ar X iv : g r - qc / 9 80 30 33 v 1 1 0 M ar 1 99 8 The many faces of superradiance

Inertial motion superradiance, the emission of radiation by an initially unexcited system moving inertially but superluminally through a medium, has long been known. Rotational superradiance, the amplification of radiation by a rotating rigid object, was recognized much later, principally in connection with black hole radiances. Here we review the principles of inertial motion superradiance and prove thermodynamically that the Ginzburg–Frank condition for superradiance coincides with the condition for superradiant amplification of already existing radiation. Examples we cite include a new type of black hole superradiance. We correct Zel’dovich’s thermodynamic derivation of the Zel’dovich–Misner condition for rotational superradiance by including the radiant entropy in the bookkeeping . We work out in full detail the electrodynamics of the Zel’dovich rotating cylinder, including a general electrodynamic proof of the Zel’dovich–Misner condition, and explicit calculations of the superradiant gain for both types of polarization. Contrary to Zel’dovich’s pessimistic conclusion we conclude that, if the cylinder is surrounded by a dielectric jacket and the whole assembly is placed inside a rotating cavity, the superradiance is measurable in the laboratory. Typeset using REVTEX E–mail: [email protected] Also at the Department of Applied Mathematics and Statistics, University of Campinas, Campinas SP, Brazil E–mail: [email protected]