Electromagnetic Interactions with an Arbitrary Loosely Bound System

Abstract

We present an electromagnetic interaction Hamiltonian for an arbitrary $N$-particle system with any mass, charge, spin, and magnetic moment which is consistent with the low-energy theorem for Compton scattering and the Drell-Hearn-Gerasimov sum rule to order ${\mathrm{}\left(\mathrm{mass}\right)\mathrm{}}^{-2\mathrm{}}$. It is related to the sum of individual Foldy-Wouthuysen interactions by a unitary transformation.