Operator Gauge Transformations
E. Corinaldesi, P. Román · American Journal of Physics · 1965
The Gupta-Bleuler formulation of the subsidiary condition of quantum electrodynamics in the interaction picture is reviewed. This formalism is used to construct a simple proof of the invariance of the S-matrix under q-number gauge transformations A′μ = Aμ+δμF(□)δνA or, equivalently, under certain well-known modifications of the photon propagator.