Energy from Black Holes can be stolen, shows an elegant new physics experiment

Rotational superradiance detected for the first time in a water vortex flow which may mimic the effect of some kind of black holes

Water waves can gain energy when they scatter from a whirlpool-like vortex. That is the conclusion of physicists in Brazil, Canada and the UK, who are the first to observe a phenomenon called "rotational superradiant scattering". The team says that the effect could be used to study black-hole physics.

Silke Weinfurtner and colleagues at the University of Nottingham, Universidade Federal do ABC and University of British Columbia observed the effect using a rectangular water tank 1.5 m long and 3 m wide. Water is pumped continuously into one corner of the tank and allowed to drain through a 4 cm-diameter hole in the middle of the tank – creating a familiar draining vortex.

From the paper’s abstract:

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% ± 8%, obtained for 3.70 Hz waves, in a 6.25-cm-deep fluid, consistent with the superradiant scattering caused by rapid rotation. We expect our experimental findings to be relevant to black-hole physics, since shallow water waves scattering on a draining fluid constitute an analogue of a black hole, as well as to hydrodynamics, due to the close relation to over-reflection instabilities.

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.

"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."

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