  Wave scattering phenomena are ubiquitous to almost all Sciences, from Biology
to Physics. When an incident wave scatters off of an obstacle, it is partially
reflected and partially transmitted. Since the scatterer absorbs part of the
incident energy, the reflected wave carries less energy than the incident one.
However, if the obstacle is rotating, this process can be reversed and waves
can be amplified, extracting energy from the scatterer. Even though this
phenomenon, known as superradiance, has been thoroughly analysed in several
theoretical scenarios (from eletromagnetic radiation scattering on a rotating
cylinder to gravitational waves incident upon a rotating black hole), it has
never been observed. Here we describe in detail the first laboratory detection
of superradiance. We observed that plane waves propagating on the surface of
water are amplified after being scattered by a draining vortex. The maximum
amplification measured in the experiment was 20%, obtained for 3.70 Hz waves,
in a 6.25 cm deep fluid. Our results are consistent with superradiant
scattering caused by rapid rotation. In particular, a draining fluid can
transfer part of its rotational energy to incident low-frequency waves. Our
experimental findings will shed new light on Black Hole Physics, since shallow
water waves scattering on a draining fluid constitute an analogue of a black
hole. We believe, especially in view of the recent observations of
gravitational waves, that our results will motivate further research (both
theoretical and experimental) on the observation of superradiance of
gravitational waves.
