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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?
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?
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.
So an antimatter galaxy would look to us exactly the same as a matter galaxy.
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.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.
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?
I found the following paper, which proposes that anti-stars can be distinguished from matter stars from the "polarization properties of their electromagnetic emissions" (see section 4.2).
http://arxiv.org/PS_cache/astro-ph/pdf/0405/0405417v3.pdf
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.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.
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.