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Antiparticle annihilation

  1. Nov 14, 2008 #1
    Hello, everyone. I have searched google and wiki, but have not been able to find the solution to my question. I was wondering why do antiparticles annihilate on contact with normal matter?

    If someone could help explain this to me or send me a link on where i can get this information it would be great?

    I am assuming that has something to do with wave cancellations, but i am unsure.
  2. jcsd
  3. Nov 14, 2008 #2


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    They do it because they must I would say. But I stress you should think of what the word "contact" means in quantum mechanics.. what does it mean?
  4. Nov 14, 2008 #3


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    "Contact" is a word that doesn't belong in this context. The proper term is "interaction".

    A positron (=anti-electron) isn't going to destroy a neutrino for example. They will hardly interact at all. If you e.g. send a positron towards a proton, it will just be repelled unless the speed is very high and the aim is perfect. If on the other hand you put a positron close to an electron, with almost the same velocity, they will often destroy each other because they attract, and they can destroy each other without violating conservation of charge.
  5. Nov 15, 2008 #4
    I was wondering this myself... There must be some theory on why they annihilate? Or is the fact that they do all based on observations?
  6. Nov 15, 2008 #5

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    I think you are getting hung up on the word "annihilate". Particles interact with each other through a number of processes. In some of them, the particle's identity is preserved at the end of the reaction. In others it's not.
  7. Nov 15, 2008 #6


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    I know it's hard to see this, but that question is actually a bit like asking if there's a theory that explains why bananas falls to the ground when you drop them. You don't need a separate theory describing "annihilation" just as you don't need a special theory for bananas. Every quantum field theory that describes particles also describes their antiparticles. "Annihilation" is just one of many ways the particles can interact. If you put a lot of antiprotons near a lot of protons, you can expect it to happen a lot just because there's an attractive "force" between particles of opposite charge.
  8. Nov 15, 2008 #7
    One way to think about it, are conserved quantum numbers. Although there are interactions such as positronium where electrons temporarily form bound states with positrons, if we look at the big particle physics picture of infinite plain waves, we look at all the things we need to conserve, like energy, momentum and the quantum numbers.
    If we start with one state and there are more states where all the conserved properties match, the system will enter the other states as well. Maybe you could think of it as tunneling.
    So since two photons have the same quantum numbers as the electron positron pair it will - during some black box interaction - leak into the photon state. You can even argue what the chances are for certain states, depending on how many there are, so if there are more photon states it will prefer those over other matter anti matter pairs.
    From the photon state it will usually not return, because the photons fly away in opposite directions and leave the setup. Photons do not have the benefit to be attracted to each other, which makes matter-anti-matter interactions more likely than photon collisions.
    Of course you can do exact QFT calculations, possibly with wave packets or positronium states, but someone else has to help you with that.
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