Superradiance without event horizons in General Relativity

TL;DR

This paper investigates superradiant scattering in non-black-hole systems within General Relativity, focusing on rotating stars and charged spheres, highlighting mechanisms beyond event horizons that enable wave amplification.

Contribution

It demonstrates superradiance can occur without event horizons, analyzing energy dissipation and wave amplification in rotating stars and charged spheres.

Findings

Superradiance is possible without event horizons.

Energy dissipation plays a key role in superradiance.

Wave amplification occurs in rotating stars and charged spheres.

Abstract

Superradiant scattering processes are studied in general relativistic systems which, unlike rotating and/or charged black holes, do not exhibit an event horizon. Inspired by Zel'dovich's seminal works on the amplification of waves by a rotating cylinder, we analyse, in the context of General Relativity, the possibility of superradiance for electromagnetic waves reflecting off a rotating star and for charged scalar perturbations impinging on a charged sphere. The role of energy dissipation in these systems is analysed and compared with the role of the event horizon in black hole superradiance.