Water swirling down a drain has exposed an elusive phenomenon long believed to appear in black holes. Light waves scattering off a rotating black hole can bounce off with more energy than they came in with, by sapping some of the black hole’s rotational energy. But the effect, predicted in 1971 and known as rotational superradiance, is so weak that it would be extremely difficult to observe in a real black hole. So scientists had never seen rotational superradiance in action, until now.
https://www.sciencenews.org/article/water-circling-drain-provides-insight-black-holes
