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Rotational superradiant scattering in a vortex flow

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When an incident wave scatters off of an obstacle, it is partially reflected and partially transmitted. In theory, if the obstacle is rotating, waves can be amplified in the process, extracting energy from the scatterer. Here we describe in detail the first laboratory detection of this phenomenon, known as superradiance. We observed that waves propagating on the surface of water can be amplified after being scattered by a draining vortex. The maximum amplification measured was 14%, obtained for 3.70 Hz waves, in a 6.25 cm deep fluid, in consistency with superradiant scattering caused by rapid rotation.  Our experimental findings will shed new light not only on Black Hole Physics, since shallow water waves scattering on a draining fluid constitute an analogue of a black hole, but also on hydrodynamics, due to its close relation to over-reflection instabilities. We believe, in view of the recent detections of gravitational waves, that our results will motivate further research on the observation of superradiance in Astrophysics.

This talk is part of the Institute of Astronomy Colloquia series.

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