1. Sep 3, 2011

### knightq

Take the scattering of photon and electron as example, we first figure out the amptitude for transition definte polarization,
say f(ϵα(k),ϵα′(k′))
If the initial photon is unpolarized and the final polarization is not observed, we need to average the initial polarization and sum over the possible final polarization.
But why we just average and sum over the two independent polarization,1/2∑α∑α′f? the initial and final polarization should be all possible combination of the independent polarizatition.