Current Commutators and Electromagnetic Mass Differences

Abstract
A version of the Jost-Lehmann-Dyson representation is used to study the forward-spin-averaged "Compton" amplitude constructed from conserved currents. Bjorken's relationship between the divergence in electromagnetic mass shifts and the equal-time commutator of a current density and a time derivative of a current density is clarified and extended to include the case where subtractions in the amplitude are necessary. The connection between equal-time commutators and some features of the radiative corrections to β decay are discussed, and some properties of Schwinger terms are developed.