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Matthew
2004-Nov-27, 03:16 AM
Now there isn't much difference between matter and anti-matter, just enough so they anhilate each other upon contact. Now as there is little differnce how do we know that the milky way is made up of matter, while MOST of the universe is made up of anti-matter (what we consider to be anti-matter).

How could we tell?

antoniseb
2004-Nov-27, 03:38 AM
Originally posted by matthew@Nov 27 2004, 03:16 AM
How could we tell?
If the matter-antimatter boundary were nearby, we wouldn't see any extra-galactic sources of heavy nuclei as cosmic rays, but would instead see anti-nuclei getting annihilated, either here, or at the boundary. We do see the former, and don't see the latter.

There are clouds of Hydrogen that fall into our galaxy. If a cloud of anti-Hydrogen were to fall in, you'd know about it from the proton-anti-proton annihilation gammas all coming from a diffuse source. Likewise we'd see something similar if a cloud of Hydrogen were to fall into a nearby anti-galaxy. We don't see this.

We DO see positron annihilation near the center of our galaxy and in a cloud rising up from the core, but we do NOT see anti-proton annihilation to go with it, so the positrons are not coming from clouds of pure anti-matter.

Matthew
2004-Nov-27, 05:50 AM
Thanks Anton.

This is probably a stupid question but: what if our area (the milky way and slightly beyond) is made up of matter while beyond THAT it is mostly anti-matter? Would the same type of theory still apply. Or would there not be any interactions that would allow us to determine this?

wstevenbrown
2004-Nov-27, 06:00 AM
Wait for a nova or supernova, and note whether the neutrinos received are normal- or anti-neutrinos. If the bulk of them are not anti-neutrinos, the nova was composed of antimatter. This is a chirality argument, and beyond my meager powers of explanation (ok, I'm a closet dyslexic). I'm told that arguments of this type are the only known way to convey the notion of 'handedness' unambiguously to an alien race who had never, perhaps, seen hands. There is no circumstance under which light alone could tell us. Good question-- Steve

antoniseb
2004-Nov-27, 01:19 PM
Originally posted by matthew@Nov 27 2004, 05:50 AM
what if our area (the milky way and slightly beyond) is made up of matter while beyond THAT it is mostly anti-matter? Would the same type of theory still apply.
Yes the same type of argument would apply.