Black Hole Superradiance in Rotating Fluids (SURF)

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Some of the most fundamental and perhaps bizarre processes expected to occur in the vicinity of black holes are out of observational reach. To address this issue we utilise analogue systems where we study fluctuations on a background flow that in the experiment reproduces an effective black hole. In the literature this line of research is referred to as analogue models for gravity, or simply analogue gravity. Analogue models provide not only a theoretical but also an experimental framework in which to verify predictions of classical and quantum fields exposed to 'extreme' spacetime geometries, such as rapidly rotating black holes. This project brings together two world-wide recognised experts in the field of analogue gravity with the aim of pushing the field in a new direction: we propose ground-breaking studies to mimic some of the bizarre processes occurring in the vicinity of rotating black holes from general relativity and rotating fluids in both water and optical systems. In particular, we will investigate both theoretically and experimentally the interaction between an input wave and a rotating black hole spacetime geometry, here recreated by the rotating fluid. This allows us to mimic a scattering process associated to rotating black hoes called superradiant scattering. From a historical viewpoint this kind of radiation is the precursor to Hawking radiation. More precisely, black hole superradiance is the scattering of waves from a rotating black hole: if the incoming wave also possesses a small amount of angular momentum, it will be reflected with an increased amplitude, i.e. it is amplified at the expense of the black hole that thus loses some of its rotational energy. It has also been pointed out that the same physics may take place in very different systems, for example light incident on a rotating metallic (or absorbing) cylinder may also be amplified upon reflection. Yet, no-one has ever attempted to experimentally investigate the underlying physics that extend beyond general relativity and are relevant to a variety of hydrodynamical and rotating systems.We aim to provide the first ever experimental evidence of this intriguing and fundamental amplification mechanism in two different hydrodynamical systems. The first is a water spout, controlled so that the correct boundary conditions are obtained and optimised for observing BH-SS. The second is a less conventional fluid that is made out of light. Light propagating in a special medium can behave as a fluid or even a superfluid. By building upon highly developed photonic technologies e.g. for the control and measurements of laser beam wavefronts, we will implement very precisely tailored and characterised experiments. One of the unique aspects of this project is the marriage between two very different lab-based systems, one using water the other using light, to tackle an outstanding problem in physics that is of relevance to astrophysics, hydrodynamic and optical systems.