The many faces of superradiance

TL;DR

This paper reviews inertial and rotational superradiance, proves thermodynamic conditions for superradiance, corrects previous derivations, and demonstrates laboratory measurability of superradiance with a rotating dielectric cylinder.

Contribution

It provides a comprehensive review, thermodynamic proof, and detailed electrodynamics analysis of superradiance, including a new black hole superradiance example and experimental feasibility.

Findings

Superradiance conditions coincide with the Ginzburg--Frank criterion.

Electrodynamics of a Zel'dovich rotating cylinder is fully detailed.

Superradiance can be measured in laboratory setups with a rotating dielectric cylinder.

Abstract

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 a 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.