Superradiant scattering from a hydrodynamic vortex

Abstract

Sound waves scattered from a hydrodynamic vortex may be amplified. Such superradiant scattering follows from the physical analogy between spinning black holes and hydrodynamic vortices, as spinning black holes have an ergoregion within which low frequency waves may be scattered with increased amplitude. While black holes also have an event horizon, a fluid's analogous sonic horizon requires the vortex to have a central drain, which may be challenging to produce experimentally. We show that in the fluid analogue, a drain is not required in order for a vortex to scatter sound superradiantly. Furthermore, this effect may occur even when the fluid density drops to zero at the vortex core, as is the case in a Bose-Einstein condensate. We also consider engineering the density profile, using repulsive light forces, to extend the validity of the hydrodynamic approximation towards the vortex core.