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How photons and static em feilds work

  1. Apr 12, 2013 #1
    The photon as I understand it is a massless particle that "must" travel at the speed of light.

    However I do not understand the idea that a photon is also the carrier of the em force.

    If there is a simple ball of static charge and a test charge comes near do photons exchange between the two charge center back and forth, confining themselves to the two charge centers
  2. jcsd
  3. Apr 13, 2013 #2


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    You can describe this force via the exchange of virtual photons, but static fields are easier to visualize without photons.
  4. Apr 13, 2013 #3


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    That's possible but missleading. Neither the potential nor the virtual photon are gauge invarian terms.

    It's more appropriate to apply physical gauge fixing, i.e. setting A° = 0, solving the Gauß constraint G|phys> = 0 and therefore derive the physical Hamiltonian H. H contains physical, transversal photons NOT carrying the static Coulomb interaction, plus a term ~ ρ(x)ρ(y)/|x-y| where ρ are the charge densities (i.e. electron fields). This works both in classical electrodynamics and in full QED.

    Therefore (virtual) photons are not required to explain the Coulomb force; they explain just dynamical fluctuations beyond the static approximation.
  5. Apr 13, 2013 #4

    Vanadium 50

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    Tom, do you think the OP is wondering about gauge fixing? Or is it more likely he read something in a popularization like "quantum mechanics is just like regular mechanics, except that it has virtual particles, which can temporarily violate the conservation of energy."?
  6. Apr 14, 2013 #5


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    I don't know. All I wanted to indicate is that the 0th (classical, static) order of the Coulomb field can be understood w/o mentioning virtual particles an w/o photons as force carriers.
    Last edited: Apr 14, 2013
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