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?(adsbygoogle = window.adsbygoogle || []).push({});

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# I QED sum over polarization states

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