Detecting Anti-matter

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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?
 

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  • #2
Nabeshin
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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?
To the best of my knowledge there would be no observable difference.
 
  • #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]
 
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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.
 
  • #5
Nabeshin
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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.
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.
 
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Anti-helium nuclei carry -2 units of charge rather than +2.
 
  • #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.
 
  • #8
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So an antimatter galaxy would look to us exactly the same as a matter galaxy.
Except that the cosmic rays coming from it would contain anti-nuclei. Which is what people have been and are continuing to search for.
 
  • #9
Nabeshin
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Ah, I see what you were talking about now. Okay.
 
  • #10
Chronos
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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.
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.
 
  • #11
a star of antimatter would emmit lots of gama rays. but I guess it would look the same.
 
  • #12
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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?
 
  • #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.
 
  • #14
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?
the solar winds would also be somewhat deadlier, I guess antimatter is not the friend of life.
 
  • #15
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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.
 
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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.
 
  • #20
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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"?
 
  • #21
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The fact that the photon, which is its own antiparticle, falls at the same rate as ordinary matter conclusively excludes the possibility of antimatter being repelled from ordinary matter by gravity.
unless, or course, gravitational time dilation (and shortening of measuring rods) is proportional to the absolute value of the strength of the field rather than to its potential. but that is another thread.
 
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  • #22
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unless, or course, gravitational time dilation (and shortening of measuring rods) is proportional to the absolute value of the strength of the field rather than to its potential. but that is another thread.
It's also nonsense.

It's bad enough to hijack the thread - does it have to be with nonsense?
 
  • #23
malawi_glenn
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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.

you are mixing two things, you are mixing the baryon asymmetry with the missing luminous matter problems. Those are two, as far as we know, unrelated problems to be solved. So before answering questions here, you should perhaps ask yourself what your level of understanding is. Have you done graduate courses in cosmology for instance?

I have seen many prospects for DM candidates, SUSY particles are among the most popular ones. Heard of the neutralino?
 

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