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Water circling a drain provides insight into black holes

Energy boost from rotational superradiance detected for the first time

11:00am, June 12, 2017
ripples off a vortex

DOWN THE DRAIN  Water ripples can ricochet off a vortex (shown), gaining energy. Scientists have now observed this effect, known as rotational superradiance, for the first time. The effect is also expected to occur in rotating black holes.

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. Now, physicists report June 12 in Nature Physics that they’ve glimpsed the effect for the first time, in a black hole doppelgänger made with a vortex of water, similar to water swirling down a bathtub drain.

“If you take a tennis ball and you throw it against a wall, you don’t expect it to come back with more energy,” says Silke Weinfurtner of the University of Nottingham in England, who led the

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