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Confused about this antimatter stuff

  1. Apr 11, 2005 #1

    liz

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    i'm getting quite confused about this antimatter stuff. ive heard that for every particle there is a corresponding antiparticle somewhere out there in the universe. but it also said somewhere else about the big bang theory, that the universe only exists because for some unexplained reason there is far more matter than antimatter. which one is correct? and what is antimatter?
    i think of particles and matter as blobs, so is an antiparticle also a blob? i read some analogy to explain antimatter as thinking of a sheet of metal and stamping circles out of it. the circles are matter. The spaces left in the sheet of metal are antimatter. but that doesnt really make sense.
    If anyone can help clarify all this stuff than thank you very much.
     
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  3. Apr 11, 2005 #2

    SpaceTiger

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    I think they probably meant that for every particle, an anti-particle can be created. It's hard to imagine how sigificant amounts of antimatter could exist in the universe. They would very quickly annihilate with matter. In the beginning, they think that there was a sea of matter and antimatter and that most of it annihilated. It didn't all annihilate, however, and they assume that was because there was more matter than antimatter (an asymmetry, as they say).
     
  4. Apr 11, 2005 #3
    Couple things to clear up:

    an antiparticle has the same mass and spin as its mate, but has opposite charge (this includes electric, leptonic, etc). So they are particles in our usual notion. The positron (the antiparticle of the electron) just has the opposite sign of electric charge and leptonic charge. When an antiparticle-particle collision occurs, the particles annihilate each other, leaving in the final state some sort of energy (i.e. photons) to satisfy conservation laws. You should look up material on the Dirac sea and Dirac's notions of holes as antimatter for a starting place. But it turns out that anti-particles fall out of Quantum Field Theory very nicely.

    Now, knowing this and seeing the predominance of matter in our local neighborhood (someone correct me if there is proof of the dominance of matter throughout the universe) of the universe, we conclude that there is an asymmetry in the amount of matter and antimatter created in the universe. I don't know if there is any other proof for the dominance of what we have arbitrarily called matter over antimatter, but the fact that you and I are here is enough at least on a local level.
    Hope this helped.
     
  5. Apr 11, 2005 #4

    SpaceTiger

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    Yeah, I'm not sure if such a proof exists either, but it's an interesting idea. If there were a part of the universe that was composed primarily of antimatter, then its boundary with the matter-dominated universe would be releasing tremendous amounts of energy (presumably in the form of gamma-rays). I bet we can rule out such a thing in the observable universe, but if such a boundary were to cross our horizon, it sounds like we'd have yet another doomsday scenario on our hands.
     
  6. Apr 11, 2005 #5
    OK, possible dumb question but if there was an area of space made up of just antimatter, would the expansion of the universe actually reduce the amount of matter/antimatter collisions along this boundary? Possibly this has already happened and that's why we don't see evidence of these regions of space?
     
  7. Apr 12, 2005 #6

    SpaceTiger

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    The current rate of expansion wouldn't be very efficient at preventing matter-antimatter annihilation at the boundary, but in an inflationary epoch, that probably wouldn't be the case. This is just speculation, but if the pre-inflationary universe had regions of matter and antimatter physically separated from one another, the process of inflation may have separated them enough to keep them around to the present day, but there would still be annihilation occurring on the boundary.

    I've been thinking about it a bit more, though, and I wouldn't expect much annihilation to be occurring at the present day. The initial post-inflation collisions at the boundary of the matter-antimatter regions would create large voids that would be expanded by the growth of perturbations (collapse by gravity). I'm now thinking that an indicator of an antimatter section of the universe would be a large void or set of voids inconsistent with the usual power spectrum.
     
  8. Apr 12, 2005 #7
    There is no difference from +matter and -matter. They just have different charges. We could all be made out of anti-matter, but we just call it matter lol. And everybody knows the Big Bang has some major problems. But some say that for some slight reason, there was 1/1000000 more matter than anti-matter in the universe after the BB, and over billions of years, matter won the battle. But its all in theory lol
     
  9. Apr 12, 2005 #8

    ZapperZ

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    This is not correct, and the word "theory" in physics doesn't mean "guess".

    CP violation is not just "in theory" but a clear and verified experimental observation. And there are MANY solid theories that link the existence of CP violation as the possible symmetry-breaking origin on why our universe has more of one kind of matter than the other.

    The BB has problems like any still-evolving idea, but OTHER models have more severe problems than the BB to account for ALL the experimental observatons.

    Zz.
     
  10. Apr 12, 2005 #9
    eNathan does have one good point, since matter and anti matter are opposites, we cannot tell which one is which unless we have some thing to compare them to.

    close your eyes, and spin around a few times, can you tell which way is east or west with out something else to compare it to.

    but then again, we are the ones who named it, so we can call it what ever we want (we as in human race)
     
  11. Apr 12, 2005 #10
    ZapperZ, we observe more matter than anti-matter, and the only explanation as of now is that there was just a little bit more matter than anti-matter after the BB. So, are you saying it's not a theory? Its certainly not a fact, and it is not just a "guess".
     
  12. Apr 12, 2005 #11

    ZapperZ

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    1. If the creation of matter-antimatter came out of energy, then there should be an equal amount of matter and antimatter. There are no other mechanism for such creation, at least within the standard BB theory.

    2. There are more matter than antimatter based on obvious observation

    3. You said

    "But some say that for some slight reason, there was 1/1000000 more matter than anti-matter in the universe after the BB, and over billions of years, matter won the battle. But its all in theory lol"

    Where is this from? If you are criticizing BB theory as having problems, then you should at least cite the standard version of it. As it stands NOW, the only candidate for the asymmetry in matter-antimatter ratio is the CP violation.

    Zz.
     
  13. Apr 12, 2005 #12
    Um, could not there be equal amount of anti-matter in the 'center of the unverse', where the BB ocurred. It could be in a massive black hole. Just a guess. All alot more matter than anti-matter was pushed out of the 'eating range' of the black hole. And alot more anti-matter than matter was kept in the 'eating range' of the black hole.

    Its just a thought not worth $0.02 probally.
     
  14. Apr 13, 2005 #13
    delete my earlier post, in the book 'A Breif History of Time', there is a hypothesis (or maybe a theory) about why there is more matter than anti-matter. Also, if i read right, the anti-matter is everywhere. electrons decay (this happens about a much as proton decay which takes 10000000000000000000000000000000 years to decay, that should be 31 zeros) into antiquarks which make up antimatter. it is viceversa for antimatter electrons. as i said, if i read right.

    also, it had how we could tell antimatter apart from matter, it said it did not follow the rules based on three comparisons. Read the book for how or why, even though it states results, it gives the names of the ones who did the experiment and about when it was done, so you can look up the experiment if you want.
     
  15. Apr 13, 2005 #14
    From everything I've read anti-matter behaves just like matter, except for opposite charges. I'm going out on a limb but couldn't it stand to reason that there are entire galaxies made of anti-matter, possibly in our observable area of the universe? If they are not in direct contact with matter galaxies, there would be no annihilations, so no massive energy would be radiating from them. Any reasons I've missed that prevent this?
     
  16. Apr 13, 2005 #15

    SpaceTiger

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    That's right, in fact that's what I was tacitly assuming when I referred to a separate antimatter portion of the universe. If matter and antimatter aren't exactly the same (as implied by an asymmetry), then there might be some process which occurs differently in an antimatter dominated universe. I don't think it would be a problem for the formation of galaxies, however. I wouldn't expect matter-antimatter galaxies to be colliding and annihilating (though that would be quite an event), but I would expect their intergalactic media to be doing so.
     
  17. Apr 13, 2005 #16

    DaveC426913

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    It is not meant to mean for every single individual particle there is a counterpart particle.

    It means for every type of particle there is a counterpart type.

    There are protons and there are antiprotons; There are electrons and there are antielectrons, etc.
     
  18. Apr 13, 2005 #17
    electrons decay into anti-quarks, and anti-elctrons decay into quarks

    after the BB, anti-electrons decayed much more, creating more matter than anti-matter.
     
  19. Apr 13, 2005 #18
    Maybe I missed something but how can a tiny electron decay into a massive anti-quark? Or a positron decay into an equally massive quark? Did I misunderstand the standard model?
     
  20. Apr 13, 2005 #19
    No you have not mis-understood the standard model (SM). In the SM protons and electrons are stable. And as far as anyone has experimentally seen, they are completely stable. Now an experiment was done where they took a large amount of pure water and watched for proton decay for a long time. This is what set the ridiculously large limit on proton lifetimes. I have never heard of any experiment where electrons were tested for decay. And if electrons are decaying, expecially into anti-quarks (only antiquarks?) they must be decaying into baryons (only color neutral particles are present in nature), which seems ridiculous to me since that kind of reaction would violate baryon AND lepton number... hard for me to imagine- that doesn't mean it is not true though. This must involve one of Hawkings GUT's.
    Cheers
     
  21. Apr 14, 2005 #20
    Here's a paper I found, re electron decay:

    http://arxiv.org/abs/hep-ph/9808252

    Keep in mind that this is all theoretical speculation, and not proven experimentally.

    But I don't think that the standard model alone can account for the observed level of baryon asymmetry in our universe, so something else is also going on. Standard model CP violation is not enough.

    BTW - a particle cannot decay into a heavier particle, if we assume energy conservation holds, so the electron cannot decay into a quark, since even the lightest quark is heavier than an electron. And I don't think there are any theoretical models out there that violate energy conservation. Non-crank models, that is.
     
    Last edited: Apr 14, 2005
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