Photon and its antiparticle

  1. Why is a photon considered to be its own antiparticle? What distinguishes a photon from an antiphoton? When photon and antiphoton collide do they annihilate?
     
  2. jcsd
  3. I'm not very good at particle physics, but I think only charged particles can have an antiparticle. The neutron has an antiparticle, but it is made of different charged quarks.
     
  4. Demystifier

    Demystifier 5,176
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    Actually, it would be more correct to say that antiphoton does not exist.
     
  5. So how do you define the antiparticle of a photon, if antiphoton doesn't exist? Which of the quantum numbers of a photon are 'opposite/different' in its antiparticle?
     
  6. SpectraCat

    SpectraCat 1,395
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    Can't the same question be asked of any "force carrier" particle? Photons are (massless) carriers of the EM force, and are emitted when charge-carrying particle/antiparticle pairs annihilate. Gluons are the (also massless) carriers of the strong force, and are emitted when color-carrying particle/antiparticle pairs annihilate. So perhaps it is generally true that these massless carrier particles have no antiparticles? I guess that may be a bit ambiguous, since QCD requires 8 distinct kinds of gluons, and I guess none of them have ever been observed directly, but I have not read that they are generally organized into gluon-antigluon pairs.

    Anyhow, what would be emitted in the annihilation of a putative photon-antiphoton pair? I mean, energy and momentum have to be conserved, right?

    EDIT: so, to further qualify my own question, I know electron-positron pair production is possible if the photon energy is high enough ... I am trying to understand the general situation that would be valid at all photon energies.
     
    Last edited: Feb 1, 2010
  7. Meir Achuz

    Meir Achuz 2,058
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    Homework Helper
    Gold Member

    Particles like the photon and the pi zero that are their own antiparticles are eigenstates of the charge conjugation multiplicative operator C. The photon has eigenvalue -1, and the pi zero has eignevalue +1.
     
  8. SpectraCat

    SpectraCat 1,395
    Science Advisor

    Ok, my understanding of QFT is far from comprehensive, so forgive me if this makes no sense, but if a photon is its own antiparticle, doesn't that mean that for a photon with and arbitrarily chosen set of properties (e.g. energy, wave-vector, polarization), there should be also exist a photon with the set of properties that would "annihilate" the original one? How can such a process conserve energy, if we ignore energies above the pair-production threshold? (I guess we can get momentum conservation if we choose counter-propagating wave-vectors.)

    I guess this amounts to what the OP was asking in the first place ...
     
  9. Meir Achuz

    Meir Achuz 2,058
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    Two photons can ( and do) annihilate into other particles in just the same way that an electron and a positron can.
     
  10. SpectraCat

    SpectraCat 1,395
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    Fine .... but I can't find any qualitative descriptions of photon-photon annihilation at energies below 1 GeV or so. It seems that if photons have antiparticles and can undergo annihilation, there shouldn't be any energy restrictions on it. So what is the antiparticle of (say) a 532 nm green photon, and what particles are produced when they annihilate?

    Again, I am just following my intuition here, so please forgive me if these are naive questions ...
     
  11. The photon is different from the QCD gluons. As opposed to the photon those actually carry a nonzero charge with respect to the force they mediate. This is because their associated gauge group SU(3) is noncommutative. Their charges are denoted by e.g. "red-antigreen", "green-antiblue", and so on. Therefore they each have an antiparticle with the opposite colour charges. The charge of the antiparticle to the red-antigreen gluon would be green-antired.

    Torquil
     
  12. From what I know of eigenstates, or rather energy eigenstates, is that they represent the states of a system that correspond to definite values of quantized energy. So does a photon and an antiphoton have different energies?
     
  13. DrDu

    DrDu 4,351
    Science Advisor

    They can have different energies, but note that this depends on the reference system.
    The two photons can only anihilate if they do not travel exactly in the same direction. But then you can find a reference system where one is red shifted and the other one blue shifted so that their energies coincide and they impact head on. This is analogous to the scattering of massive particles in the laboratory frame vs the rest frame of the center of mass.
     
  14. DrDu

    DrDu 4,351
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    They can have different energies, but note that this depends on the reference system.
    The two photons can only anihilate if they do not travel exactly in the same direction. But then you can find a reference system where one is red shifted and the other one blue shifted so that their energies coincide and they impact head on. This is analogous to the scattering of massive particles in the laboratory frame vs the rest frame of the center of mass.
     
  15. SpectraCat

    SpectraCat 1,395
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    Can you please describe such an annihilation experiment more fully, for the case of a low energy photon, such as a 532 nm photon propagating in the +x direction and plane polarized along the y-axis in some lab-fixed frame. What are the characteristics of the "antiphoton" corresponding to this particle, and what is emitted during the annihilation event to conserve energy?

    I can't see how any other particles besides more photons could be emitted in this case, and in that case, how can you tell the difference between an annihilation event and a scattering event? Of course, I am not familiar with the full "menagerie" of sub-atomic particles ... are there other candidates for emission in such an annihilation?
     
  16. jtbell

    Staff: Mentor

    The concept of "eigenstate" also applies to other quantities besides energy. An energy eigenstate has a definite energy, but (in general) indefinite values of other physical quantities. An eigenstate of some other quantity has a definite value for that quantity, but (in general) indefinite values for other physical properties (including energy). Important exeption: if the operators for two quantities commute, then the system can be in a simultaneous eigenstate of both quantitles.
     
  17. If they DID produce a scattering event: 1.) I'd wet myself before I died and, 2.) How does a packet of energy scatter into anything but more energy? Photons would have to scatter into a different EM quanta... so no, you are not suffering from a knowledge gap here.
     
  18. DrDu

    DrDu 4,351
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    Two photons may anihilate e.g. into an electron and a positron. This requires at least two photons having 511 keV, the rest energy of the two electrons formed. I think the only anihilation process possible for visible light would be the production of a neutrino and an antineutrino which are nearly massless, which is a phantastically improbable.
     
  19. What would be the mechanism of this annihilation? It sounds more like spontaneous pair production. I'm confused.
     
  20. DrDu

    DrDu 4,351
    Science Advisor

    The anihilation of two photons with the formation of an electron and a positron is just the time reverse of the anihilation of an electron and a positron to form two gamma photons.
    What else do you expect to happen in the anihilation process? The energy is conserved, so new particles have to be produced.
     
  21. ...And does this occur in nature?
     
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