Can two photons be at the same place at the same time?

  1. Since photons are bosons and do not follow the pauli exclusion principle, does this mean that two photons can be at the same place at the same time?

    Is it meaningful to talk about a photon 'colliding' with another photon?

    I mean, suppose we send two photons of the same color towards each other at an angle of 60 degrees between them, and they meet. Is that a 'collision'?

    Do the photon's directions change and just add up vectorially? I'm thinking that since the photon speeds can't add up, and must stay the same, since they already travel at the speed of light, how can momentum be conserved? Does one absorb the other? How about energy conservation?
  2. jcsd
  3. Yes. (Though technically there is a sense in which the position of a photon isn't perfectly well defined in the first place).

    Sure. Most of the time photons pass through each other without affecting each other, but sometimes (very rarely) they scatter off each other. The probability of scattering is extremely small unless the photons have extremely high energy. When scattering occurs the final directions of the photons can only be predicted probabilistically.

    But note that it's not necessary for two particles to be in exactly the same place in order for them to scatter off each other. For example, electrons can scatter off each other. In this case the scattering is caused by the electric repulsion between the two electrons.
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  4. Vanadium 50

    Vanadium 50 17,446
    Staff Emeritus
    Science Advisor
    Gold Member

    To have a sensible discussion, you need to describe what sort of measurement that you would do that would indicate "two photons in the same place at the same time". Otherwise this will turn into a discussion of what the photons are doing when we aren't looking at them, which will lead to much heat and very little light.
  5. How can photons interact with each other? they don't carry a charge. Except for if you mean radiative corrections, something like the attachment.
    However I wasn't able to find how to put the arrows :) [could there be an easier diagram?]

    Attached Files:

  6. Yeah, that's the simplest diagram for photon-photon scattering.
  7. Orodruin

    Staff: Mentor

    You put the arrows in one direction around the loop. The fermions are internal lines and you cannot really talk about them being particles/antiparticles. There are also additional diagrams where the order of the connected external lines are permuted with respect to your diagram.
  8. mathman

    mathman 6,437
    Science Advisor
    Gold Member

    The laser is based on the principle of coherence, i.e. photons joining together in the laser beam.
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