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Does Antimatter Fall Up Or Down?
KentuckyFC writes "There are enough loopholes in the general theory of relativity to allow antimatter to fall up rather than down in a gravitational field. We've never been able to make enough of the stuff to do the experiment. But at the European particle physics laboratory at CERN, where scientists have been refining the technique for making antihydrogen, researchers are designing an experiment called AEGIS that will finally settle the matter. The idea is simple — fire a beam of antihydrogen atoms and watch which way they fall — but the details are fiendish (abstract). The answer should help solve a number of important conundrums such as why there is so little antimatter in our part of the universe and what the value of the cosmological constant is."
how fast things can change:
Gravity affects matter and antimatter the same way because gravity is not a charged property and a matter particle has the same mass as its antiparticle.
So the above is no longer believed to be true?
As I rememeber, this has already been tested by drifting positrons down the length of the SLAC accelerator tube and measuring the beam deflection due to gravity (at least 20+ years ago).
Yes, anti-matter does fall down just like matter.
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But how does antimatter react to curved spacetime (could it 'roll uphill')...
That's what the experiment in the article is testing. Does antimatter react the same way to an external gravitational field as normal matter, or oppositely?
It'd be Big News if it turned out to be oppositely, though. General relativity describes gravitation in terms of space-time curvature; particles under the influence of gravity alone move along geodesics which only depend on their initial position and velocity. There isn't any way to accomodate different particles feeling gravitation differently in that framework. There are generalizations like Einstein-Cartan gravity to accomodate spin, but that just allows the connection to have an antisymmetric part, and doesn't change the fact that there's only one curvature for every particle to feel. The key axiom of GR is the equivalence principle, which states that, locally, there is no observable difference between gravity an accelerated reference frame. This requires that gravity accelerate every particle by the same amount, independent of any other particle-specific variables.
Put briefly, this has never been tested before, but it'd be a very big surprise if antimatter behaved any differently from normal matter, and would throw most current theories of gravitation out the window. It'd be like a modern-day Michelson-Morley experiment.
and how does antimatter (with mass) curve spacetime? (could it 'outdent' rather than 'indent' it)
That's a different question, and one that would be far more difficult to test. You'd need to gain a few dozen orders of magnitude of precision in measuring these things, or assemble a macroscopic chunk of antimatter somehow.
It'd also be a big surprise for a different reason. This is essentially treating antimatter as having negative mass and thus producing a repulsive gravitational effect. There's no deep reason why this would be mathematically inconsistent with GR, although it would have wacky consequences like perhaps the possibility of stable wormholes and FTL. In technical terms, it violates the weak energy condition. It's also unlikely for a different reason: conservation of momentum in GR requires inertial mass and gravitational mass to be equal, so for antimatter to produce a repulsive gravitational field like this would also require it to have negative intertial mass. It would respond oppositely to ordinary, non-gravitational forces, a positronium atom would have *negative* net mass (the electron and positron masses cancel, and the binding energy makes it negative), and a whole host of other consequences that would be readily observable but haven't been seen. Further, in quantum field theory having negative mass particles would create problems with vacuum stability.
So, both of these are possible in the sense that the experiment hasn't been done yet, so we don't know for sure they aren't true, but either one would invalidate huge swaths of physics and definitely qualify as Big News.
While this is a reasonable guess, it's about the same as guessing heavier objects fall faster. Consider electron-hole pairs in a silicon lattice. They act very much like electron-positron pairs. However, electrons fall down, and holes fall up. To me, it would seem odd if anti-matter fell down.
Why is there so much matter around, and no anti-matter? Perhaps because they repel each other? There is some evidence that nearby galaxies are made of matter and not anti-matter, but the universe is very big, and time could be effected differently by anti-matter gravity (speeding up). Why are galaxy clusters accelerating in their separation from each other? Could anti-matter still be present somewhere, causing the acceleration? Why is matter in the universe so clumped together, and not more uniformly spread out? Could there be clumps of antimatter between the clumps of matter?
Evidence suggests that there simply is no anti-matter left in the universe, but it's fun to speculate upon implications of anti-matter falling up.
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