  The momentum of a free massive particle, invariant under translation, thereby
realizes a trivial representation of the translation group. By allowing
nontrivial reps of translations, momentum changes with translation, a recipe
for force. Here the procedure is applied to the conventional construction of a
free quantum field using spacetime symmetries, yielding a more general field
with the free field as a special case. It is shown that a particle described by
the quantum field follows the classical trajectories of a massive charged
particle in electromagnetic and gravitational fields.
