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Detecting Anti-matter

  1. May 15, 2009 #1
    Suppose a galaxy out there was made entirely of anti-matter and never comes in contact with normal matter would be able to tell that it is made of anti-matter and not matter? If yes how would we do it?
     
  2. jcsd
  3. May 15, 2009 #2

    Nabeshin

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    To the best of my knowledge there would be no observable difference.
     
  4. May 16, 2009 #3

    Chronos

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    It is not impossible, but highly unlikely galaxies composed entirely of antimatter exist in the known universe. Intergalactic space is largely occupied by vast clouds of ordinary matter particles. Matter / antimatter collisions would result in very high energy gamma rays. These are not observed. See
    http://www.space.com/scienceastronomy/antimatter_sun_030929.html [Broken]
     
    Last edited by a moderator: May 4, 2017
  5. May 16, 2009 #4

    Vanadium 50

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    Chronos, there are limits to this. At sufficient distances, and we're talking at least tens of megaparsecs, this annihilation radiation is lost in the diffuse gamma ray and x-ray background. However, we also have some searches for anti-helium nuclei (produced in anti-stars) in cosmic rays to attempt to push this threshold out.
     
  6. May 16, 2009 #5

    Nabeshin

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    Is there anything distinctive about anti-helium that would make it observationally different from normal helium? It seems the crux of the OP's question is whether there's any detectable difference between matter and antimatter structures barring annihilation events.
     
  7. May 16, 2009 #6

    Vanadium 50

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    Anti-helium nuclei carry -2 units of charge rather than +2.
     
  8. May 16, 2009 #7

    Nabeshin

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    Clearly.

    But as I understand it, all atomic transitions and interactions with other antimatter (antimatter-antimatter interactions) appear exactly identical to those produced by interactions between normal matter (matter-matter interactions). So an antimatter galaxy would look to us exactly the same as a matter galaxy.
     
  9. May 16, 2009 #8

    Vanadium 50

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    Except that the cosmic rays coming from it would contain anti-nuclei. Which is what people have been and are continuing to search for.
     
  10. May 16, 2009 #9

    Nabeshin

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    Ah, I see what you were talking about now. Okay.
     
  11. May 17, 2009 #10

    Chronos

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    Agreed, diffuse annihilations would be lost in the background noise at sufficiently large distance. But even an asteroid size intruder comprosed of ordinary matter would result in a detectable emission. Furthermore, if entire galaxies of antimatter do exist, rogue antimatter bodies [stars, asteroids, gas clouds, etc.] must also surely exist. The characteristic high energy gamma bursts that would result from collisions with their counterparts have not been observed.
     
  12. May 17, 2009 #11
    a star of antimatter would emmit lots of gama rays. but I guess it would look the same.
     
  13. May 18, 2009 #12
    A photon is its own antiparticle, so it's logical to think that a star and an antistar would be observationally indistinguishable. But I read somewhere (can't remember where) that antistars may be distinguishable from anti-stars by the polarization of their photons. So the polarization of gamma-rays emitted from supernovae would be somehow different than from anti-supernovae. Can anybody confirm if this is true?
     
  14. May 18, 2009 #13

    Chronos

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    Collisions between antimatter / matter bodies is the only thing detectable. In a 50-50 universe, it would be blatantly obvious. It would also be obvious down to about 99.999%. Like most of science, nothng can be ruled out, merely ruled highly improbable.
     
  15. May 18, 2009 #14
    the solar winds would also be somewhat deadlier, I guess antimatter is not the friend of life.
     
  16. May 18, 2009 #15
    http://en.wikipedia.org/wiki/Gravitational_interaction_of_antimatter#Motivations_for_antigravity

    4 Motivations for antigravity
    Supporters argue that antimatter antigravity would explain several important physics questions. Besides the already mentioned prediction of CP violation, they argue that it explains two cosmological paradoxes. The first is the apparent local lack of antimatter: by theory antimatter and matter would repel each other gravitationally, forming separate matter and antimatter galaxies. These galaxies would also tend to repel one another, thereby preventing possible collisions and annihilations.

    This same galactic repulsion is also endorsed as a potential explanation to the observation of a flatly accelerating universe.
     
  17. May 18, 2009 #16

    Vanadium 50

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  18. May 18, 2009 #17
  19. May 20, 2009 #18
  20. May 29, 2009 #19
    The observed universe is matter, not antimatter. So no.. But..

    One theory that is funny to think about, but in some ways I don't agree with basically states the universe was 50.00001% matter and 49.99999% anti-matter, over time they all destroyed each other and what is left is 0.00001% of the original matter. :) It was an early way of explaining the observed vs theory mass of the universe difference in the Big Bang. Now we just make up something and call it 'Dark Matter' and say "Don't bother testing for it, you can't." IMHO not very scientific, but no other explanations are currently forthcoming.

    Like I said, funny to think about, and would make an interesting Sci-Fi book, but that is about all.
     
  21. May 29, 2009 #20

    Vanadium 50

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    This is PhysicsForums, though, not BadSciFiForums.

    First, if the universe were as you proposed, it would be photon-dominated. It's not - by many orders of magnitude. In science, observations are important.

    Second, you state 'Now we just make up something and call it 'Dark Matter' and say "Don't bother testing for it, you can't."'

    Who is "we"?
     
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