  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.
