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Gravitational behaviour of antimatter

  1. Jun 25, 2011 #1
    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.
     
    Last edited: Jun 25, 2011
  2. jcsd
  3. Jun 25, 2011 #2

    bcrowell

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  4. Jun 25, 2011 #3

    Bill_K

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    Antiprotons circulate in the Tevatron for ten hours or more, and have never been observed to fall up.
     
  5. Jun 25, 2011 #4

    phinds

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    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.
     
  6. Jun 25, 2011 #5
    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 earths 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?
     
    Last edited: Jun 25, 2011
  7. Jun 25, 2011 #6

    bcrowell

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    The electromagnetic forces in that situation are a gazillion times larger than the gravitational ones. There's no way to detect gravitational effects.
     
  8. Jun 25, 2011 #7
    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.
     
  9. Jun 25, 2011 #8
    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.
     
  10. Jun 25, 2011 #9

    bcrowell

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    That's 15 minutes during which they're being confined by electromagnetic fields. The electromagnetic fields might tend to mask any gravitational effect.
     
  11. Jun 25, 2011 #10

    phinds

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    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.
     
  12. Jun 25, 2011 #11
    I know. Shut them off in a small box and see what happens.
     
  13. Jun 25, 2011 #12
    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.
     
  14. Jun 25, 2011 #13
    I imagine something like this is in the works anyway.
     
  15. Jun 25, 2011 #14
    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 :biggrin:
     
    Last edited: Jun 25, 2011
  16. Jun 25, 2011 #15

    pervect

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    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.
     
  17. Jun 25, 2011 #16
    The equivalence principle is more GR flavour than Newtonian.
    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.
     
  18. Jun 25, 2011 #17
    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.
     
  19. Jun 25, 2011 #18

    turbo

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    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.
     
  20. Jun 26, 2011 #19
    Don't wanna wait. I want the correct answer yesterday, and before I'm dead and buried would be nice :) Hurry up!

    GrayGhost
     
  21. Jun 26, 2011 #20
    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.
     
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