Re: Ridiculous
Experiments have tentatively shown that antimatter has anti-gravity - it runs away from matter.
They have not. Experiments are being done, but no one has been able to do experiments which give answers to any useful level of precision, or at least they have not published the results yet if they have (here is one fairly recent result).
However it would be absolutely astonishing if antimatter repelled matter gravitationally for two reasons.
Firstly if this were true then general relativity is a dead theory: not just wrong in some limit but completely and utterly wrong in every way. This is because GR describes gravitation in terms of curvature of spacetime: objects moving under gravity follow 'straight lines' – geodesics – in spacetime. Those geodesics are a property of spacetime, which means that antimatter sees the same geodesics that matter does, and moves in the same way, if GR is true. And there are now really a large number of experimental tests of GR, all of which it has passed, slightly depressingly: it would be completely astonishing if it was wrong in this trivial way.
Secondly there are particles which are their own antiparticles, and those particles are deflected by gravity in the way GR predicts them to be: they follow geodesics as GR predicts they should. So that simply makes no sense at all if antimatter feels gravity differently than matter. Those particles, of course, are photons (and the particular geodesics they follow are null geodesics).
Biting the hand that feeds IT
