Photon polarization completeness relation
= Photon polarization completeness relation
{title2=$\sum_r\epsilon_i^{(r)}\epsilon_j^{(r)*}=\delta_{ij}-k_ik_j/|\mathbf k|^2$}
Two orthonormal transverse <polarization vectors> span the plane perpendicular to a nonzero momentum. Summing their dyadic products gives the spatial transverse projector. It has trace and rank two and acts as the identity on physical <photon> polarizations.