Gravitational behaviour of antimatter

[paweld]

Are there any experimantal evidences which imply that antimatter interacts gravitationally
in exactly the same way as matter.

I found one argument in "Feynamn lectures on gravitation":
Let's consider correction to binding energy of an electron in a atom coming from
vacuum polarization. Since this correction involves pairs of particles and antiparticles
if antimater gravitational interaction was different then matter interaction then the
ratio of gravitational to inertial mass of different substances would be slightly different.
Since we don't meassure any differences the matter and antimatter interacts gravitationally
in the same way.

I consider this type of argumentation as vague. I don't know whether we can treat
virtual particles as real (the paris are in this case virtual). Are there any direct evidences
of gravitational interaction of antimatter.

 

[bcrowell]

http://en.wikipedia.org/wiki/Gravitational_interaction_of_antimatter

 

[Bill_K]

Antiprotons circulate in the Tevatron for ten hours or more, and have never been observed to fall up.

 

[phinds]

Although, as the wiki article states, it's inconclusive and under study (not controversial, just not evidenced yet) perhaps the very recent achievement of maintaining anti-matter for something like 15 minutes (considered rather astonishing relative to all previous records) will afford some basis for experimental verification.

 

[TurtleMeister]

Wouldn't it be true that even if an anti particle had an anti gravitational field, that the particle would still fall down in the Earth's gravitational field? Since the gravitational charge (active gravitational mass) of a single particle would be so much weaker than the field generated by the earth, wouldn't the difference in acceleration between the normal particle and the anti particle be undetectable?

 

[bcrowell]

Antiprotons circulate in the Tevatron for ten hours or more, and have never been observed to fall up.

The electromagnetic forces in that situation are a gazillion times larger than the gravitational ones. There's no way to detect gravitational effects.

 

[Sam Gralla]

There isn't any experimental evidence that moons made of green cheese gravitate normally, either. We only have direct evidence that moons made of normal moon-matter behave normally. Therefore, it remains possible that green-cheese-moons in fact anti-gravitate.

 

[danR]

Although, as the wiki article states, it's inconclusive and under study (not controversial, just not evidenced yet) perhaps the very recent achievement of maintaining anti-matter for something like 15 minutes (considered rather astonishing relative to all previous records) will afford some basis for experimental verification.

15 minutes is sure a long time if you were to put them in a small container and see whether the top or bottom emitted more annihilation photons.

 

[bcrowell]

15 minutes is sure a long time if you were to put them in a small container and see whether the top or bottom emitted more annihilation photons.

That's 15 minutes during which they're being confined by electromagnetic fields. The electromagnetic fields might tend to mask any gravitational effect.

 

[phinds]

That's 15 minutes during which they're being confined by electromagnetic fields. The electromagnetic fields might tend to mask any gravitational effect.

Yeah, I wasn't sure about that, but figured that would likely be a problem. Still, it's very neat that the stuff has been held for such a relatively long time.

 

[danR]

That's 15 minutes during which they're being confined by electromagnetic fields. The electromagnetic fields might tend to mask any gravitational effect.

I know. Shut them off in a small box and see what happens.

 

[danR]

Yeah, I wasn't sure about that, but figured that would likely be a problem. Still, it's very neat that the stuff has been held for such a relatively long time.

Containment is what we don't want. Shut off the fields around a small box and surround it with detectors and see where most of the photons come from. Is there a statistical bias? The experiment might have to be run many times. A superlative vacuum would be good.

 

[danR]

I imagine something like this is in the works anyway.

 

[yuiop]

Are there any experimantal evidences which imply that antimatter interacts gravitationally in exactly the same way as matter.

I found one argument in "Feynamn lectures on gravitation":
Let's consider correction to binding energy of an electron in a atom coming from
vacuum polarization. Since this correction involves pairs of particles and antiparticles
if antimater gravitational interaction was different then matter interaction then the
ratio of gravitational to inertial mass of different substances would be slightly different.
Since we don't meassure any differences the matter and antimatter interacts gravitationally
in the same way.

I consider this type of argumentation as vague.

One aspect not covered here in any detail are the differences between passive and active gravitational mass. Using the equivalence principle we can deduce that GR predicts that positive mass, zero mass and negative mass all fall downward towards a normal mass with identical acceleration. For example let us say we have an inertial test mass inside and near the top of a non accelerating rocket that is drifting in space. When the rocket accelerates, the test mass remains stationary but to an observer inside the rocket the test mass seems to accelerate or gravitate towards the bottom of the rocket. The same result will occur for a photon (which has zero rest mass) going across the rocket or for a negative test mass (if such a thing exists). Therefore the equivalence principle implies that all masses (positive or negative) will accelerate towards a positive gravitational mass.

On the other hand, the acceleration of the rocket is equivalent to and proportional to the active gravitational mass of the gravitational source. This implies that if we had a large negative active gravitational mass that the equivalent would be a rocket accelerating backward, (from nose towards tail) and all test masses (+tive, zero or -tive) will accelerate away from a negative active gravitational mass.

Now just about everything I have read suggests that anti matter has positive rest mass just like regular matter, but since they almost never make any distinction between active and passive gravitational mass, I would say it still an open question.

So IF antimatter has negative gravitational mass (not sure if it does) then the equivalence principle predicts that a large antimatter gravitational source would repel all other matter equally, whether the other masses are normal matter or anti-matter. Now the problem is that this in turns implies that you cannot have a large antimatter source because it would not clump together like normal matter, because a cloud of negative matter is self repelling.

Just my amateurish thoughts on the matter

 

[pervect]

GR makes some well-known predictions that seem very likely to be true. One of them is that a black hole made out of anti-matter would have the same gravity as one made out of normal matter.

So your thoughts on the matter aren't GR thoughts. They seem to have a mostly Newtonian flavor to them, though that's a rather general observation.

Gravitationally repulsive material is possible, but would fall in the classification of "exotic matter". Antimatter is not belived to be exotic matter, but dark energy (if you don't mind including energy in the term matter) would be belived to be exotic.

 

[yuiop]

So your thoughts on the matter aren't GR thoughts. They seem to have a mostly Newtonian flavor to them, though that's a rather general observation.

The equivalence principle is more GR flavour than Newtonian.

Gravitationally repulsive material is possible, but would fall in the classification of "exotic matter". Antimatter is not belived to be exotic matter, but dark energy (if you don't mind including energy in the term matter) would be belived to be exotic.

I am suggesting that the equivalence principle predicts that negative test mass would fall the same as positive test mass relative to a positive mass source and that a negative mass source would repel positive and negative masses equally.

However, I am talking about negative active gravitational mass and I acknowledge that anti-matter does not necessarily have negative active gravitational mass.

 

[danR]

... Using the equivalence principle we can deduce that GR predicts that positive mass, zero mass and negative mass all fall downward towards a normal mass with identical acceleration. For example let us say we have an inertial test mass inside and near the top of a non accelerating rocket that is drifting in space. ..

This makes me think of a second experiment to the one above: put the small container in a centrifuge and this time leave the field trap on (but somewhat weak). The H- should 'fall' normally to the 'bottom' of the container. A more sophisticated geometry of photon detectors would be necessary, but the results should come in much faster.

 

[turbo]

There are experiments that aim to cool neutral anti-hydrogen at CERN and probe that spectroscopically. It's going to take a while. We are getting closer to some answers, but we have to wait.

 

[GrayGhost]

There are experiments that aim to cool neutral anti-hydrogen at CERN and probe that spectroscopically. It's going to take a while. We are getting closer to some answers, but we have to wait.

Don't want to wait. I want the correct answer yesterday, and before I'm dead and buried would be nice :) Hurry up!

GrayGhost

 

[danR]

There are experiments that aim to cool neutral anti-hydrogen at CERN and probe that spectroscopically. It's going to take a while. We are getting closer to some answers, but we have to wait.

I thought the H- was already very cool. Why not just turn off the magnetic field and let them fall or rise to the top or bottom of the vessel and see where the most annihilation photons come from? I mean falling down is a big part of gravity's effect.
________________

'To simplicate, add lightness.' --Murphy's third minus one corollary of parsimonious design.

 

[danR]

This experiment is designed to determine exactly what antimatter does in gravity:

http://aegis.web.cern.ch/aegis/home.html

I think they are doing it the hard way, however.

 

[TurtleMeister]

This experiment is designed to determine exactly what antimatter does in gravity:

http://aegis.web.cern.ch/aegis/home.html

I think they are doing it the hard way, however.

Amazing. Anyone care to wager that there will be no detectable difference in the measured value of g?

 

[turbo]

Amazing. Anyone care to wager that there will be no detectable difference in the measured value of g?

Or in inertia? There are possibilities beyond the purely gravitational.

 

[PAllen]

Or in inertia? There are possibilities beyond the purely gravitational.

Inertia is already known. Proton /anti proton colliders would detect even the slightest difference in inertial (different fields would be needed to hold the different particles in a ring).

 

[turbo]

Inertia is already known. Proton /anti proton colliders would detect even the slightest difference in inertial (different fields would be needed to hold the different particles in a ring).

I will suspend judgement on that one. Magnetic fields are many orders of magnitude more powerful than gravitational fields, and swamp them. Let's see what comes out of CERN.

 

[PAllen]

I will suspend judgement on that one. Magnetic fields are many orders of magnitude more powerful than gravitational fields, and swamp them. Let's see what comes out of CERN.

Irrelevant. Inertial mass is *defined* by the relation between non-gravitational force and acceleration. The higher the inertial mass, the more force is needed to produce a given acceleration. If there were the slightest difference in inertial mass between protons and anti-protons, colliders like tevatron would detect it.

 

[turbo]

Irrelevant. Inertial mass is *defined* by the relation between non-gravitational force and acceleration. The higher the inertial mass, the more force is needed to produce a given acceleration. If there were the slightest difference in inertial mass between protons and anti-protons, colliders like tevatron would detect it.

If that was already settled and accepted, we wouldn't have any really expensive experiments to probe equivilancy, would we?

 

[PAllen]

If that was already settled and accepted, we wouldn't have any really expensive experiments to probe equivilancy, would we?

No. What isn't settled is whether antimatter has the same inertial mass but opposite (or different) 'gravitational charge'. No one believes this is possible, but it is certainly worth testing. For normal matter, all three of the following are known to be equivalent to a high precision:

1) inertial mass
2) 'gravitational charge' (how it responds to gravity)
3) gravitational mass (strength of gravity produced).

(Normally, we assume 2 and 3 are the same, but one could conceive of theories where they are different).

For anti-matter, only (1) is directly measured to high precision.

 

[danR]

Inertia is already known. Proton /anti proton colliders would detect even the slightest difference in inertial (different fields would be needed to hold the different particles in a ring).

Ha. Good one. Therefore, I will wager \bar{H} falls. Down.

 

[TurtleMeister]

1) inertial mass
2) 'gravitational charge' (how it responds to gravity)
3) gravitational mass (strength of gravity produced).

I think you have that wrong. Gravitational charge is equivalent to active gravitational mass, which relates to the strength of the gravitational field produced by a body. Passive gravitational mass relates to how a body responds to a gravitational field produced by another body.

Ha. Good one. Therefore, I will wager \bar{H} falls. Down.

The wager is whether the measured value of g in the experiment is the same as the value we are all familiar with.