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Antiparticle of photon

  1. Oct 10, 2014 #1
    Do really is the presence of an antiparticle of photon possible in the cosmos? Does it have one?
     
  2. jcsd
  3. Oct 10, 2014 #2

    Nugatory

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    The photon is its own antiparticle.
     
  4. Oct 11, 2014 #3
    Sir,
    How can a photon be its own antiparticle, as a particle and anti particle annihilate each other????
     
  5. Oct 11, 2014 #4

    Dale

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    What is produced when a particle and its antiparticle annihilate each other?
     
  6. Oct 12, 2014 #5

    Drakkith

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    Sure, but photons rarely interact with other photons and they certainly don't annihilate with themselves. The full answer to your questions requires some understanding of the intricacies of quantum physics that are difficult to explain here on the forums if you have little knowledge of the theory.
     
  7. Oct 12, 2014 #6
    When a particle/antiparticle pair annihilates the energy and momentum have to go somewhere. Usually a pair of two photons is created that take them up.

    If you wanted to annihilate a pair of photons you would also have to create a pair of different particles. This could in principle be a different pair of photons, but such an process is very unlikely.

    For any other pair of particles the photons have to have enough energy to create the particles in the first place, i.e. the energy equivalent to their rest mass. The lightest massive particle is the electron, with a rest mass of 511 keV. You need to create an electron and a positron, so you need at least 2*511 keV. This process is exactly the inverse of the annihilation of a electron-positron pair that creates two photons.

    An effect that is often observed is called "pair creation" when a photon with energy higher than 2*511 keV annihilates with a (virtual) photon from
    the electric field near a heavy nucleus to create an electron and a positron.
     
  8. Oct 12, 2014 #7
    Sir,
    Whats your point?
     
  9. Oct 12, 2014 #8
    To explain how the photon can be its own antiparticle, and to show that there is actually experimental evidence that this is the case.

    That was the original question, wasn't it?
     
  10. Oct 12, 2014 #9

    mfb

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    The lightest massive charged particle.
    Neutrinos are lighter but without electric charge, so photons won't produce them.
    Not always. And the photon is one of those exceptions.
     
  11. Oct 12, 2014 #10

    Dale

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    I wouldn't call it an exception. When two antiparticles interact they do always annihilate each other. The result of annihilation can be any combination of particles that conserve energy, momentum, charge, spin, etc. So an electron and a positron can annihilate and produce a pair of photons. Run that same reaction backwards and you have a pair of photons annihilating to produce an electron and a positron. So I would say that it is not an exception, it is just rare since photons are not pulled to each other in the same way that positrons and electrons are.
     
  12. Oct 12, 2014 #11

    A.T.

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    Maybe it helps to consider matter and antimatter as the same thing just advancing in opposite directions in time. Since photons do not advance in time (don't age), they are neither matter nor antimatter, but a state between the two.
     
  13. Oct 12, 2014 #12

    mfb

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    I would not call t-channel scattering of electron+positron an annihilation, but it is certainly an interaction. The same process is possible with two electrons, for example.

    Yes but it requires a minimal photon energy, whereas the first reaction does not require special conditions for particle energies.
    Sometimes it can happen, but not always and even if it can it does not have to.
     
  14. Oct 12, 2014 #13

    Dale

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    Hmm, good point. I had not considered scattering, which obviously I should have.

    Interestingly, photon-photon scattering would look very much like a typical annihilation reaction. Two anti particles come in, two photons come out.
     
  15. Oct 12, 2014 #14

    Drakkith

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    Oh god no, I don't recommend this at all.
     
  16. Oct 12, 2014 #15

    Simon Bridge

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    iirc QFT insists that all charged particles have an antiparticle, and is specific about the properties they have in terms of symmetry.
    Since QFT does not impose any other restriction that would imply that uncharged particles would be their own antiparticle ... but there should be more to it than that - the particle should be fundamental rather than a composite right? So anti-hadrons end up being the anti-quark versions of the regular hadrons.

    Photons, being fundamental and chargless get to be their own antiparticle by those rules.

    But we also popularly expect a two antiparticles particle and it's antiparticle to annihilate. [edit typo]
    Certainly creation an annihilation operators are used a lot with photons, and we can run the time-reversal of common pair production reactions to get the annihilation into matter...

    ... but basically that photons are their own antiparticle seems to be the standard one in physics.
    If this seems counter-intuitive, the reason is probably the popular picture that lots of antimatter and matter in a confined space = boom.

    It's a discussion that crops up here from time to time:
    https://www.physicsforums.com/threads/are-photons-the-antiparticle-of-itself.739672/
    https://www.physicsforums.com/threads/photons-antiparticle.573929/

    And elsewhere too, eg:
    http://van.physics.illinois.edu/qa/listing.php?id=1153

    There is journal support:
    Physical Review eg. http://journals.aps.org/pr/abstract/10.1103/PhysRev.97.1387
    Nature eg. http://www.nature.com/nphys/journal/v5/n9/abs/nphys1380.html
    ... though they may be a bit oblique to this discussion. I'm sure others with better access can come up with better examples.
    Ideally we want an observation of photon-photon annihilation
    Can't tell if this[/i] is a good example.
     
    Last edited: Oct 14, 2014
  17. Oct 14, 2014 #16

    mfb

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    No. Why do you think so? I don't get why charges, and in particular the electric charge are/is often considered as special.
    All charges and all other quantum numbers have to be zero for a particle to have a chance to be its own antiparticle. In the Standard Model, this just leaves the photon, the Higgs and two out of eight gluons.
    If you also consider composite particles, then there are many more (typically particle+matching antiparticle compounds).

    Why? In the same way two particles don't annihilate, the antiparticles don't do that. Annihilations are special reactions of particles with antiparticles.
     
  18. Oct 14, 2014 #17

    Simon Bridge

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    In the first part I have not written anythig that disagrees with what you followed it with.
    The second part is a typo misstatement - thanks :)
     
  19. Oct 14, 2014 #18
    I just spent 45 minutes writing a better explanation of what I was trying to convey and was unable to send it. I'll try to get with the program here. Please bare with me. Thanks...
     
  20. Oct 16, 2014 #19
    How can a photon and anti-photon annihilate each other and produce heavier body than themselves???
     
  21. Oct 16, 2014 #20

    Orodruin

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    The only thing required is that energy and momentum are conserved. In particular, in a two-photon system, you can go to the CoM frame (as long as the photons are not going in the same direction) where the total momentum is zero. If the total energy in this frame is larger than the total mass of the particle-antiparticle pair, this is possible.
     
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