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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Anti-gravity | 1/3 | https://en.wikipedia.org/wiki/Anti-gravity | reference | science, encyclopedia | 2026-05-05T09:16:54.702608+00:00 | kb-cron |
Anti-gravity is the concept of a force that would exactly oppose the force of gravity. Under the known laws of physics, anti-gravity is not possible. Experimental measurements rule out repulsion between antihydrogen and the mass of the Earth. Anti-gravity does not refer to either the lack of weight under gravity experienced in free fall or orbit, or to balancing the force of gravity with some other force, such as electromagnetism, aerodynamic lift, or ion-propelled "lifters", which fly in the air by moving air with electromagnetic fields. Historically, anti-gravity was considered a possibilty after the discovery of antimatter. Once the nature of antimatter was more clearly established, it was clear that gravity works the same for both matter and antimatter. Anti-gravity is a recurring concept in science fiction.
== Theoretical probability ==
Under the laws of general relativity, anti-gravity is impossible except under contrived circumstances. Under that theory, and particle physics, gravity is mass-energy, a quantity believed to always be positive. It is always attractive and never repulsive. During the close of the twentieth century NASA provided funding for the Breakthrough Propulsion Physics Program (BPP) from 1996 through 2002. This program studied a number of "far out" designs for space propulsion that were not receiving funding through normal university or commercial channels. Anti-gravity-like concepts were listed under "approaches categorized as non-viable" since the study found no evidence of anti-gravity-like forces. So many inappropriate proposals were submitted that NASA developed a screening guide for reviewers.
== History == Attempts to understand why gravity is solely an attractive force go back at least as far as James Clerk Maxwell in the late nineteenth century. He noted that existence of unlike charges in electromagnetism was the root of its fundamental difference from gravity. With the discovery of general relativity and the emergence of particle physics in the twentieth century this difference seemed even more fundamental. The "charge" in the theory of gravity is mass-energy, a quantity believed to always be positive. Thus gravity seemed to always be attractive and never repulsive. Two significant possible exceptions emerged, however –in quantum physics and at cosmological scales.
=== Antimatter gravitation === In 1928 Paul Dirac produced the first relativistic quantum mechanics theory. The theory accurately predicted properties of the electron but it also has a second solution. In 1931 Robert Oppenheimer showed that Dirac's original interpretation of the second solution was incorrect and Dirac responded with a new proposal: the second solution was a positively charged "anti-electron". Dirac also said that every other particle should have an opposite charged counterpart. With the discovery of the positron in 1932 and the antiproton in 1955, this theoretical concept of antimatter was grounded in empirical evidence. Dirac's theory did not include gravitation and there remains no consistent theory that combines both quantum mechanics and general relativity. A hypothetical negative mass charge in Newton's equations or general relativity is theoretically consistent even though no observations support this concept. Since antimatter is extremely rare, the possibility remained that repulsion between matter and antimatter would lead to antigravity. By 1956 the scientific impossibility of antigravity was a subject of theoretical analysis. Three more comprehensive arguments were published soon thereafter. In 1958, Philip Morrison showed that repulsion by mass would imply failure of conservation of energy in Earth's gravitational field. In 1959, Leonard I. Schiff showed that in quantum field theory the virtual anti-electron contribution to the vacuum polarization would break the equivalence of inertial and gravitational mass contrary to the results of the Eötvös experiment. Then in 1961, Myron L. Good noted that the longest-lived K meson is a superposition of a particle and its antiparticle; if these two particles responded differently to gravity the long-lived K meson would decay. Despite these arguments, new theories motivated by issues in cosmology and uncertainties in particle physics have been proposed in which the gravitational interaction of matter and antimatter could be repulsive.
=== Experiments === Attempts to measure the gravitational force on antimatter particles is extremely challenging. For matter particles, the equivalence of inertial and gravitational mass, known as the weak equivalence principle, has been demonstrated to a precision of 10−15. However the technique used differential electrostatic accelerometers on a pair of test masses composed of titanium and of platinum, all in an orbiting satellite. Producing antimatter hydrogen atoms requires a source of antiprotons like a particle accelerator combined with a source of positrons, making a satellite, two-mass experiment impractical. In 2023, the amount of antihydrogen escaping from the top and bottom of a vertical vacuum chamber at CERN was compared, ruling out repulsive gravity between antihydrogen and Earth's mass.
== Studies, empirical claims and commercial efforts == There have been a number of studies, attempts to build anti-gravity devices, and a small number of reports of anti-gravity-like effects in popular and scientific literature. None of the examples that follow are accepted as reproducible examples of anti-gravity.
=== Thomas Townsend Brown's gravitator ===