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Longitudinal polarization in Feynman photon propagator?

  1. Dec 11, 2014 #1
    For internal photon states, is it necessary to sum over the longitudinal polarization state in addition to the transverse states? And if so, does the ordinary Feynman-gauge propagator take care of this?

  2. jcsd
  3. Dec 11, 2014 #2


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    No. However, the longitudinal states do not couple to the conserved electromagnetic current, so it doesn't matter whether you include them or not. Thus the ##k^\mu k^\nu## term in the propagator can be multiplied by an arbitrary number (this is sometimes called "generalized Feynman gauge"), and the scattering amplitudes do not depend on that number.
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