Electromagnetic Mass Shifts, Equal-Time Commutators, and Jost-Lehmann Representation

Electromagnetic mass shifts, equal-time commutators (ETC), and their relation are analyzed, using the Jost-Lehmann representation for the forward Compton amplitude. The divergences in $\delta m$ are determined by Lorentz scalars associated with the ETC (when these exist). These results are illustrated by models with currents bilinear and linear in canonical fields. Existence of the ETC implies that the Jost-Lehmann representation is unsubtracted.