Water circling a drain provides insight into 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 study. “But when you throw something at a black hole, if it’s a rotating black hole, you can actually gain energy.”

To demonstrate the effect, the scientists created a swirl of water. “The fluid has to drain in a way that looks like a black hole,” says physicist Antonin Coutant, also at Nottingham. Surface ripples reach a point of no return where they are sucked into the vortex. That’s analogous to a black hole’s event horizon, the boundary from which no light can escape. Weinfurtner, Coutant and colleagues report that water waves scattering off the vortex got a superradiant boost: They were amplified by up to 14 percent on average, depending on the frequency and direction of the waves.

For obvious reasons, researchers can’t study a real black hole in a laboratory. If they could, “we’d all be in trouble,” says physicist Sam Dolan of the University of Sheffield in England, who was not involved with the study. A water vortex is the next best thing. The result, Dolan says, “gives us more confidence that our theories about black holes are correct.”

Although rotational superradiance is a weak effect in black holes, there may be opportunities to observe it, says physicist Vítor Cardoso of Instituto Superior Técnico in Lisbon, Portugal. Superradiance affects gravitational waves as well as light waves. Ripples in spacetime stirred up by merging black holes (SN Online: 6/1/17) should be slightly amplified if those black holes are spinning. That amplification could be observed by future ultrasensitive gravitational wave detectors.