Horizons and Ergoregions in Superfluids

Abstract

Ripplons -- gravity-capillary waves on the free surface of a liquid or at the interfaces between two superfluids -- are the most favourable excitations for simulation of the general-relativistic effects related to horizons and ergoregions. The white-hole horizon for the ``relativistic'' ripplons at the surface of the shallow liquid is easily simulated using the kitchen-bath hydraulic jump. The same white-hole horizon is observed in quantum liquid -- superfluid 4He. The ergoregion for the ``non-relativistic'' ripplons is generated in the experiments with two sliding 3He superfluids. The common property experienced by all these ripplons is the Miles instability inside the ergoregion or horizon. Because of the universality of the Miles instability, one may expect that it could take place inside the horizon of the astrophysical black holes, if there is a preferred reference frame which comes from the trans-Planckian physics. If this is the case, the black hole would evapotate much faster than due to the Hawking radiation. Hawking radiation from the artificial black hole in terms of the quantum tunneling of phonons and ripplons is also discussed.