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Why would Antimatter behave differently via Gravity?

Regarding the following statement in this article:

Most important of these is whether ordinary gravity attracts or repels antimatter. In other words, does antihydrogen fall up or down?

Is this a seriously considered hypothesis? What would be the consequences on general relativity?

If this is seriously studied, can you point to some not-too-cryptic studies on the (anti ;-)matter?

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Sklivvz
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  • I want to give this link for experiment running at CERN to test the behavior of antinmatter to gravity https://home.cern/news/news/experiments/new-antimatter-gravity-experiments-begin-cern . – anna v Dec 07 '19 at 07:00

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EDIT: fixed thought experiment.

If antimatter anti-gravitates, it would be possible to build a perpetual motion machine:

1) Start with a zero-net momentum pair of photons. Have them collide, and make a particle/antiparticle pair. These can be moved around arbitrarily with zero net work, since any gravitational force on the particle will be equal and opposite to the force on the antiparticle.

2) raise them to an arbitrary height. Let them collide, producing a massive particle that is its own antiparticle, say a Higgs.

3) let this particle fall. It gains energy

4) let the particle decay upon falling. it now has the rest energy of the particle/antiparticle pair (equal to the energy of the two photons), plus any gravitational potential from falling.

5) reflect the two photons that are the decay product on antipodal mirrors attached by a wire and suspended from the ceiling. No net momentum or energy is transferred to this mirror system, and the photons can be freely merged again to repeat the process.

Zo the Relativist
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    Are you sure that there will be an energy surplus? I think even if there were no losses, the thing would just balance. – anna v May 02 '11 at 19:09
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    @anna: it gains energy on the way up (antigravitating particle accelerates up) and gains energy on the way down (photons blueshift in a gravitational field), so you'd get the rest mass of the antimatter + blueshift energy + gravitational potential energy of the antigravitating material – Zo the Relativist May 02 '11 at 19:34
  • But the end result is a downward flow of ordinary matter -- it's annihilated up high and created down below. The overall energy balance isn't completely obvious to me. – Ted Bunn May 02 '11 at 20:23
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    Thinking about it a bit more, I think you're right. If there were no gravitational blueshift of the photons on their way down, I think you'd have energy balance: the energy gained by this mechanism would be exactly the same as if you just dropped an equivalent amount of mass. But the extra energy release from the blueshift gives you something for nothing. – Ted Bunn May 02 '11 at 21:14
  • this might seem an extremely silly question to ask - but have we confirmed that both photons emitted by pair annihilation blueshift instead of redshifting when going down a gravitational potential well? - do we have an obvious violation of an observational fact if that would be the case? – lurscher May 03 '11 at 16:57
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    @lurscher: if that were the case, you'd have to have some sort of internal quantum number to the photon that tracked the fact that the photon came from pair annhillation, and not, say, brehmstrallung. To current experimental knowledge, all photons can be labeled by their polarization and their momentum, without any knowledge of their source. – Zo the Relativist May 13 '11 at 14:56
  • thanks @Jerry! concise answer; then it would affect the degeneracy of the photon states, changing the entropy of photon gases, which would represent the conflict with experimental data – lurscher May 13 '11 at 15:52
  • I've been recently trying to find a reference for experimental data regarding photon gas degeneracy. Theoretically is expected to be 2 for two polarizations, but haven't found any paper comparing the equilibrium properties in an experiment. Do you what I should look? – lurscher Sep 21 '14 at 05:38
  • Photons from the annihilation should preserve the momentum of the antimatter before annihilation, hence when one tries to focus them back (e.g. using a mirror at the top), they would transfer their doubled momenta to the mirror (which is a part of your setup). On the other hand, the antimatter and the accelerator repel each other, so the latter gains momentum in the opposite direction and moves a bit. This movement is later compensated twice by the momentum taken from the reflected photons, then followed by their "blue-shifted" momentum when they reach the bottom. Now, the blue-shift is... – gox Aug 13 '15 at 23:36
  • ...exactly compensated by movement of the accelerator. In the end, energy and momentum are most probably conserved, thus no energy gain $-$ it's more like photons in a box. Furthermore, the pool of matter at the top is not infinite etc. – gox Aug 13 '15 at 23:36
  • @gox: see edit. I fixed this. – Zo the Relativist Aug 14 '15 at 04:40
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I can't guarantee the authenticity of the article. But it seems to me quite bizarre since I fail to see how something (even antimatter) can behave differently than matter in a gravitational field without violating the equivalence principle.

A positron for example is a hole in the Dirac sea and it has the same mass as of the electron and behaves exactly similar to an electron in a gravitational field.

The only repulsive gravity that exists in GR is the cosmological constant in the Einstein's equations which is in no way related to antimatter.

  • Certainly if the equivalence principle is true, then antimatter falls down. But it's always possible that the equivalence principle isn't true! I'm not saying that's likely, but presumably people who talk about testing whether antimatter falls up or down are talking about testing the equivalence principle. – Ted Bunn May 02 '11 at 20:24
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The only problem with this is that antimatter has the same mass as its matter counterpart. So this means that is effected the same way by gravity as normal matter.

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