Emergence of an acoustic black hole in a lattice model

Abstract

We build an acoustic black hole in a lattice model: a draining vortex evolved by the local collide-and-stream rule of a lattice-Boltzmann model, with no acoustic metric imposed, the geometry being measured from the steady state. A horizon emerges, first located from the measured Mach profile and then confirmed by phonon ray tracing on the measured metric, and the ringdown after a perturbation follows approximately the quasinormal frequencies of the draining bathtub. The residual departures are traced to the cell-scale structure of the horizon, which is not an ideal absorber. We model it in two ways: as a reflective horizon on the ideal bathtub, with two free parameters (the modulus and phase of the reflectivity), and as the sonic point of the realised flow absorbing in its own frame, with no adjustable parameter. The second describes the horizon and the non-rotating spectrum best. Superradiance has two ingredients: the emergent geometry, which amplifies in a loss-free calculation on the measured metric, and the horizon, whose absorption frame sets the width and strength of the amplified band but leaves no trace in the ringdown. Both models are used to examine superradiance, which has not been observed in the present toy model: whether the lattice hole superradiates dynamically remains open. The lattice leaves its own imprint as well: a fourfold anisotropy of the sonic surface and a surface gravity set by the cell-scale structure of the horizon.