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."

Comments

It will fall down

[little1973]

According to GR, gravity is the curvature of Space-Time. As the anti-matter moves through space it has to follow this curvature. If it does not, that means GR is wrong (which may be the case, but I doubt this experiment will disprove GR).

Re:It will fall down

[Anonymous Coward]

bollocks. having a faster-than-escape velocity doesn't make things fall upwards. it merely means they don't come down fast enough to hit the earth

Re:It will fall down

[something_wicked_thi]

Oh, dear. I think you should write these guys a letter before they waste all this time and money on an experiment with such an obvious flaw!

Re:It will fall down

[fireboy1919]

Didn't you have to take high school chemistry? What you're saying has quite a few problems:

1) That determines the rate of effusion. Molecules don't just go straight up. They bounce around. What actually happens in reality is that the force holding those molecules to the earth isn't actually enough to force it to happen. But the upward movement is going to happen slowly. You can still measure the effect that gravity has while this upward movement happens.

2) Also because it has to do with effusion, a *beam* of antiparticles in a vaccuum won't be affected by it. They're not going to bounce around and have effusion effects happen; it's going to be more like a batting practice machine - balls come out and curve, and are done the moment they hit something. This is obviously what they're going to do since antimatter is quickly eliminated in the presence of matter.

3) Even if that was a problem, it's not actually a problem at all temperatures and pressures. If you wanted to do an experiment where *normal* hydrogen didn't rise, just lower the temperature.

Re:It will fall down

[Anonymous Coward]

I'm a physics PhD and this is definitely one of the experiments where there is no reason to expect things to behave differently from the theory. Any reasonable theory already allows us to put low limits on the difference in gravitational behaviour between matter and anti matter and there certainly is no theory of gravity that I know of where antimatter "falls up". There are some where it might fall differently.

There also is some direct evidence that if you have differences they are not due to gravity:

Reference e.g.:

http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/antimatter_fall.html

"The only direct experimental result on antimatter and gravity comes from Supernova 1987A. This supernova in the Large Magellanic Cloud emitted both neutrinos and antineutrinos, some of which were eventually detected on Earth. Those neutrinos and antineutrinos took 160,000 years to reach Earth, and while travelling were bent from a "straight line" path by the gravity from our own galaxy. The bending with gravity changed the time needed to reach Earth by about 5 months, yet both the neutrinos and the antineutrinos reached Earth at roughly the same time (within the same 12 second interval). This shows that the neutrinos and antineutrinos "fell" similarly, to a very high level of precision (about 1 part in a million). [4] and [5] provide some background information on this."

Re:Confused

[dnwq]

An experiment that reduces doubt. Does that work for you?

No Loopholes in General Relativity

[mbone]

There are enough loopholes in the general theory of relativity to allow antimatter to fall up rather than down in a gravitational field.

Uh, no there are not. Gravity (or geometry, same thing in the theory) depends on mass energy in General Relativity. Stuff (with mass energy) follows the metric (the local geodesic). Even photons (which are their own anti-particles) follow the geodesic - and that has certainly been tested. Equivalence principle tests also show that different sorts of nuclear matter (including neutrons) individually follow the geodesic. Anti-matter certainly has mass energy, and (with matter) can be converted to photons and is no different in the theory. In other words "there is only one type of geodesics and there are no antigeodesics for antimatter."

The original article talks about "flavors" of General Relativity. Ain't so such beasties. Period. If you go to the real original article, you find a proposal for a 1% test of the equivalence principle for antimatter, and no such claims of flavors. Now, the equivalence principle has been tested to better than parts per trillion, and part of the mass energy in ordinary matter is made up of antimatter (in virtual particle pairs), so (based on the experimental evidence) I would claim that this test will be negative and is not actually that interesting as new physics. (The articles say that these older tests are "model dependent," but they are not model dependent enough to matter for this.)

That doesn't mean that this shouldn't be done (everything should be tested in physics, and different tests are always useful), but the prediction of General Relativity is clear : if anti-matter has anti-gravity, then General Relativity is wrong. The experimental evidence is also clear : this isn't going to be accurate enough to matter. Will make for some good public relations, though.

Re:Gravitons

[rumith]

I think that the parent means that neutral bosons have no antiparticles. While neutron is a barion and thus has an antiparticle composed of (~u ~u ~d) quarks, neutral bosons (such as photon and Z0) do not have corresponding antiparticles.

Re:Confused

[courtarro]

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.

Holes are a virtual particle with no mass, whereas anti-matter has mass. Electrons actually exist, but holes are simply a place where an electron can fit. Take the example of a helium balloon in a car: if you slam on the breaks, it flies to the back of the car because all the air is rushing forward, and the balloon's relative vacuum gets pushed backward. A hole behaves similarly. Antimatter, on the other hand, has mass just like its matter counterpart and therefore isn't directly comparable to a hole.

Re:Confused

[mshannon78660]

Uh, no, actually, they don't. Because F=Gmm'/r (m being the mass of one object, and m' the other, G being the gravitational constant and r being the distance between them), and a=F/m (acceleration equals force divided by mass). When you substitute one for the other, you get a=Gm'/r - which is completely independent of the mass of the object being observed. Now, you could argue that (as you seem to) that the earth is also accelerating towards the other mass, leading to what seems to be a higher rate of falling - but (contrary beliefs about relativity aside), you can't really treat all of the acceleration as relative in that manner - that simplification of relativity only works for uniform motion - it is possible to detect acceleration (although you can also replace it with a gravitational field, but that would then invalidate our original formulas - can't change horses in mid-stream like that).