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Light interaction/interference

  1. Jul 9, 2010 #1
    Simply thinking about how certain things work, one thing leads to another; I wind up at fiber optics and am reminded of a question I thought of a little more than a year ago while I was taking physics (HS). It had to do with energy, however I'm more concerned with light at the moment, so I ask this:

    -do/can photons interact with each other?
    -does light cause distortion by itself? (does light have or is it in and of itself an electromagnetic field?)
    -I was going to ask about intersecting light but when I 'looked' at the example in my head of perpendicular intersecting beams or lasers and I imagined an infinitely vast space between two photons of the first beam and that the perpendicular beam's photons would always end up between and never intersect with or 'touch' the photons of the perpendicular path). This leads to the next.
    -can two or more photons be at the exact same place at the exact same time, or would they interfere with each other and separate?

    If anyone is curious, the original question I mentioned that I had before was: Is there a limit to how much energy can be in one place at a time? My instructor replied no, although I'm not sure how certain they were of that answer.
  2. jcsd
  3. Jul 9, 2010 #2
    Light IS an electromagnetic field. The fields have to be arranged in a certain way (self-sustaining), but beyond that light is nothing more and no less than E&M fields. I just confused myself about whether 'photons' (the particle quanta) can 'interact'; but in general E&M waves will just superpose as they pass 'through' each-other. I.e. there will be an observable difference when they are in the same place at the same time (constructive/destructive interference), but they won't alter each-other (it might not be that simple from a photon-perspective ???).

    According to classical physics there is no limit to how much energy can be in a given region--too much will collapse into a black-hole. Quantum Field Theory--I don't think--would create any limits either; but string-theory definitely would (when you get to planck energies at the planck scale). I'd say the best overall answer is that we don't really know, but if there is a limit, its REALLY big; but one way or the other it would always collapse to a black hole first.
  4. Jul 9, 2010 #3
    Particle physics treats photons as Bosons ('force' particles) of full integer spin; this is in contrast to Fermions ('matter' particles) of half integer spins. Bosons do not follow the exclusion principle so they are allowed to 'occupy the same quantum states'; another way of saying this is that they can occupy the same space; they most certainly can 'phase-through' each other.

    My QM is a little rusty, so I can't really explain interference?
  5. Jul 9, 2010 #4


    Staff: Mentor

    To first order, no.

    It is an electromagnetic field, but the equations are linear so it will not cause any distortion.

    They can be in the same state, no problem.

    If you have enough energy in one place you get a black hole. Our current theories do not have a good description of the "core" of a black hole.
  6. Jul 10, 2010 #5
    Not only can multiple photons be at the same place at the same time, but in certain cases they will "prefer" to be. For example, when a photon of the appropriate frequency is incident upon an excited atom of the appropriate kind, the incident photon will STIMULATE the atom to emit a photon in the same state as the incident photon. This effect, first suggested by Einstein, is the basis of LASERs ... Light Amplification through the Stimulated Emission of Radiation. Laser light is highly collimated; the photons diverge only very gradually, compared to the radiation from other sources. The fact that they diverge at all can be attributed to the Uncertainty Principle, which allows for a small spread in their momentum perpendicular to the laser light's propagation axis.
  7. Jul 10, 2010 #6
    This question got me thinking. Can I pick up on the answer given by DaleSpam:

    "no to first order"

    I take this a reference to the Feynman diagram expansion of quantum electodynamics. This is saying that there are no tree diagrams describing the direct interaction of 2 photon states. At higher order it is possible that a photon can turn into an electron-positron pair and hence interact with other photons some how.

    But is there not a more basic way in which photons interact?

    My understanding is this..please correct me if I've got this wrong.

    The basic Feynman diagram shows two photons moving in free-space with definite momentum. In this case then, to first order, two photon states can pass through each other without interaction (that was the point about linear equations).

    But quantum mechanics is more complex than this. Depending on how a photon is generated it can have many different spacial wave functions. One notable case then is the spacial wavefunction of a photon generated by a double slit. In this case the photon wave function interferes with itself, and this interference determines the probability of where the photon will end up.

    I guess the correct term for this is light self-interference rather than self-interaction.
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