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Positronium Questions

  1. Sep 13, 2004 #1
    The lifetime of Positronium has been measured to be about 142 ns (Nico et al. 1990), and its' ionization energy is about 6.8 eV. It appears to undergo self-annihilation thereby generating high energy photons. Hydrogen, by comparison, has an IE of 13.6 eV, double that of Positronium, but it does not annihilate itself, and has, in effect, an infinite lifetime. The radii of the two systems are effectively the same.

    When we shoot a beam of electrons at a beam of positrons we also get annihilation, but when we shoot a beam of electrons at a beam of protons, we get hydrogen atoms.

    I would appreciate any insight on the differences in the these two systems that explains the radical differences in behavior. Thanks in advance!
     
  2. jcsd
  3. Sep 13, 2004 #2
    Its not that simple as just things with unlike charge annihiliating each other, its just because positronium has a positron and an electron at its orbit, so they annihilate each other because they are anti-particles of each other. not only because they have opposite charge.

    The reason why a hydrogen atom don't get annihilated because

    1. Only an electron is around, no positrons here to annihilate the electron as well as itself.

    2. Even though, a proton and an electron don't annihilate each other, but the orbiting electron cannot be attracted to the proton and fall down to the bottom of the potential well simply because the uncertainty relation says that it is not possible.

    Am I understanding your question correctly here? or is it that you're asking something else?
     
  4. Sep 13, 2004 #3
    Put another way, there are two kinds of matter: "regular" matter, which is composed of the familiar protons, electrons, and neutrons (along with other quark combos and leptons and whatnot), and antimatter. Every "regular" particle has an associated antiparticle, which has the same mass but the opposite charge. The antielectron is the positron, and pretty much all the others are just named anti- plus the regular counterpart (antiprotons, antineutrons, etc.). Well, whenever a particle encounters an antiparticle counterpart (and only that counterpart, nothing else will do), they annihilate one another in a flash of EM radiation, as high energy gamma waves.

    Protons and electrons don't annihilate one another simply because they are not antiparticles of one another. Furthermore, the mass of the proton is much, much greater than the mass of an electron, so it doesn't really make sense that the electron could annihilate the proton, does it? On the other hand, the positron is the electron's antiparticle. They have the same mass, but their charges are opposite one another. Therefore, they will annihilate if they run into each other.
     
  5. Sep 14, 2004 #4
    Protons and electrons don't annihilate one another simply because they are not antiparticles of one another. Furthermore, the mass of the proton is much, much greater than the mass of an electron, so it doesn't really make sense that the electron could annihilate the proton, does it? On the other hand, the positron is the electron's antiparticle. They have the same mass, but their charges are opposite one another. Therefore, they will annihilate if they run into each other.

    So what prevents part of the proton charge from 'annihilating' all of the electron charge?
     
  6. Sep 14, 2004 #5
    Nothing "prevents" it, since the electron and the proton can combine to form a neutron. That's not the whole story though. Particle-antiparticle annihilation requires a "perfect match" between the two particles - opposite electric charge is not enough. Electric charge is a locally conserved quantum number, and each fundamental particle carries its own charge. A particle cannot pass that charge onto something else (or have that charge "destroyed" by an opposite charge) while retaining its identity ; the particle itself must be destroyed at the interaction point for that to happen. The creation of neutral bound states (ie hydrogen atom) doesn't count either, since the constituents retain their separate charges. While such a system is neutral "as a whole" (ie when seen from afar), the charge distribution becomes apparent when the system is examined at close range.
     
  7. Sep 14, 2004 #6
    Its like neo in the matrix and what the oracle said, agent smith is his evil twin, everything like neo but just the direct opposite. In the end, both neo and smith were killed (am I right) ? And only smith and neo can kill each other, no other tom dick and harry can do that.
     
  8. Sep 14, 2004 #7
    Okay, so here's a couple more things.

    There are some rules called conservation rules that tell us how things work. No one really knows why these things are the way they are, they just haven't seen them being broken yet - and they've looked very very hard. Here's one:

    "The total number of (electrons + electron neutrinos) - the total number of (positrons + electron anti-neutrinos) is constant*"

    So if you put a proton and an electron together, you have to conserve the number of electrons or electron neutrinos. Which means you have to end up with an electron or an electron neutrino.

    Here's another conservation rule.

    "The total charge in the universe is constant"

    So we have two options: Either we end up with an electron or an electron neutrino. But if we think about the charge involved, for it all to add up we have to have a total charge of 0. Well there is a way we can do that. We could get something like:

    e + p -> nu_e + n

    or electron + proton -> electron neutino + neutron

    Here's a new conservation law, you might have seen it at school:

    "The total energy in the universe is constant"

    If you know that energy and mass are the same thing, then you can work out the energy on the left hand side:
    E = m(electron) + m(proton) = 0.5 + 938.3 = 938.8 units of energy
    on the right hand side:
    E = m(neutrino) + m(neutron) = 0 + 939.5 = 939.5 units of energy

    So this reaction *is* possible, but our particles need to borrow some energy from somewhere. This happens sometimes - it's a special type of radioactive decay.

    There are other wild ideas - the electron and proton could turn into a neutrino and a photon, perhaps? But these are all excluded by other laws of physics like the ones above, but ones that I'm not going to detail.

    Compare this to the positron and the electron - now we have an electron and a positron. If we have photons at the end, then we get a net number of electrons + positrons of zero. But that's what we started with. Also, the total charge adds to zero - so that's fine. Finally, the annihilation is allowed by energy conservation, as the photons have no mass.

    Here's a question for you. When electrons and positrons annihilate, they always annihilate to at least two photons. Why do you think that is?


    *This rule, called "lepton number conservation", has in fact been broken - but only in a very specific case called "neutrino oscillations" which I won't delve into here. Try google...
     
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