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Electromagnetic field vs. photon(s)

  1. Jul 13, 2007 #1
    Inspired by another thread, where the discussion drifts toward the (Schroedinger) wave function of photons, I rather like to focus on the electromagnetic (EM) field and its relation to photon(s):

    What actually is the relation between an EM field and one or more photons. Which, if any, of the following statements comes anywhere near a true statement?
    • A photon is best described by an EM field.
    • Entangled photons share the same EM field.
    • An EM field gives rise to a photon when it interacts with, say, an electron.
    • In the latter case the EM field ceases to exist.
    • No, it depends: if all its energy is used up, it disappears, but if there is energy left, it continues to exist, and this is where the rest of it represents a photon that what previously entangled with the photon swallowed by the electron.
  2. jcsd
  3. Jul 13, 2007 #2
    hm ... I'm not sure I'd agree with any of those statements!

    I usually think of the EM field as the classical field satisfying the Maxwell equations, and the photon as the carrier of the field in Quantum Electrodynamics. Of course, in the "first quantized" picture of the Schroedinger Eq., the EM field is still kind of classical.

    In any case, rather than your statements, I would say, "an EM field may by thought of as the exchange of an infinite number of virtual photons." In other words, if two charge particles interact, then their interaction consists of the exchange of photons. That's what it means to say that one of them "feels" the field of the other; they're throwing photons back and forth.

    If you have an EM field, you must have photons, and vice versa; and if you don't, you don't.

    In QED, by the way and in case you didn't know this, the full effect of the interaction between these two particles is described as the sum of the effects of individual interactions involving at least two photons, but also many more, up to infinity. The contributions from the interactions involving more photons become increasingly smaller, however, so they can generally be neglected.

    Does that help at all?

    - Bruce
  4. Jul 13, 2007 #3
    Ok, so

    two charge[d?] particles interact => photons are exchanged​

    Can we turn this around and say
    photon from computer screen hits my retina => charged particles in retina and computer screen interact?​

    This bit about exchange of an infinite number virtual photons sound, eeeehm, ....:confused:

  5. Jul 13, 2007 #4
    Absolutely you can say that - the photons from computer screen interact with the electrons on atoms on your retina, no question about it.

    With light (i.e. an EM wave) it's not as difficult to think about photons since you already have something moving from one place to another with a well defined velocity. With a static EM field it's harder to picture, but this is where the exchange of "virtual" photons comes in.

    If you want to know more about that exchange of infinite numbers of photons, do a web search for Quantum Electrodynamics or Quantum Field Theory. There are a lot of intro. level explanations out there.

    - Bruce
  6. Jul 16, 2007 #5


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  7. Jul 18, 2007 #6

    We discussed this topic many times before :


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