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The Elegant Universe

  1. Apr 3, 2006 #1
    I am currently reading The Elegant Universe by Brian Greene and have a question about a topic in it. On p.9 he is talking about muons, electrons, positrons, and all that other good stuff. He explains that when particles and their anti-particles come into contact with eachother, they "annihilate" one another. He then goes on to say that everything in the universe is made up of these particles and their anti-matter particles. My question is as follows: If the particles and their anti-matter particles annihilate one another upon contact, how could something in the universe have a particle and its anti-partner in the same "thing"? Wouldn't they just annihilate eachother and therefore wouldn't be around in the "thing" anymore? I know I am probably missing something really obvious, but humour me anyway.

    p.s. If you have the book on you, like I said before, it is on page 9 (along with a chart). Thanx in advance.
  2. jcsd
  3. Apr 3, 2006 #2


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    What he means is that everything is made of either some of those particles, or their antimatter counterparts. It is not that you have both matter and antimatter within every object.

    By far, most of the universe is made out of matter only (as opposed to antimatter), and this is actually somewhat of a mistery today because, even when we now know that there is an asymmetry in how nature treats matter and antimatter (which, in itself, is rather surprising), the ammount to which that asymmetry occurs does not account for the amount of matter we see in the universe.
  4. Apr 3, 2006 #3


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    An heretical comment on antimatter.

    First, our definitions of matter and antimatter are rather arbitrary. In general, a particle of matter and a particle of antimatter will annihilate to produce pure energy only if the two particles are of the same type. For example, an electron will annihilate with a positron. But an electron will NOT annihilate with, for example, an anti up quark.

    Thus the relation "anti" is one that we can define exactly only between two particles of the same type. Given a tau and an anti tau, we can verify that they form a matter / antimatter pair because they annihilate into pure energy.

    So matter / antimatter is a relation that divides the particles into doublets (of matter and antimatter) and singlets, rather than a property like charge that all particles can be compared one against another.

    For example, suppose we find that dark matter is composed of two particles, and that these two particles can annihilate to form pure energy, which we can detect, but there are no reactions between either of the dark matter components that we can detect with normal matter. Then there is no way that we can choose which of the two particles is the matter and which is the antimatter.

    If one redefines the particles so that one calls the electron and the down quarks "antimatter", and the positron and anti down quark matter, while leaving the up quark and neutrino definitions alone, one will find that when one plots the particles according to their weak hypercharge and electric charge quantum numbers, the antimatter particles will group together, and the matter particles will group together. In the standard scheme, they alternate in a peculiar fashion.

  5. Apr 4, 2006 #4
    thanx, of course I was missing something really obvious. Just out of curiosity where in the universe, if at all, does a particle and its anti particle come into contact with eachother? Does this happen a lot in the universe? Have scientists ever witnessed it naturally or artificially? Does it occur on Earth? And also, is a great amount of energy released in the "annihilation" process relative to fusion? The more questions answered the better. Once again thanx in advance.
  6. Apr 4, 2006 #5


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    It is occuring right now at a gazillion times a second at the Fermilab's Tevatron, and it occured a gazillion times a second at the old LEP at CERN. And it will occur a larger gazillion times a second when the LHC is completed.

  7. Apr 4, 2006 #6


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    And it occurs a gazillion times a second near the event horizon of any old blackhole :biggrin:
  8. Apr 4, 2006 #7


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    Not directly. Fermilab collides protons vs antiprotons, but LHC will collide protons vs protons.
  9. Apr 4, 2006 #8
    I would like to add a small comment. At the level of quarks, deep inside protons and neutrons (so I'm talking about ordinary matter around us) the role of particle/antiparticle annihilation is crucial and happens all the time. The proton has a core where quasi free quarks do not ineract much with each other (asymptotic freedom). This core is surrounded by a cloud, the meson cloud, where gluons constantly fluctuate into virtual quark/antiquarks pairs.
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