Electron interaction with the angular momentum of the electromagnetic field

We give a simple derivation and explanation of a recently proposed new relativistic interaction between the electron and the angular momentum of the electromagnetic field in quantum electrodynamics (QED). Our derivation is based on the work of Moller, who pointed out that, in special relativity, a particle with spin must always have a finite extension. We generalize Mollers classical result to the quantum regime and show that it leads to a new contribution to the energy, which is the special relativistic interaction term. In addition, we show that all spin terms arising from the Dirac equation may be obtained by this method.