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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?


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.

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    $\begingroup$ Are you sure that there will be an energy surplus? I think even if there were no losses, the thing would just balance. $\endgroup$ – anna v May 2 '11 at 19:09
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    $\begingroup$ @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 $\endgroup$ – Jerry Schirmer May 2 '11 at 19:34
  • $\begingroup$ 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. $\endgroup$ – Ted Bunn May 2 '11 at 20:23
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    $\begingroup$ 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. $\endgroup$ – Ted Bunn May 2 '11 at 21:14
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    $\begingroup$ @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. $\endgroup$ – Jerry Schirmer May 13 '11 at 14:56

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.

  • $\begingroup$ 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. $\endgroup$ – Ted Bunn May 2 '11 at 20:24

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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    $\begingroup$ Who tells you that? $\endgroup$ – Fabian May 2 '11 at 21:16

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