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The gravitational interaction of antimatter with matter or antimatter has not been conclusively observed by physicists. While the overwhelming consensus among physicists is that antimatter will attract both matter and antimatter at the same rate that matter attracts matter, there is a strong desire to confirm this experimentally. The CPT theorem asserts that antimatter should attract antimatter in the same way that matter attracts matter. However, there are several theories about how antimatter gravitationally interacts with normal matter.
When antimatter was first discovered in 1932, physicists wondered about how it would react to gravity. Initial analysis focused on whether antimatter should react the same as matter or react oppositely. Several theoretical arguments arose which convinced physicists that antimatter would react exactly the same as normal matter. They inferred that a gravitational repulsion between matter and antimatter was implausible as it would violate CPT invariance, conservation of energy, result in vacuum instability, and result in CP violation. It was also theorized that it would be inconsistent with the results of the Eötvös test of the weak equivalence principle.
Many of these early theoretical objections were later overturned.
Antimatter's rarity and tendency to annihilate when brought into contact with matter makes its study a technically demanding task.
Many scientists consider the best experimental evidence in favor of normal gravity to come from the observations of neutrinos from Supernova 1987A. In this landmark event, three neutrino detectors around the world simultaneously observed a cascade of neutrinos emanating from a supernova in the Large Magellanic Cloud. Although the supernova happened about 164,000 light years away, both neutrinos and antineutrinos may have been detected virtually simultaneously. If both were actually observed, then any difference in the gravitational interaction would have to be very small. However, neutrino detectors cannot distinguish perfectly between neutrinos and antineutrinos. Some physicists conservatively estimate that there is less than a 10% chance that no regular neutrinos were observed at all. Others estimate even lower probabilities, some as low as 1%. Unfortunately, this accuracy is unlikely to be improved by duplicating the experiment any time soon. The last known supernova to occur at such a close range prior to Supernova 1987A was around 1867.
Physicist William Fairbank attempted a laboratory experiment to directly measure the gravitational acceleration of both electrons and positrons. However, their charge-to-mass ratio is so large that electromagnetic effects overwhelmed the experiment. Furthermore, antiparticles must be kept separate from their normal counterparts or they will quickly annihilate. Worse still, production methods typically result in high-energy antimatter particles which are unsuitable for observation of gravitational effects in a laboratory environment. In recent years, the production of cold antihydrogen has become possible at the ATHENA and ATRAP experiments at CERN. Antihydrogen, which is electrically neutral, should make it possible to directly measure the gravitational attraction of antimatter particles to the matter Earth. CERN is aiming to do this.
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In 1958, Philip Morrison argued that antigravity would violate conservation of energy. If matter and antimatter responded oppositely to a gravitational field, then it would take no energy to change the height of a particle-antiparticle pair. However, when moving through a gravitational potential, the frequency and energy of light is shifted. Morrison argued that energy would be created by producing matter and antimatter at one height and then annihilating it higher up, since the photons used in production would have less energy than the photons yielded from annihilation.
However, it was later found that antigravity would still not violate the second law of thermodynamics.
If one can invent a theory in which matter and antimatter repel one another, what does it predict for things which are neither matter nor antimatter? Photons are their own antiparticles, and in all respects behave exactly symmetrically with respect to matter and antimatter particles. In a large number of laboratory and astronomical tests, (gravitational redshift and gravitational lensing, for example) photons are observed to be attracted to matter, exactly in accordance with the theory of General Relativity. It is possible to find atoms and nuclei whose elementary particle contents are the same, but whose masses are different. For example, Helium-4 weighs less than 2 atoms of deuterium due to binding-energy differences. The gravitational force constant is observed to be the same, up to the limits of experimental precision, for all such different materials, suggesting that "binding energy"—which, like the photon, has no distinction between matter and antimatter—experiences the same gravitational forces as matter. This is again in accordance with the theory of General Relativity, and difficult to reconcile with any theory predicting that matter and antimatter repel.
Later in 1958, L. Schiff used quantum field theory to argue that antigravity would be inconsistent with the results of the Eötvös experiment.
However, the renormalization technique used in Schiff's analysis is heavily criticized, and his work is seen as inconclusive.
In 1961, Myron L. Good argued that antigravity would result in the observation of an unacceptably high amount of CP violation in the anomalous regeneration of kaons. At the time, CP violation had not yet been observed. However, Good's argument is criticized for being expressed in terms of absolute potentials. By rephrasing the argument in terms of relative potentials,
Gabriel Chardin found that it resulted in an amount of Kaon regeneration which agrees with observation. He argues that antigravity is in fact a potential explanation for CP violation. Antimatter antigravity [might also] explains two cosmological paradoxes. The first is the apparent local lack of antimatter: by theory antimatter and matter would repel each other gravitationally, forming separate matter and antimatter galaxies. These galaxies would also tend to repel one another, thereby preventing possible collisions and annihilations. These two facts combine to imply that the universe gradually arranges itself into a gravitational dipole at the largest scale.
This same galactic repulsion is also endorsed as a potential explanation to the observation of a flatly accelerating universe. If gravity was always attractive, the expansion of the universe might be expected to decelerate and eventually contract into a big crunch. Using redshift observations, astronomers and physicists estimate that instead, the size of the universe is expanding and the rate of expansion is accelerating at an approximately constant rate. Several theories have been proposed to explain this observation within the context of an always-attractive gravity. On the other hand, supporters of antigravity argue that if mutually repulsive, equal amounts of matter and antimatter would precisely offset any attraction.
CERN physicist Dragan Slavkov Hajdukovic has proposed an explanation for the problem of galaxy rotational speeds (currently explained by dark matter models) based on antimatter antigravity. Assuming that a particle and its antiparticle have the gravitational charge of the opposite sign, the physical vacuum may be considered as a fluid of virtual gravitational dipoles. Following this hypothesis, he presents indications that dark matter may not exist at all and that the phenomena for which it was invoked might be explained by the gravitational polarization of the quantum vacuum by the known baryonic matter.
Several authors have pursued the idea of antigravity, either assuming that antimatter has negative gravitational mass, and thus is self-attractive, or that it is even self-repulsive.
In a recent paper, Villata argued that there is no need to change the sign of the gravitational mass of antimatter (which would represent a violation of the weak equivalence principle) to get repulsion between matter and antimatter; but he showed that antigravity appears as a prediction of general relativity, once it is assumed that this theory is CPT invariant and that, consequently, matter is transformed into antimatter by these three joint operations (charge conjugation, parity, and time reversal). In general relativity, the equation of motion for a matter test particle in a matter-generated gravitational field is composed of four elements. If we CPT-transform all the four elements, we get an identical equation describing the motion of an antimatter test particle in an antimatter-generated gravity field, since all the four changes of sign cancel one another. Thus, this CPT symmetry ensures the same self-attractive gravitational behavior for both matter and antimatter. But, if we transform only one of the two components, either the field or the particle (represented by the remaining three elements), we get a change of sign that converts the original gravitational attraction into repulsion, so that matter and antimatter repel each other. The equation for a massless particle (e.g. a photon) is formally equal to that for material particles. Therefore, a (retarded) photon will be repelled by an antimatter gravity field, as well as a CPT-transformed photon, i.e. an advanced photon, will be repelled by matter. (As a consequence, the energy of a retarded-advanced photon pair in a gravitational field would be conserved, thus invalidating the Morrison argument against antigravity.) This may provide a test for the theory of antigravity:
the presence of antimatter in cosmic voids suggested by Villata might be revealed by its gravitational effect on the radiation coming from background sources, in a sort of antigravitational lensing.>>
