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This is inspired by another question I found on here about antimatter bending spacetime the "other way." The answers say that physicists believe antimatter will act the same as regular matter in terms of gravitation, but this has no been experimentally shown yet. If it were shown that this is not the case--that antimatter and matter do behave differently in terms of gravitation--what would the implications be, and would any currently prevailing theories need to be amended to reflect it? After all, there must be a reason physicists believe that gravitation is the same for both yet their charges (and other features I think) are opposite.

Qmechanic
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user28375028
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  • Understand that there is no antimatter yet, you are talking about anti-charged stuff. REAL antimatter is where m -> (-m). In other words REAL antimatter planet will repel from usual matter planet. Anti matter has negative mass. – Asphir Dom Jul 21 '15 at 14:03
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    We've measured the response of antimatter to an external gravity field. It falls down at exactly the same rate as ordinary matter. – Zo the Relativist Jul 21 '15 at 14:03
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    @AsphirDom: it's pretty well established what antimatter is in particle physics. – Zo the Relativist Jul 21 '15 at 14:04
  • @JerrySchirmer it is wrong naming. I will fix that. – Asphir Dom Jul 21 '15 at 14:05
  • Related: http://physics.stackexchange.com/q/9371/2451 , http://physics.stackexchange.com/q/9375/2451 , http://physics.stackexchange.com/q/83307/2451 and links therein. – Qmechanic Nov 28 '16 at 14:26
  • @JerrySchirmer: have we measured it? I knew experiments were being attempted but not that any good results had been found. (FWIW I would be deeply astonished if it turned out that AM didn't behave under gravity as M does!) –  Nov 28 '16 at 15:58
  • @tfb: thanks to Partons, we know that different atoms are made up of different matter/antimatter ratios, and with that, we can then set up an Eötvos experiment to measure the relative gravitational force on these different objects, and the result is a null experiment. You can also do these neutron diffraction experiments (where you redshift neutrons in a gravitational field) with anti-neutrons, and this also gives the result you would expect if antineutrons fell down. – Zo the Relativist Nov 28 '16 at 17:48

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We know that antimatter has positive energy, because in particle experiments (see pair production) the created antiparticles carry positive energy away from the interaction. Also in annihilation the particle and antiparticle energy both convert positively into the outgoing products.

Positive energy gets to the stress-energy tensor of General Relativity as the source of gravity. If the experiments would show that antiparticles bend spacetime negatively, we would have to change the way we build the stress-energy tensor. Antimatter would have to be included with negative sign, even though its energy is positive. That would be a monstrous problem.

mpv
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I think the results would be extremely dramatic.

In particular, there are particles which are their own antiparticles: photons for example. But we know that EM fields contribute to the RHS of the field equations of GR and photons are the quanta of EM fields.

So if antiparticles behave differently than particles in GR, then either particle physics (or QFT?) fails and photons are not in fact their own antiparticles, or GR fails.

(Or, of course, neither fail, because antimatter behaves the same way as matter gravitationally: that's why it's so important to do the experiments!)

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Surprisingly little which is currently established would change: our current model of particle physics, the Standard Model, does not deal with gravity; our current model of gravity, General Relativity, does not deal with quantum things.

A bunch of things which are not currently established could change dramatically, but we can expect that these will be confined to mostly those things which try to connect gravity and particle physics (loop quantum gravity, string theory, grand unified theories). Some of the fields which are informed by those theories, like models of what happened during the Big Bang, might also need to be updated.

CR Drost
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    I disagree with this pretty strongly. If antimatter gravitates negatively, it is basically negative energy in Einstein's equation. Unless there's a quantum proof tht bulk antimatter is impossible (and I don't think that there is, CP not being an exact symmetry aside), you've now found a way to violate the central assumption necessary to derive most of the consistency arguments in GR. Also, there's a simple thought experiment to show that if antimatter antigravitates, then you can build a perpetual motion machine. – Zo the Relativist Jul 21 '15 at 14:02
  • Those are both really interesting. I don't know what the "simple thought experiment" is so I suppose it has to be something about blueshifting: you pair-produce an electron and positron at height $h$, let the positron go to $h + \delta h$, annihilate it with an electron at that height, redirect the photons back down to $h$, split the energy perfectly to kick the pair electron up to $h + \delta h$ to "replace" the other one, and I guess you're saying that the light which fell into the gravity well is slightly blueshifted, so it has more energy total than the incident photon? – CR Drost Jul 21 '15 at 15:24
  • I guess the only way that I see to get around any of that would be to have an "antigrav photon" which has all of the same properties as the photon for the purposes of the Standard Model but which redshifts rather than blueshifts. Since annihilation always produces a pair of gammas to conserve momentum in the rest frame anyway, you'd have to say that one of these photons is "normal" and one is "antigrav". – CR Drost Jul 21 '15 at 15:39
  • at that point, you're no longer talking about the standard model. In the standard model, the photon is its own antiparticle. And yes, that's the thought experiment. – Zo the Relativist Jul 21 '15 at 17:16