7.2 KiB
| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Black hole | 5/13 | https://en.wikipedia.org/wiki/Black_hole | reference | science, encyclopedia | 2026-05-05T13:31:52.244381+00:00 | kb-cron |
The simplest static black holes have mass but neither electric charge nor angular momentum. Contrary to the popular notion of a black hole "sucking in everything" in its surroundings, from far away, the external gravitational field of a black hole is identical to that of any other body of the same mass. While a black hole can theoretically have any positive mass, its charge and angular momentum are limited by its mass, with this limit being greater for more massive black holes. The net electric charge
Q
{\displaystyle Q}
and the total angular momentum
J
{\displaystyle J}
satisfy the inequality
Q
2
4
π
ϵ
0
+
c
2
J
2
G
M
2
≤
G
M
2
{\displaystyle {\frac {Q^{2}}{4\pi \epsilon _{0}}}+{\frac {c^{2}J^{2}}{GM^{2}}}\leq GM^{2}}
for a black hole of mass
M
{\displaystyle M}
, where
ϵ
0
{\displaystyle \epsilon _{0}}
is the vacuum permittivity constant,
c
{\displaystyle c}
is the speed of light and
G
{\displaystyle G}
is the gravitational constant. Black holes with the maximum possible combination of charge and spin satisfying this inequality are called extremal black holes. Adding a low-mass object with a lot of charge or angular momentum to an extremal black hole would create a so-called naked singularities, a singularity outside of a black hole. Because these singularities make the universe inherently unpredictable, many physicists believe they could not exist. The weak cosmic censorship hypothesis, proposed by Penrose, rules out the formation of such singularities, when they are created through the gravitational collapse of realistic matter. This hypothesis remains an important area of study because has not yet been proven and it relates to many aspects of general relativity and quantum gravity. The total mass of a nearby black hole can be estimated by analysing the motion of the stars or gas surrounding it. The mass of distant supermassive black holes can be inferred from Doppler broadening of spectral lines emitted by rapidly orbiting gas, a technique called reverberation mapping.
=== Spin and angular momentum === All black holes spin, often fast—One stellar black hole, GRS 1915+105, has been estimated to spin at over 1,000 revolutions per second. The Milky Way's central black hole Sagittarius A* rotates at about 90% of the maximum possible rate. The spin rate can be inferred from measurements of atomic spectral lines in the X-ray range. As gas near the black hole plunges inward, high energy X-ray emission from electron-positron pairs illuminates the gas further out, appearing red-shifted due to relativistic effects. Depending on the spin of the black hole, this plunge happens at different radii from the hole, with different degrees of redshift. Astronomers can use the gap between the x-ray emission of the outer disk and the redshifted emission from plunging material to determine the spin of the black hole. A newer way to estimate spin is based on the temperature of gases accreting onto the black hole. The method requires an independent measurement of the black hole mass and inclination angle of the accretion disk followed by computer modelling. Gravitational waves from coalescing binary black holes can also provide the spin of both progenitor black holes and the merged hole, but such events are rare. A spinning black hole has angular momentum. The Kerr metric is the solution of Einstein's field equations for a rotating black hole. In addition to the Schwarzschild radius,
r
S
{\displaystyle r_{\textrm {S}}}
, it includes a rotational parameter,
a
=
J
M
=
2
J
r
S
,
{\displaystyle a={\frac {J}{M}}={\frac {2J}{r_{\textrm {S}}}},}
where
J
{\displaystyle J}
is the black hole angular momentum. An extremal black hole has
|
J
|
=
|
M
2
{\displaystyle |J|=|M^{2}}
, corresponding to
a
=
1
{\displaystyle a=1}
. The supermassive black hole in the center of the Messier 87 (M87) galaxy appears to have rotational parameter of 0.90±0.05, very close to the maximum theoretical value.
=== Charge === Black holes are believed to have an approximately neutral charge. For example, Michal Zajaček, Arman Tursunov, Andreas Eckart, and Silke Britzen found the electric charge of Sagittarius A* to be at least ten orders of magnitude below the theoretical maximum. If a black hole were to become charged, particles with an opposite sign of charge would be pulled in by the extra electromagnetic force, while particles with the same sign of charge would be repelled, neutralising the black hole. This effect operates even more rapidly if the black hole is also spinning. A spinning black hole in a magnetic field creates an electric field which would interact with charged particles. Since black holes have so few measurable intrinsic properties, techniques for measuring charge are of interest to astrophysics even if the values may be very small. The charge Q for a nonspinning black hole is bounded by
Q
≤
4
π
ϵ
0
G
M
,
{\displaystyle Q\leq {\sqrt {4\pi \epsilon _{0}G}}M,}
where G is the gravitational constant and M is the black hole's mass.
== Classification ==