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Photon-photon collision confusion

  1. May 10, 2009 #1
    okay, i have to ask a stupid question. a short while ago, there was a thread discussing photon - photon collisions. being an amateur, i had always understood that two photons could not collide, and i stuck in a comment to that effect, indicating that since photons, once they are emitted, only exist as a probability function, and have no exact location until they are absorbed.

    i was corrected, and told that the wiki article supporting what i had said was wrong, and pointed at some papers referring to current experiments with photon-photon collisions.

    so, apparently, photons can collide with each other, but i simply cannot understand how that corresponds with what i have been taught, regarding the photon probability function and no precise location of a photon. for photons to collide, they must have some precise location, but to have a location, the probability function must be collapsed, which seems in conflict with the basic premise of QED.

    can someone please try to provide some explanation of this for me? what am i misunderstanding? thanks.
  2. jcsd
  3. May 10, 2009 #2


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    You have to stop thinking of collisions in the quantum world as two tiny balls colliding as in pool or snooker...

    You have wave-packages, which is enough to build up a scattering formalism (you can do it with plane waves too if you want)

    If you don't think electron-electron scattering is strange, or electron-photon scattering; then you should not think photon-photon scattering be strange...
  4. May 10, 2009 #3
    Not having a precise location is a synonym to a wave. Two waves on water surface, created with different sources, propagate differently (scatter) after collision if the wave equation is non linear. It is the case of QED. But the QED photon non linearity is very very weak so most likely photons will not "see" each other while "collision".

  5. May 13, 2009 #4
    Looking at the QED Lagrangian we can see that photons don't interact at tree level, but they can interact at some order at loop level through virtual electrons. So the possibility for two photons two scatter is there. So they should be able to interact, right?
  6. May 13, 2009 #5
    Yes, exactly.
  7. May 13, 2009 #6


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    Staff: Mentor

    And a Google search on "Delbrück scattering" should lead you to some information...
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