Antimatter isn't bound by anti-energy, so doesn't that mean that even if elementary antimatter particles had negative mass, the total mass of an anti-atom would still be overwhelmingly positive?
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$\begingroup$ You are looking at this from the benefit of hindsight, of modern instruction. Back when all of these knowledge were newly minted, people did not know that almost all masses are due to the energetic binding of gluons and quarks. They simply looked at $\vec F=-G\frac{Mm}{r^2}\hat{\vec r}$ and saw that if $Mm<0$, then they would repel. Nowadays, we know that both $M$ and $m$ are really $E/c^2$ and both matter and antimatter have positive energies. $\endgroup$– naturallyInconsistentCommented Jan 18 at 11:32
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$\begingroup$ More on negative mass of antimatter. $\endgroup$– Qmechanic ♦Commented Jan 18 at 11:46
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$\begingroup$ @naturallyInconsistent but this experiment was performed recently and the paper was published in 2023. Shouldn't they all have the same benefit of hindsight? $\endgroup$– Vilim LendvajCommented Jan 18 at 12:32
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2$\begingroup$ Relying solely on hindsight about accepted physics to determine if an experiment should be done pretty much rules out finding new physics. The fully expected answer is that anti-hydrogen would behave the same as hydrogen. If it hadn't, well, that would have been really exciting. As an old advisor of mine said, "If you know the result of an experiment, you'd better do it." $\endgroup$– Jon CusterCommented Jan 18 at 13:40
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$\begingroup$ @JonCuster That's a very valid point, but it doesn't change the fact that the experiment was mischaracterized. It's not true that antiatoms would fall up if it elementary antiparticles had negative gravitational mass. Something would have to be much much more wrong with our understanding of the universe to make antiatoms fall up. There was some doubt that elementary antiparticles might have negative gravitational mass, and they still might, since this experiment proved nothing. I don't think anyone has even prepared a candidate theory that could explain antiatoms actually falling upwards. $\endgroup$– Vilim LendvajCommented Jan 18 at 14:24
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Your point about binding energy is valid, and this is one reason why almost no physicist thought that anti-hydrogen would fall up. We just wanted to check, however, since the most exciting thing for a physicist is to discover that their expectations are wrong. The ultimate goal of anti-hydrogen gravity experiments, however, is to test if anti-hydrogen and hydrogen fall at very slightly different speeds, which is more plausible.
Your argument about binding energy is similar to one that Schiff made in 1959 and which has been discussed in many subsequent papers, most recently by Caldwell and Dvali. Binding energy is just one reason why it was very difficult to come up with a serious theory in which hydrogen would fall up, so the consensus expectation for many decades has been that anti-hydrogen would fall down. Nevertheless, there have been a few "antigravity" ideas. Several are listed on Wikipedia, and here are a few more:
- "Gravity, antimatter and the Dirac-Milne universe"
- "Antimatter gravity and the Universe"
- "Gravitational matter-antimatter impact interactions"
None of these papers, however, were strong refutations against the expectation that antiprotons fall down.
What is possible, however, is that antimatter might fall at a very slightly different speed, as reviewed recently in "Testing Fundamental Physics in Antihydrogen Experiments". A long-range force that did not couple to gluons but did couple oppositely to quarks and antiquarks could produce a difference in the gravity felt by protons and anti-protons of $\Delta g/g \sim 10^{-2}$ For example, there might be a weak long-range "Fifth Force" coupling to B-L (where B & L are baryon and lepton number). Unfortunately, existing experimental tests of the Weak Equivalence Principle almost certainly already imply that $\Delta g/g \lesssim 10^{-7}$.
answered Jan 18 at 19:20