  Although the thermal and radiative effects associated with a two-level
quantum system undergoing acceleration are now widely understood and accepted,
a surprising amount of controversy still surrounds the simpler and older
problem of an accelerated classical charge. We argue that the analogy between
these systems is more than superficial: There is a sense in which a "UD
detector" in a quantized scalar field effectively acts as a classical source
for that field if the splitting of its energy levels is so small as to be
ignored. After showing explicitly that a detector with unresolved inner
structure does behave as a structureless scalar source, we use that analysis to
rederive the scalar version of a previous analysis of the accelerated
electromagnetic charge, without appealing to the troublesome concept of
"zero-energy particles." Then we recover these results when the detector energy
gap is taken to be zero from the beginning. This vindicates the informal
terminology "zero-frequency Rindler modes" as a shorthand for "Rindler modes
with arbitrarily small energy." In an appendix, the mathematical behavior of
the normal modes in the limit of small frequency is examined in more detail
than before. The vexed (and somewhat ambiguous) question of whether
coaccelerating observers detect the acceleration radiation can then be studied
on a sound basis.
