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The nature of photons

  1. Jan 19, 2008 #1
    Can anyone tell me the true nature of a photon? One physist tells me they don´t exist, another says they vary in size and now I am told they are paired masses. When I originally asked if one could split the photon, I thought it was partially material, i.e. consisting of colour with an element of weight and partially made out of, well, energy. I would also like to know if photons can be artifically manipulated, especially with regard to their size? I choose to call them wave packets. Would this be correct?
  2. jcsd
  3. Jan 19, 2008 #2
    If you are in the territory of semi-classical physicists, that would be correct. But if you are in the territory of QFT physicists, never call them "wave packets", they may shoot you...
  4. Jan 19, 2008 #3


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    Of course the photon exists :P

    A photon is a photon, and we can assign models to describe a photon.

    In the standard model the photon is a massless spin 1 particle with no internal structure (a point-particle, just as the quarks and leptons) etc..
  5. Jan 19, 2008 #4


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    Hmmm. Yes, but you'll need to master quite a lot of quantum field theory to
    understand the entire answer. :-(

    Well, the theory of quanta of the electromagnetic field (quantum electrodynamics)
    is one of the most stunningly accurate theories on Earth, so it's hard to guess
    (out of context) what that physicist meant.

    A photon is an eigenstate of the "number operator" in quantum electrodynamics.
    It is therefore also an eigenstate of the energy-momentum operator. I.e: it has
    determinate energy and momentum. But by the Heisenberg uncertainty principle,
    that implies its position is totally indeterminate. However, one can prepare quantum
    states of the electromagnetic field which have a minimum (but non-zero) uncertainty
    in momentum and position simultaneously (these roughly correspond to a laser beam).
    For such states the number of photons is indeterminate. Heuristically, you might
    think of these states as wave packets, -- but only heuristically. Only the full theory
    of quantum electrodynamics (or rather, the Standard Model) can tell the whole story.

    Although the quantized EM field is structure-less (i.e., not composed of anything more
    fundamental as far as we know) it can be misleading to say a photon is a "point particle",
    because that implies it exists at one point of space only. However, as explained above, the
    position of a single photon is totally indeterminate.

    I'm guessing you were told about how a (high-energy) photon can turn into
    an electron-positron pair. That's not quite the whole story. A photon in isolation
    doesn't do this (it would violate conservation of energy-momentum). However,
    during interactions with other things, it can occur. But this doesn't mean
    that the photon is "composed" of an electron and positron.

    The colour corresponds to the frequency [itex]\nu[/itex], which also corresponds to the
    energy E via Planck's relationship: [itex]E = h \nu[/itex] where h is Planck's constant.

    I don't know what you mean by "partially material". The photon's rest mass is zero.

    Fundamental particles are not "made" out of energy. Rather, "energy" is one of their
    properties that we can measure experimentally.

    People who work in quantum optics prepare strange states of light all the time,
    in all sorts of configurations. But these states consist of a superposition of photons.
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