  We revisit the Fulling-Davies-Unruh effect in the context of two-level single
and entangled atomic systems that are either uniformly accelerated or static in
a thermal bath. We consider the interaction between the systems and a massless
scalar field, covering the scenarios of free space as well as within a cavity.
Through the calculation of atomic transition rates, it is found that in free
space there is an equivalence between a uniformly accelerated atom with respect
to an observer with that of a single atom which is static with respect to the
observer and immersed in a thermal bath, as long as the temperature of the
thermal bath matches the Unruh temperature. This equivalence breaks down in the
presence of a cavity. For two-atom systems, we consider the initial state to be
in a general pure entangled form. We find that in this case, the equivalence
between the accelerated and static thermal bath scenarios holds only under
specific limiting conditions in free space but breaks down completely in a
cavity set-up.
