What would the consequences be if it was experimentally shown that antimatter bends spacetime opposite from matter?

Question

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.

Answer 1

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.

Answer 2

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!)

Answer 3

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.