The gravitational interaction of 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 matter attracts matter (and antimatter), there is a strong desire to confirm this experimentally. For example, if gravitational interactions between antimatter and matter were found to be repulsive then there would potentially be a violation of conservation of energy
, one of the most fundamental laws of physics (and allow for the possibility of antigravity
Antimatter's rarity and tendency to annihilate when brought into contact with matter makes its study a technically demanding task. Most methods for the creation of antimatter (specifically antihydrogen) result in high energy atoms unsuitable for gravity related study. In recent years the ATHENA and ATRAP consortia have successfully created low energy antihydrogen; but observations have thus far been methodically limited to annihilation events which yield little to no gravitational data.
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:
- Normal gravity: Standard theory asserts that antimatter should fall in exactly the same manner as normal matter.
- Antigravity: Early theoretical analysis also focused on whether antimatter might instead repel with the same magnitude. This should not be confused with the many other speculative phenomena which may also be called 'antigravity'.
- Gravivector & Graviscalar: Later difficulties in creating quantum gravity theories have led to the idea that antimatter may react with a slightly different magnitude.
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 experiment, three neutrino detectors around the world simultaneously observed a cascade of neutrinos emanating from a supernova
in a nearby galaxy. 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 cannot be improved by duplicating the experiment any time soon. The last supernova
to occur at such a close range happened in 1604.
Physicist William Fairbank attempted a laboratory experiment to directly measure the gravitational acceleration of both electrons
. However, their charge-to-mass ratio
is so large that electromagnetic effects overwhelmed the experiment.
It is difficult to directly observe gravitational forces at the particle level. At these small distances, electric forces tend to overwhelm the much weaker gravitational interaction. Furthermore, antiparticles must be kept separate from their normal counterparts or they will quickly annihilate. Worse still, the methods of production of antimatter typically have very energetic results unsuitable for observations. Understandably, this has made it difficult to directly measure the gravitational reaction of antimatter.
Cold neutral hydrogen experiments
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.
The antimatter gravity debate
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.
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 then the photons yielded from annihilation. However, it was later found that antigravity would still not violate the second law of thermodynamics
The equivalence principle
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 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.
Motivations for antigravity
Supporters argue that antimatter antigravity would explain several important physics questions. Besides the already mentioned prediction of CP violation, they argue that it 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.
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.
- A.P. Mills Jr., M Leventhal, Can we measure gravitational free fall of cold Rydberg state positronium?, Nuclear Instruments and Methods in Physics Research B 192 (2002) pp102-106. (experiment proposal.)
- Thomas J. Phillips, A Technique for Directly Measuring the Gravitational Acceleration of Antihydrogen, (undated).
- Thomas J. Phillips, Antimatter Gravity Experiment, (undated). (proposal for an experiment to measure the gravitational attraction of antihydrogen using interferometric techniques.)
- Micheal Martin Nieto, T. Goldman, John D. Anderson, Eunice L. Lau, J Perez-Mercader, Theoretical Motivation for Gravitation Experments on Ultra-low Energy Antiprotons and AntiHydrogen, (1994), ArXiv hep-ph/9412234
- G. Chardin, Motivations for antigravity in General Relativity, Hyperfine Interactions, volume 109, issue 1 - 4, March 1997, pages 83 - 94.