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The gravitational interaction of antimatter (with Matter or Antimatter ) have not been conclusively observed by physicists. While the overwhelming consensus (amongst physicists) is that antimatter will attract both matter and antimatter at the same rate matter attracts matter, there is a strong desire to confirm this experimentally. If the '''gravitational interactions between antimatter and antimatter''' were found to be Repulsive it would be a clear violation of CPT Symmetry ; and if the '''gravitational interactions between antimatter and matter''' were found to be repulsive it would be a potential violation of Conservation Of Energy – arguably the most fundamental law of physics. The rarity of antimatter, and its tendency towards Annihilation (when brought into contact with matter), makes the study of antimatter a difficult task. Most methods for the creation of antimatter (specifically Antihydrogen ) result in high-energy atoms not well suited to Gravity related study. In recent years the ATHENA and ATRAP consortiums have successfully created low-energy antihydrogen; but observations have thus far been methodically limited to annihilation events which yield little to no gravitational data. THREE THEORIES 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:
EXPERIMENT Supernova 1987A 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 eminating 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. Fairbank's experiments Physicist William Fairbank attempted a laboratory experiment to directly measure the gravitational acceleration of both Electron s and Positron s. 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. In recent years, the production of cold Antihydrogen , which is electrically neutral, has renewed the hope of possibly measuring the gravitational acceleration of antimatter. 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 Eotvos test of the Weak Equivalence Principle . Many of these early theoretical objections were later overturned. MORRISON'S ARGUMENT In 1958, P. 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 of light is shifted. Thus Morrison showed how a process could be constructed which would manufacture energy from the vacuum. However, it was later found that antigravity would still not violate the Second Law Of Thermodynamics . SCHIFF'S ARGUMENT Later in 1958, L. Schiff used quantum field theory to argue that antigravity would be inconsistent with the results of the Eotvos experiment. However, the renormalization technique used in Schiff's analysis is heavily criticized, and his work is seen as inconclusive. GOOD'S ARGUMENT 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 Kaon s. 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 infact 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 deccelerate and eventually contract into a Big Crunch . Using Redshift observations, astronomers and physicists estimate that the size of the universe is instead 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. FOOTNOTES |
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