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I QED sum over polarization states

  1. Sep 8, 2018 #1
    Hello! In the calculation of the QED matrix element, it says in the book I read that we have to sum over the polarization states of the photon: $$\sum_\lambda \epsilon_\mu^\lambda\epsilon_\nu^{\lambda *}=-g_{\mu\nu}$$ I am a bit confused why do we do a summation over the orthonormal basis describing the polarization and we don't do an integral over all possible polarization states? Is it any reason why the photon would leave in an eigenstate and not in a linear combination of polarization basis?
  2. jcsd
  3. Sep 13, 2018 #2
    Thanks for the thread! This is an automated courtesy bump. Sorry you aren't generating responses at the moment. Do you have any further information, come to any new conclusions or is it possible to reword the post? The more details the better.
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