  The linear coupling of a rotating heat bath to a quantum field is studied in
the framework of the Markovian master equation for the field's non-unitary time
evolution. The bath's rotation induces population inversion for the field's
low-energy modes. For bosons, this leads to superradiance, an irreversible
process in which some of the bath's kinetic energy is extracted by spontaneous
and stimulated emission. We find the energy and entropy balance for such
systems and apply our results to the theory of black hole radiation. We also
comment on how this relates to classical self-oscillations, including shear
flow instabilities in hydrodynamics.
