect causing the convective flow; for example, thermal convection. Convection may also take place in soft solids or mixtures where particles can flow. Convection cannot take place in most solids because neither bulk


id: convection-20260505-1
file: data/en.wikipedia.org/wiki/Convection-1.md
category: reference
title: Convection
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Convection
date_saved: 2026-05-05T10:54:52.394392+00:00
instance: kb-cron
parent: convection-20260505-0
summary:
Natural convection is a flow whose motion is caused by some parts of a fluid being heavier than other parts. In most cases this leads to natural circulation: the ability of a fluid in a system to circulate continuously under gravity, with transfer of heat energy. The driving force for natural convection is gravity. In a column of fluid, pressure increases with depth from the weight of the overlying fluid. The pressure at the bottom of a submerged object then exceeds that at the top, resulting in a net upward buoyancy force equal to the weight of the displaced fluid. Objects of higher density than that of the displaced fluid then sink. For example, regions of warmer low-density air rise, while those of colder high-density air sink. This creates a circulating flow: convection. Gravity drives natural convection. Without gravity, convection does not occur, so there is no convection in free-fall (inertial) environments, such as that of the orbiting International Space Station. Natural convection can occur when there are hot and cold regions of either air or water, because both water and air become less dense as they are heated. But, for example, in the world's oceans it also occurs due


id: convection-20260505-2
file: data/en.wikipedia.org/wiki/Convection-2.md
category: reference
title: Convection
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Convection
date_saved: 2026-05-05T10:54:52.394392+00:00
instance: kb-cron
parent: convection-20260505-0
summary:
== Examples and applications == Systems of natural circulation include tornadoes and other weather systems, ocean currents, and household ventilation. Some solar water heaters use natural circulation. The Gulf Stream circulates as a result of the evaporation of water. In this process, the water increases in salinity and density. In the North Atlantic Ocean, the water becomes so dense that it begins to sink down. Convection occurs on a large scale in atmospheres, oceans, planetary mantles, and it provides the mechanism of heat transfer for a large fraction of the outermost interiors of the Sun and all stars. Fluid movement during convection may be invisibly slow, or it may be obvious and rapid, as in a hurricane. On astronomical scales, convection of gas and dust is thought to occur in the accretion disks of black holes, at speeds which may closely approach that of light. === Demonstration experiments === Thermal convection in liquids can be demonstrated by placing a heat source (for example, a Bunsen burner) at the side of a container with a liquid. Adding a dye to the water (such as food colouring) will enable visualisation of the flow. Another common experiment to demonstrate thermal convection in liquids


id: convection-20260505-3
file: data/en.wikipedia.org/wiki/Convection-3.md
category: reference
title: Convection
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Convection
date_saved: 2026-05-05T10:54:52.394392+00:00
instance: kb-cron
parent: convection-20260505-0
summary:
Warm air has a lower density than cool air, so warm air rises within cooler air, similar to hot air balloons. Clouds form as relatively warmer air carrying moisture rises within cooler air. As the moist air rises, it cools, causing some of the water vapor in the rising packet of air to condense. When the moisture condenses, it releases energy known as latent heat of condensation which allows the rising packet of air to cool less than its surrounding air, continuing the cloud's ascension. If enough instability is present in the atmosphere, this process will continue long enough for cumulonimbus clouds to form, which support lightning and thunder. Generally, thunderstorms require three conditions to form: moisture, an unstable airmass, and a lifting force (heat). All thunderstorms, regardless of type, go through three stages: the developing stage, the mature stage, and the dissipation stage. The average thunderstorm has a 24 km (15 mi) diameter. Depending on the conditions present in the atmosphere, these three stages take an average of 30 minutes to go through. === Oceanic circulation === Solar radiation affects the oceans: warm water from the Equator tends to circulate toward the poles, while cold polar water heads towards


id: convection-20260505-4
file: data/en.wikipedia.org/wiki/Convection-4.md
category: reference
title: Convection
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Convection
date_saved: 2026-05-05T10:54:52.394392+00:00
instance: kb-cron
parent: convection-20260505-0
summary:
The convection zone of a star is the range of radii in which energy is transported outward from the core region primarily by convection rather than radiation. This occurs at radii which are sufficiently opaque that convection is more efficient than radiation at transporting energy. Granules on the photosphere of the Sun are the visible tops of convection cells in the photosphere, caused by convection of plasma in the photosphere. The rising part of the granules is located in the center where the plasma is hotter. The outer edge of the granules is darker due to the cooler descending plasma. A typical granule has a diameter on the order of 1,000 kilometers and each lasts 8 to 20 minutes before dissipating. Below the photosphere is a layer of much larger "supergranules" up to 30,000 kilometers in diameter, with lifespans of up to 24 hours. === Water convection at freezing temperatures === Water is a fluid that does not obey the Boussinesq approximation. This is because its density varies nonlinearly with temperature, which causes its thermal expansion coefficient to be inconsistent near freezing temperatures. The density of water reaches a maximum at 4 °C and decreases as the temperature deviates. This


id: convection-20260505-5
file: data/en.wikipedia.org/wiki/Convection-5.md
category: reference
title: Convection
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Convection
date_saved: 2026-05-05T10:54:52.394392+00:00
instance: kb-cron
parent: convection-20260505-0
summary:
Δ ρ = ρ 0 β Δ T {\displaystyle \Delta \rho =\rho _{0}\beta \Delta T} where ρ 0 {\displaystyle \rho _{0}} is the reference density, typically picked to be the average density of the medium, β {\displaystyle \beta } is the coefficient of thermal expansion, and Δ T {\displaystyle \Delta T} is the temperature difference across the medium. The general diffusivity, D {\displaystyle D} , is redefined as a thermal diffusivity, α {\displaystyle \alpha } . D = α {\displaystyle D=\alpha } Inserting these substitutions produces a Rayleigh number that can be used to predict thermal convection. Ra = ρ 0 g β Δ T L 3 α μ {\displaystyle {\textbf {Ra}}={\frac {\rho _{0}g\beta \Delta TL^{3}}{\alpha \mu }}} === Turbulence === The tendency of a particular naturally convective system towards turbulence relies on the Grashof number (Gr). G r = g β Δ T L 3 ν 2 {\displaystyle Gr={\frac {g\beta \Delta TL^{3}}{\nu ^{2}}}} In very sticky, viscous fluids (large ν), fluid motion is restricted, and natural convection will be non-turbulent. Following the treatment of the previous subsection, the typical fluid velocity is of the order of g Δ ρ L 2 / μ {\displaystyle g\Delta \rho L^{2}/\mu } ,


id: convection-20260505-6
file: data/en.wikipedia.org/wiki/Convection-6.md
category: reference
title: Convection
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Convection
date_saved: 2026-05-05T10:54:52.394392+00:00
instance: kb-cron
parent: convection-20260505-0
summary:
Convection, especially Rayleigh–Bénard convection, where the convecting fluid is contained by two rigid horizontal plates, is a convenient example of a pattern-forming system. When heat is fed into the system from one direction (usually below), at small values it merely diffuses (conducts) from below upward, without causing fluid flow. As the heat flow is increased, above a critical value of the Rayleigh number, the system undergoes a bifurcation from the stable conducting state to the convecting state, where bulk motion of the fluid due to heat begins. If fluid parameters other than density do not depend significantly on temperature, the flow profile is symmetric, with the same volume of fluid rising as falling. This is known as Boussinesq convection. As the temperature difference between the top and bottom of the fluid becomes higher, significant differences in fluid parameters other than density may develop in the fluid due to temperature. An example of such a parameter is viscosity, which may begin to significantly vary horizontally across layers of fluid. This breaks the symmetry of the system, and generally changes the pattern of up- and down-moving fluid from stripes to hexagons, as seen in the illustration. Such hexagons are one example of


id: coriolis-force-20260505-0
file: data/en.wikipedia.org/wiki/Coriolis_force-0.md
category: reference
title: Coriolis force
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Coriolis_force
date_saved: 2026-05-05T10:54:53.676521+00:00
instance: kb-cron
parent: coriolis-force-20260505-0
summary:
In physics, the Coriolis force is a pseudo-force that acts on objects in motion within a frame of reference that rotates with respect to an inertial frame. In a reference frame with clockwise rotation, the force acts to the left of the motion of the object. In one with anticlockwise (or counterclockwise) rotation, the force acts to the right. Deflection of an object due to the Coriolis force is called the Coriolis effect. Though recognized previously by others, the mathematical expression for the Coriolis force appeared in an 1835 paper by French scientist Gaspard-Gustave de Coriolis, in connection with the theory of water wheels. Early in the 20th century, the term Coriolis force began to be used in connection with meteorology. On Earth, the Coriolis force will affect the movement of air and water over long distance. Because of Earth's rotation, the moving object does not travel in a straight line relative to the Earth's surface but follows a curved path. This deflection, to the right in the Northern Hemisphere and to the left in the Southern Hemisphere, significantly influences large-scale circulation of winds and ocean currents. Newton's laws of motion describe the motion of an object in an inertial


id: coriolis-force-20260505-1
file: data/en.wikipedia.org/wiki/Coriolis_force-1.md
category: reference
title: Coriolis force
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Coriolis_force
date_saved: 2026-05-05T10:54:53.676521+00:00
instance: kb-cron
parent: coriolis-force-20260505-0
summary:
Italian scientist Giovanni Battista Riccioli and his assistant Francesco Maria Grimaldi described the effect in connection with artillery in the 1651 Almagestum Novum, writing that rotation of the Earth should cause a cannonball fired to the north to deflect to the east. In 1674, Claude François Milliet Dechales described in his Cursus seu Mundus Mathematicus how the rotation of the Earth should cause a deflection in the trajectories of both falling bodies and projectiles aimed toward one of the planet's poles. Riccioli, Grimaldi, and Dechales all described the effect as part of an argument against the heliocentric system of Copernicus. In other words, they argued that the Earth's rotation should create the effect, and so failure to detect the effect was evidence for an immobile Earth. Their arguments were correct, but the effect of Coriolis force is so small that it was not measured until the 19th century. The Coriolis acceleration equation was derived by Euler in 1749, and the effect was described in the tidal equations of Pierre-Simon Laplace in 1778. Gaspard-Gustave de Coriolis published a paper in 1835 on the energy yield of machines with rotating parts, such as waterwheels. That paper considered the supplementary forces that are


id: coriolis-force-20260505-2
file: data/en.wikipedia.org/wiki/Coriolis_force-2.md
category: reference
title: Coriolis force
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Coriolis_force
date_saved: 2026-05-05T10:54:53.676521+00:00
instance: kb-cron
parent: coriolis-force-20260505-0
summary:
if the velocity is parallel to the rotation axis, the Coriolis force is zero. For example, on Earth, this situation occurs for a body at the equator moving north or south relative to the Earth's surface. (At any latitude other than the equator, however, the north–south motion would have a component perpendicular to the rotation axis and a force specified by the inward or outward cases mentioned below). if the velocity is straight inward to the axis, the Coriolis force is in the direction of local rotation. For example, on Earth, this situation occurs for a body at the equator falling downward, as in the Dechales illustration above, where the falling ball travels further to the east than does the tower. Note also that heading north in the Northern Hemisphere would have a velocity component toward the rotation axis, resulting in a Coriolis force to the east (more pronounced the further north one is). if the velocity is straight outward from the axis, the Coriolis force is against the direction of local rotation. In the tower example, a ball launched upward would move toward the west. if the velocity is in the direction of rotation, the Coriolis force is outward


id: coriolis-force-20260505-3
file: data/en.wikipedia.org/wiki/Coriolis_force-3.md
category: reference
title: Coriolis force
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Coriolis_force
date_saved: 2026-05-05T10:54:53.676521+00:00
instance: kb-cron
parent: coriolis-force-20260505-0
summary:
The figure describes a more complex situation where the tossed ball on a turntable bounces off the edge of the carousel and then returns to the tosser, who catches the ball. The effect of Coriolis force on its trajectory is shown again as seen by two observers: an observer (referred to as the "camera") that rotates with the carousel, and an inertial observer. The figure shows a bird's-eye view based upon the same ball speed on forward and return paths. Within each circle, plotted dots show the same time points. In the left panel, from the camera's viewpoint at the center of rotation, the tosser (smiley face) and the rail both are at fixed locations, and the ball makes a very considerable arc on its travel toward the rail, and takes a more direct route on the way back. From the ball tosser's viewpoint, the ball seems to return more quickly than it went (because the tosser is rotating toward the ball on the return flight). On the carousel, instead of tossing the ball straight at a rail to bounce back, the tosser must throw the ball toward the right of the target and the ball then seems to the


id: coriolis-force-20260505-4
file: data/en.wikipedia.org/wiki/Coriolis_force-4.md
category: reference
title: Coriolis force
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Coriolis_force
date_saved: 2026-05-05T10:54:53.676521+00:00
instance: kb-cron
parent: coriolis-force-20260505-0
summary:
a C = − 2 Ω × v = 2 ω ( − v u 0 v e ) . {\displaystyle \mathbf {a} _{\mathrm {C} }=-2{\boldsymbol {\Omega }}\times \mathbf {v} =2\,\omega \,{\begin{pmatrix}-v_{\mathrm {u} }\\0\\v_{\mathrm {e} }\end{pmatrix}}\ .} Ω in this case is parallel to the north–south axis. Accordingly, an eastward motion (that is, in the same direction as the rotation of the sphere) provides an upward acceleration known as the Eötvös effect, and an upward motion produces an acceleration due west. === Meteorology and oceanography === Perhaps the most important impact of the Coriolis effect is in the large-scale dynamics of the oceans and the atmosphere. In meteorology and oceanography, it is convenient to postulate a rotating frame of reference wherein the Earth is stationary. In accommodation of that provisional postulation, the centrifugal and Coriolis forces are introduced. Their relative importance is determined by the applicable Rossby numbers. Tornadoes have high Rossby numbers, so, while tornado-associated centrifugal forces are quite substantial, Coriolis forces associated with tornadoes are for practical purposes negligible. Because surface ocean currents are driven by the movement of wind over the water's surface, the Coriolis force also affects the movement of ocean currents and cyclones as well.


id: coriolis-force-20260505-5
file: data/en.wikipedia.org/wiki/Coriolis_force-5.md
category: reference
title: Coriolis force
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Coriolis_force
date_saved: 2026-05-05T10:54:53.676521+00:00
instance: kb-cron
parent: coriolis-force-20260505-0
summary:
The practical impact of the "Coriolis effect" is mostly caused by the horizontal acceleration component produced by horizontal motion. There are other components of the Coriolis effect. Westward-traveling objects are deflected downwards, while eastward-traveling objects are deflected upwards. This is known as the Eötvös effect. This aspect of the Coriolis effect is greatest near the equator. The force produced by the Eötvös effect is similar to the horizontal component, but the much larger vertical forces due to gravity and pressure suggest that it is unimportant in the hydrostatic equilibrium. However, in the atmosphere, winds are associated with small deviations of pressure from the hydrostatic equilibrium. In the tropical atmosphere, the order of magnitude of the pressure deviations is so small that the contribution of the Eötvös effect to the pressure deviations is considerable. In addition, objects traveling upwards (i.e. out) or downwards (i.e. in) are deflected to the west or east respectively. This effect is also the greatest near the equator. Since vertical movement is usually of limited extent and duration, the size of the effect is smaller and requires precise instruments to detect. For example, idealized numerical modeling studies suggest that this effect can directly affect tropical large-scale wind


id: coriolis-force-20260505-6
file: data/en.wikipedia.org/wiki/Coriolis_force-6.md
category: reference
title: Coriolis force
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Coriolis_force
date_saved: 2026-05-05T10:54:53.676521+00:00
instance: kb-cron
parent: coriolis-force-20260505-0
summary:
The train travels toward the west: In that case, it moves against the direction of rotation. Therefore, on the Earth's rotating frame the Coriolis term is pointed inwards towards the axis of rotation (down). This additional force downwards should cause the train to be heavier while moving in that direction.If one looks at this train from the fixed non-rotating frame on top of the center of the Earth, at that speed it remains stationary as the Earth spins beneath it. Hence, the only force acting on it is gravity and the reaction from the track. This force is greater (by 0.34%) than the force that the passengers and the train experience when at rest (rotating along with Earth). This difference is what the Coriolis effect accounts for in the rotating frame of reference. The train comes to a stop: From the point of view on the Earth's rotating frame, the velocity of the train is zero, thus the Coriolis force is also zero and the train and its passengers recuperate their usual weight.From the fixed inertial frame of reference above Earth, the train now rotates along with the rest of the Earth. 0.34% of the force of gravity provides the


id: coriolis-force-20260505-7
file: data/en.wikipedia.org/wiki/Coriolis_force-7.md
category: reference
title: Coriolis force
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Coriolis_force
date_saved: 2026-05-05T10:54:53.676521+00:00
instance: kb-cron
parent: coriolis-force-20260505-0
summary:
==== Laboratory testing of draining water under atypical conditions ==== In 1962, Ascher Shapiro performed an experiment at MIT to test the Coriolis force on a large basin of water, 2 meters (6 ft 7 in) across, with a small wooden cross above the plug hole to display the direction of rotation, covering it and waiting for at least 24 hours for the water to settle. Under these precise laboratory conditions, he demonstrated the effect and consistent counterclockwise rotation. The experiment required extreme precision, since the acceleration due to Coriolis effect is only 3 × 10 − 7 {\displaystyle 3\times 10^{-7}} that of gravity. The vortex was measured by a cross made of two slivers of wood pinned above the draining hole. It takes 20 minutes to drain, and the cross starts turning only around 15 minutes. At the end it is turning at 1 rotation every 3 to 4 seconds. He reported that, Both schools of thought are in some sense correct. For the everyday observations of the kitchen sink and bath-tub variety, the direction of the vortex seems to vary in an unpredictable manner with the date, the time of day, and the particular household of the experimenter.


id: coriolis-force-20260505-8
file: data/en.wikipedia.org/wiki/Coriolis_force-8.md
category: reference
title: Coriolis force
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Coriolis_force
date_saved: 2026-05-05T10:54:53.676521+00:00
instance: kb-cron
parent: coriolis-force-20260505-0
summary:
To demonstrate the Coriolis effect, a parabolic turntable can be used. On a flat turntable, the inertia of a co-rotating object forces it off the edge. However, if the turntable surface has the correct paraboloid (parabolic bowl) shape (see the figure) and rotates at the corresponding rate, the force components shown in the figure make the component of gravity tangential to the bowl surface exactly equal to the centripetal force necessary to keep the object rotating at its velocity and radius of curvature (assuming no friction). (See banked turn.) This carefully contoured surface allows the Coriolis force to be displayed in isolation. Discs cut from cylinders of dry ice can be used as pucks, moving around almost frictionlessly over the surface of the parabolic turntable, allowing effects of Coriolis on dynamic phenomena to show themselves. To get a view of the motions as seen from the reference frame rotating with the turntable, a video camera is attached to the turntable so as to co-rotate with the turntable, with results as shown in the figure. In the left panel of the figure, which is the viewpoint of a stationary observer, the gravitational force in the inertial frame pulling the object toward


id: coriolis-force-20260505-9
file: data/en.wikipedia.org/wiki/Coriolis_force-9.md
category: reference
title: Coriolis force
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Coriolis_force
date_saved: 2026-05-05T10:54:53.676521+00:00
instance: kb-cron
parent: coriolis-force-20260505-0
summary:
The definition of the Coriolis effect from the Glossary of Meteorology The Coriolis Effect — a conflict between common sense and mathematics PDF-file. 20 pages. A general discussion by Anders Persson of various aspects of the coriolis effect, including Foucault's Pendulum and Taylor columns. The coriolis effect in meteorology PDF-file. 5 pages. A detailed explanation by Mats Rosengren of how the gravitational force and the rotation of the Earth affect the atmospheric motion over the Earth surface. 2 figures 10 Coriolis Effect Videos and Games- from the About.com Weather Page Coriolis Force – from ScienceWorld Coriolis Effect and Drains An article from the NEWTON web site hosted by the Argonne National Laboratory. Catalog of Coriolis videos Coriolis Effect: A graphical animation, a visual Earth animation with precise explanation An introduction to fluid dynamics SPINLab Educational Film explains the Coriolis effect with the aid of lab experiments Do bathtubs drain counterclockwise in the Northern Hemisphere? Archived 15 May 2008 at the Wayback Machine by Cecil Adams. Bad Coriolis. An article uncovering misinformation about the Coriolis effect. By Alistair B. Fraser, emeritus professor of meteorology at Pennsylvania State University The Coriolis Effect: A (Fairly) Simple Explanation, an explanation for the layperson Observe


id: critical-phenomena-20260505-0
file: data/en.wikipedia.org/wiki/Critical_phenomena-0.md
category: reference
title: Critical phenomena
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Critical_phenomena
date_saved: 2026-05-05T10:54:54.979917+00:00
instance: kb-cron
parent: critical-phenomena-20260505-0
summary:
In physics, critical phenomena is the collective name associated with the physics of critical points. Most of them stem from the divergence of the correlation length, but also the dynamics slows down. Critical phenomena include scaling relations among different quantities, power-law divergences of some quantities (such as the magnetic susceptibility in the ferromagnetic phase transition) described by critical exponents, universality, fractal behaviour, and ergodicity breaking. Critical phenomena take place in second order phase transitions, although not exclusively. The critical behavior is usually different from the mean-field approximation which is valid away from the phase transition, since the latter neglects correlations, which become increasingly important as the system approaches the critical point where the correlation length diverges. Many properties of the critical behavior of a system can be derived in the framework of the renormalization group. In order to explain the physical origin of these phenomena, we shall use the Ising model as a pedagogical example. == Critical point of the 2D Ising model == Consider a 2 D {\displaystyle 2D} square array of classical spins which may only take two positions: +1 and −1, at a certain temperature T {\displaystyle T} , interacting through the Ising classical Hamiltonian: H =


id: critical-phenomena-20260505-1
file: data/en.wikipedia.org/wiki/Critical_phenomena-1.md
category: reference
title: Critical phenomena
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Critical_phenomena
date_saved: 2026-05-05T10:54:54.979917+00:00
instance: kb-cron
parent: critical-phenomena-20260505-0
summary:
== Mathematical tools == The main mathematical tools to study critical points are renormalization group, which takes advantage of the Russian dolls picture or the self-similarity to explain universality and predict numerically the critical exponents, and variational perturbation theory, which converts divergent perturbation expansions into convergent strong-coupling expansions relevant to critical phenomena. In two-dimensional systems, conformal field theory is a powerful tool which has discovered many new properties of 2D critical systems, employing the fact that scale invariance, along with a few other requisites, leads to an infinite symmetry group. == Critical point in renormalization group theory == The critical point is described by a conformal field theory. According to the renormalization group theory, the defining property of criticality is that the characteristic length scale of the structure of the physical system, also known as the correlation length ξ, becomes infinite. This can happen along critical lines in phase space. This effect is the cause of the critical opalescence that can be observed as a binary fluid mixture approaches its liquid–liquid critical point. In systems in equilibrium, the critical point is reached only by precisely tuning a control parameter. However, in some non-equilibrium systems, the critical point is an attractor


id: cross-ventilation-20260505-0
file: data/en.wikipedia.org/wiki/Cross_ventilation-0.md
category: reference
title: Cross ventilation
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Cross_ventilation
date_saved: 2026-05-05T10:54:56.230155+00:00
instance: kb-cron
parent: cross-ventilation-20260505-0
summary:
Cross ventilation is a natural phenomenon where wind enters an opening, such as a window, flows directly through the space, and exits through an opening on the opposite side of the building (where the air pressure is lower). This produces a cool stream of air and a current across the room from the exposed area to the sheltered area. Cross ventilation is powered by the wind and thus requires no energy input, in addition to being the most effective method of wind ventilation. A commonly used technique to remove pollutants and heat in an indoor environment, cross ventilation can also decrease or even obviate the need for air conditioning and can improve indoor air quality. Other terms used for the effect include cross-breeze, cross-draft, wind effect ventilation and cross-flow ventilation. In Germany they have been called lüften and Stoßlüften. == Process == The phenomenon occurs when openings in an environment (including vehicles) or building (houses, factories, sheds, etc) are set on opposite or adjoining walls, which allow air to enter and exit, thus creating a current of air across the interior environment. Windows or vents positioned on opposite sides of the room allow passive breezes a pathway through the structure,


id: cross-ventilation-20260505-1
file: data/en.wikipedia.org/wiki/Cross_ventilation-1.md
category: reference
title: Cross ventilation
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Cross_ventilation
date_saved: 2026-05-05T10:54:56.230155+00:00
instance: kb-cron
parent: cross-ventilation-20260505-0
summary:
Single-sided ventilation: This method depends on the pressure contrasts between different openings within the occupied space. For rooms that only feature a single opening, the ventilation is impelled by turbulence, thereby creating a pumping activity on that lone opening, causing small inflows and outflows. Single-sided ventilation has a weak effect. It is preferable when cross ventilation is not achievable, where it uses windows or vents at the other side of the space to control air pressure. Cross ventilation (single spaces): Being unsophisticated and efficacious, this type of ventilation is a horizontal process that is driven by pressure differences between the windward and leeward sides of the occupied indoor environment. Ventilation here is generally provided using windows and vents at either side of a building where the variation in pressure draw air in and out. Cross ventilation (double-banked spaces): Involving banked rooms, this method features openings in the hallway structure. The openings allow a way for noise to move between spaces. It can provide a much higher air-exchange rate in comparison with single-sided ventilation. Stack ventilation: This ventilation is a vertical process and it's beneficiary for taller buildings with central atriums. It draws cooler air in at a lower level, whereby


id: diffraction-20260505-0
file: data/en.wikipedia.org/wiki/Diffraction-0.md
category: reference
title: Diffraction
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Diffraction
date_saved: 2026-05-05T10:54:57.453682+00:00
instance: kb-cron
parent: diffraction-20260505-0
summary:
Diffraction is the deviation of waves from straight-line propagation due to an obstacle or through an aperture, without any change in their energy. Diffraction is the same physical effect as interference, but interference is typically used for the superposition of a few waves, while the term diffraction is used when many waves are superposed. The term diffraction pattern is used to refer to an image or map of the different directions of the waves after they have been diffracted. Diffraction patterns are pronounced when a wave from a coherent source (such as a laser) encounters a slit/aperture as shown in the first image. In classical physics, diffraction is described by the Huygens–Fresnel principle that treats each point in a propagating wavefront as a collection of individual spherical wavelets. The patterns are due to the summation over different points on the wavefront (or, equivalently, each wavelet) that travel by paths of different lengths to the registering surface. If there are multiple closely spaced openings, a complex pattern of varying intensity can result. Other types of apertures or obstacles lead to different patterns, some of which are described later on this page. These effects occur when a light wave travels through a


id: diffraction-20260505-1
file: data/en.wikipedia.org/wiki/Diffraction-1.md
category: reference
title: Diffraction
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Diffraction
date_saved: 2026-05-05T10:54:57.453682+00:00
instance: kb-cron
parent: diffraction-20260505-0
summary:
== Basics == Diffraction is a general phenomenon of waves, occurring whenever a wave encounters some form of obstruction. The obstruction may be solidly blocking the wave, or transparent and shifting the phase of the wave without any change in energy (elastic scattering). The waves beyond the obstacle interfere leading to a diffraction pattern. The diffraction pattern cannot be predicted by the straight line trajectories of geometrical optics. For a simple example, a strong light source blocked by a solid object does not show a crisp dark shadow when examined carefully. The appropriate model for diffraction depends upon the character of the waves and the obstructions. The simplest model uses the Huygens–Fresnel principle. The Huygens part visualizes propagation of a wave by considering every point on a wavefront as a source for a secondary spherical wave, these being called Huygens wavelets. The Fresnel part is the superposition (linear sum) of these secondary waves and their consequent interference. In the absence of obstacles, the Huygens's principle alone predicts wavefront propagation. When some of the secondary waves are blocked by an obstacle, the remainder will create both wavefront propagation in the unblocked direction and waves behind the obstacle which form the diffraction


id: diffraction-20260505-2
file: data/en.wikipedia.org/wiki/Diffraction-2.md
category: reference
title: Diffraction
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Diffraction
date_saved: 2026-05-05T10:54:57.453682+00:00
instance: kb-cron
parent: diffraction-20260505-0
summary:
An illuminated slit that is wider than a wavelength produces interference effects in the space downstream of the slit. Assuming that the slit behaves as though it has a large number of point sources spaced evenly across the width of the slit, the interference effects can be calculated. If the incident light is a single wavelength and coherent, these sources all have the same phase. Light in the space downstream of the slit is made up of contributions from each of these point sources. If the relative phases of contributions from each point source vary due to differences in the path lengths, the resulting intensity will vary. Imagining the slit as the y axis with the z across the slit and x pointing downstream, intensity minima and maxima can be seen along z for large values of x. For points very close to x {\displaystyle x} all the point sources are in phase. This is the undiffracted beam, forming a maximum. Moving away fom the x {\displaystyle x} axis, the path length from the point sources in the center and those on the edges of the slit differ. When the path difference equals λ / 2 {\displaystyle \lambda /2} ,


id: diffraction-20260505-3
file: data/en.wikipedia.org/wiki/Diffraction-3.md
category: reference
title: Diffraction
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Diffraction
date_saved: 2026-05-05T10:54:57.453682+00:00
instance: kb-cron
parent: diffraction-20260505-0
summary:
where θ i {\displaystyle \theta _{i}} is the angle at which the light is incident, d {\displaystyle d} is the separation of grating elements, and m {\displaystyle m} is an integer which can be positive or negative. The light diffracted by a grating is found by summing the light diffracted from each of the elements, and is essentially a convolution of diffraction and interference patterns. The figure shows the light diffracted by 2-element and 5-element gratings where the grating spacings are the same; it can be seen that the maxima are in the same position, but the detailed structures of the intensities are different. === General case for far field === A more mathematical approach involves treating the problem as a summation over spherical waves derived from the relevant wave equation; see for instance Born and Wolf for details. The wave that emerges from a point source has an amplitude ψ {\displaystyle \psi } at location r {\displaystyle \mathbf {r} } that is given by the solution of the frequency domain wave equation for a point source (the Helmholtz equation), ∇ 2 ψ + k 2 ψ = δ ( r ) , {\displaystyle \nabla ^{2}\psi +k^{2}\psi =\delta (\mathbf {r}


id: diffraction-20260505-4
file: data/en.wikipedia.org/wiki/Diffraction-4.md
category: reference
title: Diffraction
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Diffraction
date_saved: 2026-05-05T10:54:57.453682+00:00
instance: kb-cron
parent: diffraction-20260505-0
summary:
== Bragg diffraction == Diffraction from a large three-dimensional periodic structure such as many thousands of atoms in a crystal is called Bragg diffraction. It is similar to what occurs when waves are scattered from a diffraction grating. Bragg diffraction is a consequence of interference between waves reflecting from many different crystal planes. The condition of constructive interference is given by Bragg's law: m λ = 2 d sin ⁡ θ , {\displaystyle m\lambda =2d\sin \theta ,} where λ {\displaystyle \lambda } is the wavelength, d {\displaystyle d} is the distance between crystal planes, θ {\displaystyle \theta } is the angle of the diffracted wave, and m {\displaystyle m} is an integer known as the order of the diffracted beam. Bragg diffraction may be carried out using either electromagnetic radiation of very short wavelength like X-rays or matter waves like neutrons whose wavelength is on the order of (or much smaller than) the atomic spacing. The pattern produced gives information of the separations of crystallographic planes d {\displaystyle d} , allowing one to deduce the crystal structure. For completeness, Bragg diffraction is a limit for a large number of atoms with X-rays or neutrons, and is rarely valid for electron


id: dislon-20260505-0
file: data/en.wikipedia.org/wiki/Dislon-0.md
category: reference
title: Dislon
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Dislon
date_saved: 2026-05-05T10:54:58.714495+00:00
instance: kb-cron
parent: dislon-20260505-0
summary:
A dislon is a quantized field associated with the quantization of the lattice displacement in crystalline solids. It is a localized collective excitation of a crystal dislocation. == Description == Dislons are special quasiparticles that emerge from the quantization of the lattice displacement field around a dislocation in a crystal. They exhibit unique particle statistics depending on the dimension of quantization. In one-dimensional quantization, dislons behave as bosonic quasiparticles. However, in three-dimensional quantization, the topological constraint of the dislocation leads to a breakdown of the canonical commutation relation, resulting in the emergence of two independent bosonic fields known as the d-field and f-field. == Interaction == Dislons interact with other particles such as electrons and phonons. In the presence of multiple dislocations, the electron-dislon interaction can affect the electrical conductivity of the system. The distance-dependent interaction between electrons and dislocations leads to oscillations in the electron self-energy away from the dislocation core. == Applications == The study of dislons provides insights into various phenomena in materials science, including the variation of superconducting transition temperatures in dislocated crystals. Dislons play a role in understanding the interaction between dislocations and phonons, affecting thermal transport properties in the presence of dislocations. == See


id: doppler-broadening-20260505-0
file: data/en.wikipedia.org/wiki/Doppler_broadening-0.md
category: reference
title: Doppler broadening
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Doppler_broadening
date_saved: 2026-05-05T10:54:59.980028+00:00
instance: kb-cron
parent: doppler-broadening-20260505-0
summary:
In atomic physics, Doppler broadening is broadening of spectral lines due to the Doppler effect caused by a distribution of velocities of atoms or molecules. Different velocities of the emitting (or absorbing) particles result in different Doppler shifts, the cumulative effect of which is the emission (absorption) line broadening. This resulting line profile is known as a Doppler profile. A particular case is the thermal Doppler broadening due to the thermal motion of the particles. Then, the broadening depends only on the frequency of the spectral line, the mass of the emitting particles, and their temperature, and therefore can be used for inferring the temperature of an emitting (or absorbing) body being spectroscopically investigated. == Derivation (non-relativistic case) == When a particle moves (e.g., due to the thermal motion) towards the observer, the emitted radiation is shifted to a higher frequency. Likewise, when the emitter moves away, the frequency is lowered. In the non-relativistic limit, the Doppler shift is f = f 0 ( 1 + v c ) , {\displaystyle f=f_{0}\left(1+{\frac {v}{c}}\right),} where f {\displaystyle f} is the observed frequency, f 0 {\displaystyle f_{0}} is the frequency in the rest frame, v {\displaystyle v} is the velocity of the


id: dresselhaus-effect-20260505-0
file: data/en.wikipedia.org/wiki/Dresselhaus_effect-0.md
category: reference
title: Dresselhaus effect
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Dresselhaus_effect
date_saved: 2026-05-05T10:55:01.257959+00:00
instance: kb-cron
parent: dresselhaus-effect-20260505-0
summary:
The Dresselhaus effect is a phenomenon in solid-state physics in which spin–orbit interaction causes energy bands to split. It is usually present in crystal systems lacking inversion symmetry. The effect is named after Gene Dresselhaus, who discovered this splitting in 1955. Spin–orbit interaction is a relativistic coupling between the electric field produced by an ion-core and the resulting dipole moment arising from the relative motion of the electron, and its intrinsic magnetic dipole proportional to the electron spin. In an atom, the coupling weakly splits an orbital energy state into two states: one state with the spin aligned to the orbital field and one anti-aligned. In a solid crystalline material, the motion of the conduction electrons in the lattice can be altered by a complementary effect due to the coupling between the potential of the lattice and the electron spin. If the crystalline material is not centro-symmetric, the asymmetry in the potential can favour one spin orientation over the opposite and split the energy bands into spin aligned and anti-aligned subbands. The Rashba spin–orbit coupling has a similar energy band splitting, but the asymmetry comes either from the bulk asymmetry of uniaxial crystals (e.g. of wurtzite type) or the spatial


id: droplet-cluster-20260505-0
file: data/en.wikipedia.org/wiki/Droplet_cluster-0.md
category: reference
title: Droplet cluster
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Droplet_cluster
date_saved: 2026-05-05T10:55:02.492156+00:00
instance: kb-cron
parent: droplet-cluster-20260505-0
summary:
Droplet cluster is a self-assembled levitating monolayer of microdroplets usually arranged into a hexagonally ordered structure over a locally heated thin (about 1 mm) layer of water. The droplet cluster is typologically similar to colloidal crystals. The phenomenon was observed for the first time in 2004, and it has been extensively studied after that. Growing condensing droplets with a typical diameter of 0.01 mm – 0.2 mm levitate at an equilibrium height, where their weight is equilibrated by the drag force of the ascending air-vapor jet rising over the heated spot. At the same time, the droplets are dragged towards the center of the heated spot; however, they do not merge, forming an ordered hexagonal (densest packed) pattern due to an aerodynamic repulsive pressure force from gas flow between the droplets. The spot is usually heated by a laser beam or another source of heat to 60 °C – 95 °C, although the phenomenon was observed also at temperatures slightly above 20 °C. The height of levitation and the distance between the droplets are of the same order as their diameters. Due to complex nature of aerodynamic forces between the microdroplets in an ascending jet, the droplets do not coalesce


id: dynamic-scaling-20260505-0
file: data/en.wikipedia.org/wiki/Dynamic_scaling-0.md
category: reference
title: Dynamic scaling
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Dynamic_scaling
date_saved: 2026-05-05T10:55:03.760471+00:00
instance: kb-cron
parent: dynamic-scaling-20260505-0
summary:
Dynamic scaling (sometimes known as Family–Vicsek scaling) is a litmus test that shows whether an evolving system exhibits self-similarity. In general a function is said to exhibit dynamic scaling if it satisfies: f ( x , t ) ∼ t θ φ ( x t z ) . {\displaystyle f(x,t)\sim t^{\theta }\varphi \left({\frac {x}{t^{z}}}\right).} Here the exponent θ {\displaystyle \theta } is fixed by the dimensional requirement [ f ] = [ t θ ] {\displaystyle [f]=[t^{\theta }]} . The numerical value of f / t θ {\displaystyle f/t^{\theta }} should remain invariant despite the unit of measurement of t {\displaystyle t} is changed by some factor since φ {\displaystyle \varphi } is a dimensionless quantity. Many of these systems evolve in a self-similar fashion in the sense that data obtained from the snapshot at any fixed time is similar to the respective data taken from the snapshot of any earlier or later time. That is, the system is similar to itself at different times. The litmus test of such self-similarity is provided by the dynamic scaling. == History == The term "dynamic scaling" as one of the essential concepts to describe the dynamics of critical phenomena seems to originate


id: electrofreezing-20260505-0
file: data/en.wikipedia.org/wiki/Electrofreezing-0.md
category: reference
title: Electrofreezing
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Electrofreezing
date_saved: 2026-05-05T10:55:05.009538+00:00
instance: kb-cron
parent: electrofreezing-20260505-0
summary:
Electrofreezing is the tendency of a material to solidify upon being exposed to an external electric field. Electrofreezing was initially introduced by Dufour in 1892. Examples are the electrofreezing of liquid ammonia supposed to be naturally occurring during electrical storms in Jupiter-like planets, and ice χ supposedly being a form of high pressure ice. Depending on the material, freezing occur only at certain field intensities, above which electric fields are strong enough to induce chemical reactions. == References ==


id: electromagnetic-induction-20260505-0
file: data/en.wikipedia.org/wiki/Electromagnetic_induction-0.md
category: reference
title: Electromagnetic induction
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Electromagnetic_induction
date_saved: 2026-05-05T10:55:06.297489+00:00
instance: kb-cron
parent: electromagnetic-induction-20260505-0
summary:
Electromagnetic induction or magnetic induction is the production of an electromotive force (emf) across an electrical conductor in a changing magnetic field. Michael Faraday is generally credited with the discovery of induction in 1831, and James Clerk Maxwell mathematically described it as Faraday's law of induction. Lenz's law describes the direction of the induced field. Faraday's law was later generalized to become the Maxwell–Faraday equation, one of the four Maxwell equations in his theory of electromagnetism. Electromagnetic induction has found many applications, including electrical components such as inductors and transformers, and devices such as electric motors and generators. == History == Electromagnetic induction was discovered by Michael Faraday, published in 1831. It was discovered independently by Joseph Henry in 1832. In Faraday's first experimental demonstration, on August 29, 1831, he wrapped two wires around opposite sides of an iron ring or "torus" (an arrangement similar to a modern toroidal transformer). Based on his understanding of electromagnets, he expected that, when current started to flow in one wire, a sort of wave would travel through the ring and cause some electrical effect on the opposite side. He plugged one wire into a galvanometer, and watched it as he connected the other


id: electromagnetic-induction-20260505-1
file: data/en.wikipedia.org/wiki/Electromagnetic_induction-1.md
category: reference
title: Electromagnetic induction
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Electromagnetic_induction
date_saved: 2026-05-05T10:55:06.297489+00:00
instance: kb-cron
parent: electromagnetic-induction-20260505-0
summary:
=== Faraday's law and relativity === Faraday's law describes two different phenomena: the motional emf generated by a magnetic force on a moving wire (see Lorentz force), and the transformer emf that is generated by an electric force due to a changing magnetic field (due to the differential form of the Maxwell–Faraday equation). James Clerk Maxwell drew attention to the separate physical phenomena in 1861. This is believed to be a unique example in physics of where such a fundamental law is invoked to explain two such different phenomena. Albert Einstein noticed that the two situations both corresponded to a relative movement between a conductor and a magnet, and the outcome was unaffected by which one was moving. This was one of the principal paths that led him to develop special relativity. == Applications == The principles of electromagnetic induction are applied in many devices and systems, including: === Electrical generator === The emf generated by Faraday's law of induction due to relative movement of a circuit and a magnetic field is the phenomenon underlying electrical generators. When a permanent magnet is moved relative to a conductor, or vice versa, an electromotive force is created. If the wire is connected


id: electromagnetic-induction-20260505-2
file: data/en.wikipedia.org/wiki/Electromagnetic_induction-2.md
category: reference
title: Electromagnetic induction
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Electromagnetic_induction
date_saved: 2026-05-05T10:55:06.297489+00:00
instance: kb-cron
parent: electromagnetic-induction-20260505-0
summary:
Only five laminations or plates are shown in this example, so as to show the subdivision of the eddy currents. In practical use, the number of laminations or punchings ranges from 40 to 66 per inch (16 to 26 per centimetre), and brings the eddy current loss down to about one percent. While the plates can be separated by insulation, the voltage is so low that the natural rust/oxide coating of the plates is enough to prevent current flow across the laminations. This is a rotor approximately 20 mm in diameter from a DC motor used in a CD player. Note the laminations of the electromagnet pole pieces, used to limit parasitic inductive losses. === Parasitic induction within conductors === In this illustration, a solid copper bar conductor on a rotating armature is just passing under the tip of the pole piece N of the field magnet. Note the uneven distribution of the lines of force across the copper bar. The magnetic field is more concentrated and thus stronger on the left edge of the copper bar (a,b) while the field is weaker on the right edge (c,d). Since the two edges of the bar move with the same velocity,


id: electron-emission-20260505-0
file: data/en.wikipedia.org/wiki/Electron_emission-0.md
category: reference
title: Electron emission
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Electron_emission
date_saved: 2026-05-05T10:55:07.536321+00:00
instance: kb-cron
parent: electron-emission-20260505-0
summary:
In physics, electron emission is the ejection of an electron from the surface of matter, or, in beta decay (β− decay), where a beta particle (a fast energetic electron or positron) is emitted from an atomic nucleus transforming the original nuclide to an isobar. == Radioactive decay == In Beta decay (β− decay), radioactive decay results in a beta particle (fast energetic electron or positron in β+ decay) being emitted from the nucleus == Surface emission == Thermionic emission, the liberation of electrons from an electrode by virtue of its temperature Schottky emission, due to the: Schottky effect or field enhanced thermionic emission Field electron emission, emission of electrons induced by an electrostatic field === Devices === An electron gun or electron emitter, is an electrical component in some vacuum tubes that uses surface emission == Others == Exoelectron emission, a weak electron emission, appearing only from pretreated objects Photoelectric effect, the emission of electrons when electromagnetic radiation, such as light, hits a material == See also == Positron emission, (of a positron or "antielectron") is one aspect of β+ decay Electron excitation, the transfer of an electron to a higher atomic orbital == References ==


id: ergosphere-20260505-0
file: data/en.wikipedia.org/wiki/Ergosphere-0.md
category: reference
title: Ergosphere
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Ergosphere
date_saved: 2026-05-05T10:55:08.890537+00:00
instance: kb-cron
parent: ergosphere-20260505-0
summary:
In astrophysics, the ergosphere is a region located just outside a rotating black hole, between its event horizon and a further external surface predicted by the Kerr metric which describes these objects. Its name was proposed by Remo Ruffini and John Archibald Wheeler during the Les Houches lectures in 1971 and is derived from Ancient Greek ἔργον (ergon) 'work'. It received this name because it is theoretically possible to extract energy and mass from this region. The ergosphere touches the event horizon at the poles of a rotating black hole and extends to a greater radius at the equator. A black hole with modest angular momentum has an ergosphere with a shape approximated by an oblate spheroid, while faster spins produce a more pumpkin-shaped ergosphere. The equatorial (maximal) radius of an ergosphere is the Schwarzschild radius, the radius of a non-rotating black hole. The polar (minimal) radius is also the polar (minimal) radius of the event horizon which can be as little as half the Schwarzschild radius for a maximally rotating black hole. == Rotation == As a black hole rotates, it twists spacetime in the direction of the rotation at a speed that decreases with distance from the event


id: event-horizon-20260505-0
file: data/en.wikipedia.org/wiki/Event_horizon-0.md
category: reference
title: Event horizon
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Event_horizon
date_saved: 2026-05-05T10:55:10.114290+00:00
instance: kb-cron
parent: event-horizon-20260505-0
summary:
In astrophysics, an event horizon is a boundary in spacetime beyond which no signal can ever reach a given observer. Wolfgang Rindler coined the term in the 1950s. In 1784, John Michell proposed that gravity can be strong enough in the vicinity of massive compact objects that even light cannot escape. At that time, the Newtonian theory of gravitation and the so-called corpuscular theory of light were dominant. In these theories, if the escape velocity of the gravitational influence of a massive object exceeds the speed of light, then light originating inside or from it can escape temporarily but will return. In 1958, David Finkelstein used general relativity to introduce a stricter definition of a local black hole event horizon as a boundary beyond which events of any kind cannot affect an outside observer, leading to information and firewall paradoxes, encouraging the re-examination of the concept of local event horizons and the notion of black holes. Several theories were subsequently developed, some with and some without event horizons. One of the leading developers of theories to describe black holes, Stephen Hawking, suggested that an apparent horizon should be used instead of an event horizon, saying, "Gravitational collapse produces apparent horizons


id: event-horizon-20260505-1
file: data/en.wikipedia.org/wiki/Event_horizon-1.md
category: reference
title: Event horizon
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Event_horizon
date_saved: 2026-05-05T10:55:10.114290+00:00
instance: kb-cron
parent: event-horizon-20260505-0
summary:
=== Interacting with a cosmic horizon === In the case of a horizon perceived by a uniformly accelerating observer in empty space, the horizon seems to remain a fixed distance from the observer no matter how its surroundings move. Varying the observer's acceleration may cause the horizon to appear to move over time or may prevent an event horizon from existing, depending on the acceleration function chosen. The observer never touches the horizon and never passes a location where it appeared to be. In the case of a horizon perceived by an occupant of a de Sitter universe, the horizon always appears to be a fixed distance away for a non-accelerating observer. It is never contacted, even by an accelerating observer. == Event horizon of a black hole == One of the best-known examples of an event horizon derives from general relativity's description of a black hole, a celestial object so dense that no nearby matter or radiation can escape its gravitational field. Often, this is described as the boundary within which the black hole's escape velocity is greater than the speed of light. However, a more detailed description is that within this horizon, all lightlike paths (paths that light


id: event-horizon-20260505-2
file: data/en.wikipedia.org/wiki/Event_horizon-2.md
category: reference
title: Event horizon
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Event_horizon
date_saved: 2026-05-05T10:55:10.114290+00:00
instance: kb-cron
parent: event-horizon-20260505-0
summary:
=== Interacting with black hole horizons === A misconception concerning event horizons, especially black hole event horizons, is that they represent an immutable surface that destroys objects that approach them. In practice, all event horizons appear to be some distance away from any observer, and objects sent towards an event horizon never appear to cross it from the sending observer's point of view (as the horizon-crossing event's light cone never intersects the observer's world line). Attempting to make an object near the horizon remain stationary with respect to an observer requires applying a force whose magnitude increases unboundedly (becoming infinite) the closer it gets. In the case of the horizon around a black hole, observers stationary with respect to a distant object will all agree on where the horizon is. While this seems to allow an observer lowered towards the hole on a rope (or rod) to contact the horizon, in practice this cannot be done. The proper distance to the horizon is finite, so the length of rope needed would be finite as well, but if the rope were lowered slowly (so that each point on the rope was approximately at rest in Schwarzschild coordinates), the proper acceleration (G-force)


id: fermat-s-principle-20260505-0
file: data/en.wikipedia.org/wiki/Fermat's_principle-0.md
category: reference
title: Fermat's principle
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Fermat's_principle
date_saved: 2026-05-05T10:55:11.341862+00:00
instance: kb-cron
parent: fermat-s-principle-20260505-0
summary:
Fermat's principle, also known as the principle of least time, is the link between ray optics and wave optics. Fermat's principle states that the path taken by a ray between two given points is the path that can be traveled in the least time. First proposed by the French mathematician Pierre de Fermat in 1662, as a means of explaining the ordinary law of refraction of light (Fig. 1), Fermat's principle was initially controversial because it seemed to ascribe knowledge and intent to nature. Not until the 19th century was it understood that nature's ability to test alternative paths is merely a fundamental property of waves. If points A and B are given, a wavefront expanding from A sweeps all possible ray paths radiating from A, whether they pass through B or not. If the wavefront reaches point B, it sweeps not only the ray path(s) from A to B, but also an infinitude of nearby paths with the same endpoints. Fermat's principle describes any ray that happens to reach point B; there is no implication that the ray "knew" the quickest path or "intended" to take that path. In its original "strong" form, Fermat's principle states that the path


id: fermat-s-principle-20260505-1
file: data/en.wikipedia.org/wiki/Fermat's_principle-1.md
category: reference
title: Fermat's principle
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Fermat's_principle
date_saved: 2026-05-05T10:55:11.341862+00:00
instance: kb-cron
parent: fermat-s-principle-20260505-0
summary:
If the corridor of paths reinforcing a ray path from A to B is substantially obstructed, this will significantly affect the energy reaching B from A – unlike a similar-sized obstruction outside any such corridor. Thus the ray path marks an energy path – as does a beam. Suppose that a wavefront expanding from point A passes point P, which lies on a ray path from point A to point B. By definition, all points on the wavefront have the same propagation time from A. Now let the wavefront be blocked except for a window, centered on P, and small enough to lie within the corridor of paths that reinforce the ray path from A to B. Then all points on the unobstructed portion of the wavefront will have, nearly enough, equal propagation times to B, but not to points in other directions, so that B will be in the direction of peak intensity of the beam admitted through the window. So the ray path marks the beam. And in optical experiments, a beam is routinely considered as a collection of rays or (if it is narrow) as an approximation to a ray (Fig. 3). === Analogies === According to


id: fermat-s-principle-20260505-2
file: data/en.wikipedia.org/wiki/Fermat's_principle-2.md
category: reference
title: Fermat's principle
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Fermat's_principle
date_saved: 2026-05-05T10:55:11.341862+00:00
instance: kb-cron
parent: fermat-s-principle-20260505-0
summary:
By (i), the ray path is a path of stationary traversal time from P to W′; and by (ii), it is a path of stationary traversal time from a point on W to P′. So Huygens's construction implicitly defines a ray path as a path of stationary traversal time between successive positions of a wavefront, the time being reckoned from a point-source on the earlier wavefront. This conclusion remains valid if the secondary wavefronts are reflected or refracted by surfaces of discontinuity in the properties of the medium, provided that the comparison is restricted to the affected paths and the affected portions of the wavefronts. Fermat's principle, however, is conventionally expressed in point-to-point terms, not wavefront-to-wavefront terms. Accordingly, the example may be modified by supposing that the wavefront which becomes surface W at time t, and which becomes surface W′ at the later time t + Δt, is emitted from point A at time 0. Let P be a point on W (as before), and B a point on W′. And let A, W, W′, and B be given, so that the problem is to find P. If P satisfies Huygens's construction, so that the secondary wavefront from P is


id: fermat-s-principle-20260505-3
file: data/en.wikipedia.org/wiki/Fermat's_principle-3.md
category: reference
title: Fermat's principle
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Fermat's_principle
date_saved: 2026-05-05T10:55:11.341862+00:00
instance: kb-cron
parent: fermat-s-principle-20260505-0
summary:
where n r = c / v r {\displaystyle \,n_{\mathrm {r} }=c/v_{\mathrm {r} }\,} is the ray index – that is, the refractive index calculated on the ray velocity instead of the usual phase velocity (wave-normal velocity). For an infinitesimal path, d S = n r d s , {\displaystyle dS=n_{\mathrm {r} \,}ds\,,\,} indicating that the optical length is the physical length multiplied by the ray index: the OPL is a notional geometric quantity, from which time has been factored out. In terms of OPL, the condition for Γ to be a ray path (Fermat's principle) becomes This has the form of Maupertuis's principle in classical mechanics (for a single particle), with the ray index in optics taking the role of momentum or velocity in mechanics. In an isotropic medium, for which the ray velocity is also the phase velocity, the usual refractive index n may be substituted for nr. === Relation to Hamilton's principle === If x, y, z are Cartesian coordinates and an overdot denotes differentiation with respect to s, Fermat's principle (2) may be written δ S = δ ∫ A B n r d x 2 + d y 2 + d z 2 = δ


id: fermat-s-principle-20260505-4
file: data/en.wikipedia.org/wiki/Fermat's_principle-4.md
category: reference
title: Fermat's principle
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Fermat's_principle
date_saved: 2026-05-05T10:55:11.341862+00:00
instance: kb-cron
parent: fermat-s-principle-20260505-0
summary:
1. The principle that you take as the basis of your demonstration, namely that nature always acts in the shortest and simplest ways, is merely a moral principle and not a physical one; it is not, and cannot be, the cause of any effect in nature .... For otherwise we would attribute knowledge to nature; but here, by "nature", we understand only this order and this law established in the world as it is, which acts without foresight, without choice, and by a necessary determination. 2. This same principle would make nature irresolute ... For I ask you ... when a ray of light must pass from a point in a rare medium to a point in a dense one, is there not reason for nature to hesitate if, by your principle, it must choose the straight line as soon as the bent one, since if the latter proves shorter in time, the former is shorter and simpler in length? Who will decide and who will pronounce? Fermat, being unaware of the mechanistic foundations of his own principle, was not well placed to defend it, except as a purely geometric and kinematic proposition. The wave theory of light, first proposed


id: fermat-s-principle-20260505-5
file: data/en.wikipedia.org/wiki/Fermat's_principle-5.md
category: reference
title: Fermat's principle
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Fermat's_principle
date_saved: 2026-05-05T10:55:11.341862+00:00
instance: kb-cron
parent: fermat-s-principle-20260505-0
summary:
In fact Laplace was aware that Fermat's principle follows from Huygens's construction in the case of refraction from an isotropic medium to an anisotropic one; a geometric proof was contained in the long version of Laplace's report, printed in 1810. Young's claim was more general than Laplace's, and likewise upheld Fermat's principle even in the case of extraordinary refraction, in which the rays are generally not perpendicular to the wavefronts. Unfortunately, however, the omitted middle sentence of the quoted paragraph by Young began "The motion of every undulation must necessarily be in a direction perpendicular to its surface ..." (emphasis added), and was therefore bound to sow confusion rather than clarity. No such confusion subsists in Augustin-Jean Fresnel's "Second Memoir" on double refraction (Fresnel, 1827), which addresses Fermat's principle in several places (without naming Fermat), proceeding from the special case in which rays are normal to wavefronts, to the general case in which rays are paths of least time or stationary time. (In the following summary, page numbers refer to Alfred W. Hobson's translation.) For refraction of a plane wave at parallel incidence on one face of an anisotropic crystalline wedge (pp. 291–2), in order to find the "first ray


id: ferromagnetism-20260505-0
file: data/en.wikipedia.org/wiki/Ferromagnetism-0.md
category: reference
title: Ferromagnetism
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Ferromagnetism
date_saved: 2026-05-05T10:55:12.614744+00:00
instance: kb-cron
parent: ferromagnetism-20260505-0
summary:
Ferromagnetism is a property of certain materials (such as iron) that results in a significant, observable magnetic permeability, and in many cases, a significant magnetic coercivity, allowing the material to form a permanent magnet. Ferromagnetic materials are noticeably attracted to a magnet, which is a consequence of their substantial magnetic permeability. Magnetic permeability describes the induced magnetization of a material due to the presence of an external magnetic field. For example, this temporary magnetization inside a steel plate accounts for the plate's attraction to a magnet. Whether or not that steel plate then acquires permanent magnetization depends on both the strength of the applied field and on the coercivity of that particular piece of steel (which varies with the steel's chemical composition and any heat treatment it may have undergone). In physics, multiple types of material magnetism have been distinguished. Ferromagnetism (along with the similar effect ferrimagnetism) is the strongest type and is responsible for the common phenomenon of everyday magnetism. A common example of a permanent magnet is a refrigerator magnet. Substances respond weakly to magnetic fields by three other types of magnetism—paramagnetism, diamagnetism, and antiferromagnetism—but the forces are usually so weak that they can only be detected by


id: ferromagnetism-20260505-1
file: data/en.wikipedia.org/wiki/Ferromagnetism-1.md
category: reference
title: Ferromagnetism
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Ferromagnetism
date_saved: 2026-05-05T10:55:12.614744+00:00
instance: kb-cron
parent: ferromagnetism-20260505-0
summary:
=== Unusual materials === Most ferromagnetic materials are metals, since the conducting electrons are often responsible for mediating the ferromagnetic interactions. It is therefore a challenge to develop ferromagnetic insulators, especially multiferroic materials, which are both ferromagnetic and ferroelectric. A number of actinide compounds are ferromagnets at room temperature or exhibit ferromagnetism upon cooling. PuP is a paramagnet with cubic symmetry at room temperature, but which undergoes a structural transition into a tetragonal state with ferromagnetic order when cooled below its TC = 125 K. In its ferromagnetic state, PuP's easy axis is in the ⟨100⟩ direction. In NpFe2 the easy axis is ⟨111⟩. Above TC ≈ 500 K, NpFe2 is also paramagnetic and cubic. Cooling below the Curie temperature produces a rhombohedral distortion wherein the rhombohedral angle changes from 60° (cubic phase) to 60.53°. An alternate description of this distortion is to consider the length c along the unique trigonal axis (after the distortion has begun) and a as the distance in the plane perpendicular to c. In the cubic phase this reduces to c/a = 1.00. Below the Curie temperature, the lattice acquires a distortion c a − 1 = − ( 120 ± 5 ) × 10


id: ferromagnetism-20260505-2
file: data/en.wikipedia.org/wiki/Ferromagnetism-2.md
category: reference
title: Ferromagnetism
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Ferromagnetism
date_saved: 2026-05-05T10:55:12.614744+00:00
instance: kb-cron
parent: ferromagnetism-20260505-0
summary:
When two nearby atoms have unpaired electrons, whether the electron spins are parallel or antiparallel affects whether the electrons can share the same orbit as a result of the quantum mechanical effect called the exchange interaction. This in turn affects the electron location and the Coulomb (electrostatic) interaction and thus the energy difference between these states. The exchange interaction is related to the Pauli exclusion principle, which says that two electrons with the same spin cannot also be in the same spatial state (orbital). This is a consequence of the spin–statistics theorem and that electrons are fermions. Therefore, under certain conditions, when the orbitals of the unpaired outer valence electrons from adjacent atoms overlap, the distributions of their electric charge in space are farther apart when the electrons have parallel spins than when they have opposite spins. This reduces the electrostatic energy of the electrons when their spins are parallel compared to their energy when the spins are antiparallel, so the parallel-spin state is more stable. This difference in energy is called the exchange energy. In simple terms, the outer electrons of adjacent atoms, which repel each other, can move further apart by aligning their spins in the same direction.


id: ferromagnetism-20260505-3
file: data/en.wikipedia.org/wiki/Ferromagnetism-3.md
category: reference
title: Ferromagnetism
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Ferromagnetism
date_saved: 2026-05-05T10:55:12.614744+00:00
instance: kb-cron
parent: ferromagnetism-20260505-0
summary:
Thus, a piece of iron in its lowest energy state ("unmagnetized") generally has little or no net magnetic field. However, the magnetic domains in a material are not fixed in place; they are simply regions where the spins of the electrons have aligned spontaneously due to their magnetic fields, and thus can be altered by an external magnetic field. If a strong-enough external magnetic field is applied to the material, the domain walls will move via a process in which the spins of the electrons in atoms near the wall in one domain turn under the influence of the external field to face in the same direction as the electrons in the other domain, thus reorienting the domains so more of the dipoles are aligned with the external field. The domains will remain aligned when the external field is removed, and sum to create a magnetic field of their own extending into the space around the material, thus creating a "permanent" magnet. The domains do not go back to their original minimum energy configuration when the field is removed because the domain walls tend to become 'pinned' or 'snagged' on defects in the crystal lattice, preserving their parallel orientation. This


id: field-effect-semiconductor-20260505-0
file: data/en.wikipedia.org/wiki/Field_effect_(semiconductor)-0.md
category: reference
title: Field effect (semiconductor)
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Field_effect_(semiconductor)
date_saved: 2026-05-05T10:55:13.888296+00:00
instance: kb-cron
parent: field-effect-semiconductor-20260505-0
summary:
In physics, the field effect refers to the modulation of the electrical conductivity of a material by the application of an external electric field. In a metal, the electron density that responds to applied fields is so large that an external electric field can penetrate only a very short distance into the material. However, in a semiconductor the lower density of electrons (and possibly holes) that can respond to an applied field is sufficiently small that the field can penetrate quite far into the material. This field penetration alters the conductivity of the semiconductor near its surface, and is called the field effect. The field effect underlies the operation of the Schottky diode and of field-effect transistors, notably the MOSFET, the JFET and the MESFET. == Surface conductance and band bending == The change in surface conductance occurs because the applied field alters the energy levels available to electrons to considerable depths from the surface, and that in turn changes the occupancy of the energy levels in the surface region. A typical treatment of such effects is based upon a band-bending diagram showing the positions in energy of the band edges as a function of depth into the material. An


id: flowability-20260505-0
file: data/en.wikipedia.org/wiki/Flowability-0.md
category: reference
title: Flowability
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Flowability
date_saved: 2026-05-05T10:55:15.173423+00:00
instance: kb-cron
parent: flowability-20260505-0
summary:
Flowability, also known as powder flow is a property that defines an ability of a powdered material to flow, related to cohesion. Powder flowability depends on many traits: the shape and size of the powder particles due to intermolecular force, porosity electrostatic activity hygroscopy bulk density angle of repose presence of glidants oxidation rate (of a metallic powder) humidity ISO 4490:2018 norm (and its precedent, ISO 4490:2014) standardizes a method for determining the flow rate of metallic powders. It uses a normalized/calibrated funnel, named Hall flowmeter. == See also == Fluid mechanics Soil mechanics Cohesion (geology) angle of repose == References ==


id: free-neutron-decay-20260505-0
file: data/en.wikipedia.org/wiki/Free_neutron_decay-0.md
category: reference
title: Free neutron decay
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Free_neutron_decay
date_saved: 2026-05-05T10:55:16.386179+00:00
instance: kb-cron
parent: free-neutron-decay-20260505-0
summary:
A free neutron refers to a neutron that is not bound to an atomic nucleus. When embedded in a stable nuclide, neutrons have not been observed to decay. Free neutrons decay with a mean lifetime of 878.4±0.5 s (nearly 15 minutes), which corresponds with a half-life of around 608 seconds. The free neutron decays via the weak interaction and may be called the simplest example of beta decay. The decay results in the stable resultant products of a proton, electron and an electron antineutrino. This is expressed as a decay equation: The decay parameters of such a common baryon are of significance across multiple areas of physics, in particular particle physics and cosmology. Highly precise quantitative measurements of the free neutron mean lifetime by two different methods vary by more than the errors in the measurement techniques. This issue has been called the neutron lifetime puzzle. == History == The neutron was discovered in 1932. In a 1935 article, James Chadwick and Maurice Goldhaber speculated that the free neutron would be unstable, due to its estimated mass being larger than that of a hydrogen atom. In 1948 free neutron decay was first observed, along with a rudimentary estimation of its


id: free-neutron-decay-20260505-1
file: data/en.wikipedia.org/wiki/Free_neutron_decay-1.md
category: reference
title: Free neutron decay
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Free_neutron_decay
date_saved: 2026-05-05T10:55:16.386179+00:00
instance: kb-cron
parent: free-neutron-decay-20260505-0
summary:
=== Neutron lifetime puzzle === While the neutron lifetime has been studied for decades, there is currently a lack of agreement on its exact value, due to different results from two experimental methods, "bottle" versus "beam" methods. This has been called variously the neutron lifetime puzzle, neutron lifetime problem, or neutron lifetime anomaly. The neutron lifetime puzzle was realized after the refinement of experiments with ultracold neutrons. The error margins of the two methods previously overlapped, but increasing refinement in technique has failed to demonstrate convergence to a single value. The difference in mean lifetime values obtained as of 2014 was approximately 9 s. Further, a 2018 lattice QCD calculation of the lifetime value was not sufficiently precise to support one over the other. To give a concrete example of the discrepancy for two specific experiments, in 2013 a beam method experiment found a lifetime of τ n = 887.7 ± 1.2 s t a t ± 1.8 s y s t s . {\displaystyle \ \tau _{n}=887.7\pm 1.2_{\mathsf {stat}}\ \pm {1.8}_{\mathsf {syst}}\ {\mathsf {s}}~.} In 2021 a bottle method experiment found a lifetime of τ n = 877.75 ± 0.28 s t a t − 0.16 + 0.22 s


id: fundamental-interaction-20260505-0
file: data/en.wikipedia.org/wiki/Fundamental_interaction-0.md
category: reference
title: Fundamental interaction
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Fundamental_interaction
date_saved: 2026-05-05T10:55:17.649266+00:00
instance: kb-cron
parent: fundamental-interaction-20260505-0
summary:
In physics, the fundamental interactions or fundamental forces are interactions in nature that appear not to be reducible to more basic interactions. There are four fundamental interactions known to exist: gravity, electromagnetism, weak interaction, and strong interaction. The gravitational and electromagnetic interactions produce long-range forces whose effects can be seen directly in everyday life. The strong and weak interactions produce forces at subatomic scales and govern nuclear interactions inside atoms. Some scientists hypothesize that a fifth force might exist, but these hypotheses remain speculative. Each of the known fundamental interactions can be described mathematically as a field. The gravitational interaction is attributed to the curvature of spacetime, described by Einstein's general theory of relativity. The other three are discrete quantum fields, and their interactions are mediated by elementary particles described by the Standard Model of particle physics. Within the Standard Model, the strong interaction is carried by a particle called the gluon and is responsible for quarks binding together to form hadrons, such as protons and neutrons. As a residual effect, it creates the nuclear force that binds the latter particles to form atomic nuclei. The weak interaction is carried by particles called W and Z bosons, and also acts


id: fundamental-interaction-20260505-1
file: data/en.wikipedia.org/wiki/Fundamental_interaction-1.md
category: reference
title: Fundamental interaction
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Fundamental_interaction
date_saved: 2026-05-05T10:55:17.649266+00:00
instance: kb-cron
parent: fundamental-interaction-20260505-0
summary:
Two fermions go in → interaction by boson exchange → two changed fermions go out. The exchange of bosons always carries energy and momentum between the fermions, thereby changing their speed and direction. The exchange may also transport a charge between the fermions, changing the charges of the fermions in the process (e.g., turn them from one type of fermion to another). Since bosons carry one unit of angular momentum, the fermion's spin direction will flip from +1⁄2 to −1⁄2 (or vice versa) during such an exchange (in units of the reduced Planck constant). Since such interactions result in a change in momentum, they can give rise to classical Newtonian forces. In quantum mechanics, physicists often use the terms "force" and "interaction" interchangeably; for example, the weak interaction is sometimes referred to as the "weak force". According to the present understanding, there are four fundamental interactions or forces: gravitation, electromagnetism, the weak interaction, and the strong interaction. Their magnitude and behaviour vary greatly, as described in the table below. Modern physics attempts to explain every observed physical phenomenon by these fundamental interactions. The fundamental interactions can be compared using dimensionless coupling constants that characterize the intensity or "strength" of the


id: fundamental-interaction-20260505-2
file: data/en.wikipedia.org/wiki/Fundamental_interaction-2.md
category: reference
title: Fundamental interaction
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Fundamental_interaction
date_saved: 2026-05-05T10:55:17.649266+00:00
instance: kb-cron
parent: fundamental-interaction-20260505-0
summary:
Electromagnetism is the force that acts between electrically charged particles. This phenomenon includes the electrostatic force acting between charged particles at rest, and the combined effect of electric and magnetic forces acting between charged particles moving relative to each other. Electromagnetism has an infinite range, as gravity does, but is vastly stronger. It is the force that binds electrons to atoms, and it holds molecules together. It is responsible for everyday phenomena like light, magnets, electricity, and friction. Electromagnetism fundamentally determines all macroscopic, and many atomic-level, properties of the chemical elements. In a four kilogram (~1 gallon) jug of water, there is of total electron charge. Thus, if we place two such jugs a meter apart, the electrons in one of the jugs repel those in the other jug with a force of This force is many times larger than the weight of the planet Earth. The atomic nuclei in one jug also repel those in the other with the same force. However, these repulsive forces are canceled by the attraction of the electrons in jug A with the nuclei in jug B and the attraction of the nuclei in jug A with the electrons in jug B, resulting in


id: fundamental-interaction-20260505-3
file: data/en.wikipedia.org/wiki/Fundamental_interaction-3.md
category: reference
title: Fundamental interaction
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Fundamental_interaction
date_saved: 2026-05-05T10:55:17.649266+00:00
instance: kb-cron
parent: fundamental-interaction-20260505-0
summary:
The pions were understood to be oscillations of vacuum condensates; Jun John Sakurai proposed the rho and omega vector bosons to be force carrying particles for approximate symmetries of isospin and hypercharge; Geoffrey Chew, Edward K. Burdett and Steven Frautschi grouped the heavier hadrons into families that could be understood as vibrational and rotational excitations of strings. While each of these approaches offered insights, no approach led directly to a fundamental theory. Murray Gell-Mann along with George Zweig first proposed fractionally charged quarks in 1961. Throughout the 1960s, different authors considered theories similar to the modern fundamental theory of quantum chromodynamics (QCD) as simple models for the interactions of quarks. The first to hypothesize the gluons of QCD were Moo-Young Han and Yoichiro Nambu, who introduced the quark color charge. Han and Nambu hypothesized that it might be associated with a force-carrying field. At that time, however, it was difficult to see how such a model could permanently confine quarks. Han and Nambu also assigned each quark color an integer electrical charge, so that the quarks were fractionally charged only on average, and they did not expect the quarks in their model to be permanently confined. In 1971, Murray Gell-Mann


id: fundamental-interaction-20260505-4
file: data/en.wikipedia.org/wiki/Fundamental_interaction-4.md
category: reference
title: Fundamental interaction
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Fundamental_interaction
date_saved: 2026-05-05T10:55:17.649266+00:00
instance: kb-cron
parent: fundamental-interaction-20260505-0
summary:
== Unification == The fundamental forces may become unified into a single force at very high energies and on a minuscule scale, the Planck scale. Particle accelerators cannot produce the enormous energies required to experimentally probe this regime. The weak and electromagnetic forces have already been unified with the electroweak theory of Sheldon Glashow, Abdus Salam, and Steven Weinberg, for which they received the 1979 Nobel Prize in physics. Numerous theoretical efforts have been made to systematize the existing four fundamental interactions on the model of electroweak unification. Grand Unified Theories (GUTs) are proposals to show that each of the three fundamental interactions described by the Standard Model is a different manifestation of a single interaction with symmetries that break down and create separate interactions below some extremely high level of energy. GUTs are also expected to predict some of the relationships between constants of nature that the Standard Model treats as unrelated and gauge coupling unification for the relative strengths of the electromagnetic, weak, and strong forces. (GUT). Some attempts at GUTs hypothesize "shadow" particles, such that every known matter particle associates with an undiscovered force particle, and vice versa, altogether supersymmetry (SUSY). Other theorists seek to quantize the


id: gravitational-singularity-20260505-0
file: data/en.wikipedia.org/wiki/Gravitational_singularity-0.md
category: reference
title: Gravitational singularity
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Gravitational_singularity
date_saved: 2026-05-05T10:55:18.898698+00:00
instance: kb-cron
parent: gravitational-singularity-20260505-0
summary:
A gravitational singularity, spacetime singularity, or simply singularity, is a theoretical condition in which gravity is predicted to be so intense that spacetime itself would break down catastrophically. As such, a singularity is by definition no longer part of the regular spacetime and cannot be determined by "where" or "when". Gravitational singularities exist at a junction between general relativity and quantum mechanics; therefore, the properties of the singularity cannot be described without an established theory of quantum gravity. Trying to find a complete and precise definition of singularities in the theory of general relativity, the best theory of gravity available, remains a difficult problem. A singularity in general relativity can be defined by the scalar invariant curvature becoming infinite or, better, by a geodesic being incomplete. General relativity predicts that any object collapsing beyond its Schwarzschild radius would form a black hole, inside which a singularity will form. A black hole singularity is, however, covered by an event horizon, so it is never in the causal past of any outside observer, and at no time can it be objectively said to have formed. General relativity also predicts that the initial state of the universe, at the beginning of the Big


id: gravitational-singularity-20260505-1
file: data/en.wikipedia.org/wiki/Gravitational_singularity-1.md
category: reference
title: Gravitational singularity
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Gravitational_singularity
date_saved: 2026-05-05T10:55:18.898698+00:00
instance: kb-cron
parent: gravitational-singularity-20260505-0
summary:
Observers falling into a Schwarzschild black hole (i.e., non-rotating and not charged) cannot avoid being carried into the singularity once they cross the event horizon. As they fall further into the black hole, they will be torn apart by the growing tidal forces in a process sometimes referred to as spaghettification or the noodle effect. Eventually, they will reach the singularity and be crushed into an infinitely small point. Although in theory, the interior of a Schwarzschild black hole curves inwards towards a sharp point at the singularity, this model is only true when the spacetime inside the black hole had not been perturbed. Any perturbations, such as those caused by matter or radiation falling in, would cause space to oscillate chaotically near the singularity. Any matter falling in would experience intense tidal forces rapidly changing in direction, all while being compressed into an increasingly small volume. In the case of a charged or rotating black hole, it is possible to avoid the singularity. Extending these solutions as far as possible reveals the hypothetical possibility of exiting the black hole into a different spacetime with the black hole acting as a wormhole. The possibility of travelling to another universe is,


id: gravitational-singularity-20260505-2
file: data/en.wikipedia.org/wiki/Gravitational_singularity-2.md
category: reference
title: Gravitational singularity
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Gravitational_singularity
date_saved: 2026-05-05T10:55:18.898698+00:00
instance: kb-cron
parent: gravitational-singularity-20260505-0
summary:
Until the early 1990s, it was widely believed that general relativity hides every singularity behind an event horizon, making naked singularities impossible. This is referred to as the cosmic censorship hypothesis. However, in 1991, physicists Stuart Shapiro and Saul Teukolsky performed computer simulations of a rotating plane of dust that indicated that general relativity might allow for "naked" singularities. What these objects would actually look like in such a model is unknown. Nor is it known whether singularities would still arise if the simplifying assumptions used to make the simulation were removed. However, it is hypothesized that light entering a singularity would similarly have its geodesics terminated, thus making the naked singularity look like a black hole. Disappearing event horizons exist in the Kerr metric, which is a spinning black hole in a vacuum, if the angular momentum ( J {\displaystyle J} ) is high enough. Transforming the Kerr metric to Boyer–Lindquist coordinates, it can be shown that the coordinate (which is not the radius) of the event horizon is, r ± = μ ± ( μ 2 − a 2 ) 1 / 2 {\displaystyle r_{\pm }=\mu \pm \left(\mu ^{2}-a^{2}\right)^{1/2}} , where μ = G M / c 2


id: group-velocity-dispersion-20260505-0
file: data/en.wikipedia.org/wiki/Group-velocity_dispersion-0.md
category: reference
title: Group-velocity dispersion
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Group-velocity_dispersion
date_saved: 2026-05-05T10:55:20.281680+00:00
instance: kb-cron
parent: group-velocity-dispersion-20260505-0
summary:
In optics, group-velocity dispersion (GVD) is a characteristic of a dispersive medium, used most often to determine how the medium affects the duration of an optical pulse traveling through it. Formally, GVD is defined as the derivative of the inverse of group velocity of light in a material with respect to angular frequency, GVD ( ω 0 ) ≡ ∂ ∂ ω ( 1 v g ( ω ) ) ω = ω 0 , {\displaystyle {\text{GVD}}(\omega _{0})\equiv {\frac {\partial }{\partial \omega }}\left({\frac {1}{v_{g}(\omega )}}\right)_{\omega =\omega _{0}},} where ω {\displaystyle \omega } and ω 0 {\displaystyle \omega _{0}} are angular frequencies, and the group velocity v g ( ω ) {\displaystyle v_{g}(\omega )} is defined as v g ( ω ) ≡ ∂ ω / ∂ k {\displaystyle v_{g}(\omega )\equiv \partial \omega /\partial k} . The units of group-velocity dispersion are [time]2/[distance], often expressed in fs2/mm. Equivalently, group-velocity dispersion can be defined in terms of the medium-dependent wave vector k ( ω ) {\displaystyle k(\omega )} according to GVD ( ω 0 ) ≡ ( ∂ 2 k ∂ ω 2 ) ω = ω 0 , {\displaystyle {\text{GVD}}(\omega _{0})\equiv \left({\frac {\partial ^{2}k}{\partial \omega ^{2}}}\right)_{\omega =\omega _{0}},} or in terms


id: group-velocity-dispersion-20260505-1
file: data/en.wikipedia.org/wiki/Group-velocity_dispersion-1.md
category: reference
title: Group-velocity dispersion
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Group-velocity_dispersion
date_saved: 2026-05-05T10:55:20.281680+00:00
instance: kb-cron
parent: group-velocity-dispersion-20260505-0
summary:
Δ T = d ( 1 v g ( ω 2 ) − 1 v g ( ω 1 ) ) . {\displaystyle \Delta T=d\left({\frac {1}{v_{g}(\omega _{2})}}-{\frac {1}{v_{g}(\omega _{1})}}\right).} The expression can be approximated as a Taylor expansion, giving Δ T = d ( 1 v g ( ω 1 ) + ∂ ∂ ω ( 1 v g ( ω ′ ) ) ω ′ = ω 1 ( ω 2 − ω 1 ) − 1 v g ( ω 1 ) ) , {\displaystyle \Delta T=d\left({\frac {1}{v_{g}(\omega _{1})}}+{\frac {\partial }{\partial \omega }}\left({\frac {1}{v_{g}(\omega ')}}\right)_{\omega '=\omega _{1}}(\omega _{2}-\omega _{1})-{\frac {1}{v_{g}(\omega _{1})}}\right),} or Δ T = d × GVD ( ω 1 ) × ( ω 2 − ω 1 ) . {\displaystyle \Delta T=d\times {\textrm {GVD}}(\omega _{1})\times (\omega _{2}-\omega _{1}).} From here it is possible to imagine scaling this expression up two pulses to infinitely many. The frequency difference ω 2 − ω 1 {\displaystyle \omega _{2}-\omega _{1}} must be replaced by the bandwidth, and the time delay Δ T {\displaystyle \Delta T} evolves into the induced chirp. == Group-delay dispersion == A closely related yet independent quantity is the group-delay dispersion (GDD), defined such that group-velocity dispersion


id: hanle-effect-20260505-0
file: data/en.wikipedia.org/wiki/Hanle_effect-0.md
category: reference
title: Hanle effect
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Hanle_effect
date_saved: 2026-05-05T10:55:21.498118+00:00
instance: kb-cron
parent: hanle-effect-20260505-0
summary:
The Hanle effect (also known as zero-field level crossing) is a reduction in the polarization of light emitted by atoms excited by polarized light, when subjected to a magnetic field. Typically, when atoms are excited by linearly polarized light, the emitted fluorescence is also polarized. If a weak transverse magnetic field is applied, the emitted light becomes depolarized (or its polarization rotates). This effect can be utilized in experiments to measure the lifetime of excited states and detect the presence of magnetic fields. == History == The first experimental evidence for the effect came from Robert W. Wood, and Lord Rayleigh. The effect is named after Wilhelm Hanle, who was the first to explain the effect, in terms of classical physics, in Zeitschrift für Physik in 1924. Initially, the causes of the effect were controversial, and many theorists mistakenly thought it was a version of the Faraday effect. Attempts to understand the phenomenon were important in the subsequent development of quantum physics. An early theoretical treatment of level crossing effect was given by Gregory Breit. == Theory == === Classical model === The classical explanation for this effect involves the Lorentz oscillator model, which treats the electron bound to the


id: heat-treating-20260505-0
file: data/en.wikipedia.org/wiki/Heat_treating-0.md
category: reference
title: Heat treating
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Heat_treating
date_saved: 2026-05-05T10:55:22.810075+00:00
instance: kb-cron
parent: heat-treating-20260505-0
summary:
Heat treating (or heat treatment) is a group of industrial, thermal and metalworking processes used to alter the physical, and sometimes chemical properties of a material. The most common application is metallurgical. Heat treatments are also used in the manufacture of many other materials, such as glass. Heat treatment involves the use of heating or chilling, normally to extreme temperatures, to achieve the desired result, such as the softening or hardening of a material. Heat treatment techniques include annealing, case hardening, precipitation strengthening, tempering, carburizing, normalizing and quenching. Although the term heat treatment applies only to processes where the heating and cooling are done for the specific purpose of altering properties intentionally, heating and cooling often occur incidentally during other manufacturing processes, such as hot forming or welding. == Physical processes == Metallic materials consist of a microstructure of small crystals called "grains" or crystallites. The nature of the grains (i.e. grain size and composition) is one of the most effective factors that can determine the overall mechanical behavior of the metal. Heat treatment provides an efficient way to manipulate the properties of the metal by controlling the rate of diffusion and the rate of cooling within the microstructure. Heat


id: heat-treating-20260505-1
file: data/en.wikipedia.org/wiki/Heat_treating-1.md
category: reference
title: Heat treating
tags: science, encyclopedia
source_url: https://en.wikipedia.org/wiki/Heat_treating
date_saved: 2026-05-05T10:55:22.810075+00:00
instance: kb-cron
parent: heat-treating-20260505-0
summary:
=== Hypoeutectoid alloys === A hypoeutectic alloy has two separate melting points. Both are above the eutectic melting point for the system but are below the melting points of any constituent forming the system. Between these two melting points, the alloy will exist as part solid and part liquid. The constituent with the higher melting point will solidify first. When completely solidified, a hypoeutectic alloy will often be in a solid solution. Similarly, a hypoeutectoid alloy has two critical temperatures, called "arrests". Between these two temperatures, the alloy will exist partly as the solution and partly as a separate crystallizing phase, called the "pro eutectoid phase". These two temperatures are called the upper (A3) and lower (A1) transformation temperatures. As the solution cools from the upper transformation temperature toward an insoluble state, the excess base metal will often be forced to "crystallize-out", becoming the pro eutectoid. This will occur until the remaining concentration of solutes reaches the eutectoid level, which will then crystallize as a separate microstructure. For example, a hypoeutectoid steel contains less than 0.77% carbon. Upon cooling a hypoeutectoid steel from the austenite transformation temperature, small islands of proeutectoid-ferrite will form. These will continue to grow and the


