  In this note we discuss the invariance under general changes of reference
frame of all the physical predictions of particle detector models in quantum
field theory in general and, in particular, of those used in quantum optics to
model atoms interacting with light. We find explicitly how the light-matter
interaction Hamiltonians change under general coordinate transformations, and
analyze the subtleties of the Hamiltonians commonly used to describe the
light-matter interaction when relativistic motion is taken into account.
