Terralthra wrote:Isn't it up for debate whether the gravitational effects of antimatter are truly identical to matter? CPT predicts that antimatter will attract other antimatter in the same way matter attracts matter, but whether that carries over to matter/antimatter gravitational interaction is unclear. As far as I know, no experiment has been able to detect gravitational force from antimatter due to sensitivity of the small amounts of antimatter created. It would appear to be equally likely that there is a positive/negative-type interaction between the two, thus while you could have, so to speak, anti-stars and anti-planets and anti-nebulae, they would repulse normal matter gravitationally, neatly explaining why we don't see evidence of antimatter stellar phenomena - they behave just like normal matter phenomena until and unless they interact, which they wouldn't on any large scale due to gravitic repulsion.
This kind of thing runs right into the equivalence principle. It is known, for example, that magnetic fields affect antimatter exactly as if the only difference between it and regular matter is its electric charge. Since the only other difference between matter and antimatter is its spin (applying the CPT operator to a particle transforms it into its antiparticle and flips the spin state), this means that the inertial mass of matter and antimatter is identical. By the equivalence principle, this also means that the gravitational masses of antimatter and normal matter are identical. If antimatter and matter mass acted as different gravitational charges, this would break the equivalence principle and tear down GR with it. Quantum gravity may still do this in whatever form it takes, and it will certainly destroy GR on small scales, but I doubt a wholesale violation of the equivalence principle will end up coming out of it.
Sorry for the necro, but it looks as if scientists at CERN are actively studying this, arguing (sanely) that weak equivalence by itself doesn't allow us to rule out antigravity, since there's still a lot unexplained in the universe.
Description and first application of a new technique to measure the gravitational mass of antihydrogen wrote:There are many compelling experimental and theoretical arguments that suggest that the gravitational mass of antimatter cannot differ from the gravitational or inertial mass of normal matter, that is, that the weak equivalence principle holds. For instance, one such argument comes from the absence of anomalies in Eötvös experiments conducted with differing atoms4; the differing number of virtual particle–antiparticle pairs in such atoms might have caused gravitational anomalies to occur. However, all of these arguments are indirect and are not universally accepted; they rely on assumptions about the gravitational interactions of virtual antimatter, on postulates such as CPT invariance, or on other theoretical premises. Although these arguments may well be correct, in a world in which physicists have only recently discovered that we cannot account for most of the matter and energy in the universe, it would be presumptuous to categorically assert that the gravitational mass of antimatter necessarily equals its inertial mass. Moreover, the baryogenesis problem suggests that our understanding of antimatter is incomplete; gravitational asymmetries have been proposed as an explanation (Note that ref. 7 ultimately rejected gravity as a solution to the baryogenesis problem because of a thermodynamic proof of the weak equivalence principle. This proof was later challenged10.)
Their results? Inconclusive as yet, but they can't rule it out.
