Detecting Anti-matter

kumar1988

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?

Nabeshin

To the best of my knowledge there would be no observable difference.

Chronos

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/scienceastronom...un_030929.html

Vanadium 50

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.

Nabeshin

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.

Vanadium 50

Anti-helium nuclei carry -2 units of charge rather than +2.

Nabeshin

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.

Vanadium 50

Except that the cosmic rays coming from it would contain anti-nuclei. Which is what people have been and are continuing to search for.

Nabeshin

Ah, I see what you were talking about now. Okay.

Chronos

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.

cosmicpencil

a star of antimatter would emmit lots of gama rays. but I guess it would look the same.

Maria76

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?

Chronos

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.

cosmicpencil

the solar winds would also be somewhat deadlier, I guess antimatter is not the friend of life.

granpa

http://en.wikipedia.org/wiki/Gravita...or_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.

Vanadium 50

Is, despite being in Wikipedia, unadulterated crackpottery (and also not relevant to the 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.

Maria76

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/p.../0405417v3.pdf