 Feynman gauge

Feynman gauge is the $\xi =1$ choice in the covariant gauge family with gauge-fixing term $-({\partial }_{\mu }{A}^{\mu }{)}^2/(2\xi )$. With metric signature $(+---)$ and the usual vacuum prescription, the free photon propagator becomesIts simple numerator makes diagram calculations convenient, while keeping Lorentz covariance manifest. The four components of $A_{\mu }$ are still not four independently observable photon polarizations; physical external photons have two transverse polarizations. The Ward identity ensures cancellation of unphysical contributions in complete gauge-invariant amplitudes. Feynman gauge is a computational choice, and changing the gauge requires consistent changes throughout the propagators and related terms.