Water circling drain experiments offer insight into black holes

This artist’s concept depicts a supermassive black hole
at the center of a galaxy. The blue color here represents
radiation pouring out from material very close to the black
hole. The grayish structure surrounding the black hole, called
a torus, is made up of gas and dust. Credit: NASA/JPL-Caltech

A small international team of researchers has found that
water waves created due to scattering from a spinning vortex
can show rotational superradiance—an effect astrophysicists
have predicted likely to occur in black holes, but which has
never been replicated in a lab experiment. In their paper
published in the journal Nature Physics, the group
explains how they observed and measured waves propagating on
the surface of water near a draining vortex and what they
found.

As the researchers explain, when a wave strikes an obstacle, it
tends to scatter, as can be observed at virtually any seashore.
But more difficult to see is that some of the wave is reflected
as well as partially transmitted. This led to a theory back in
1954 by Robert Dicke that suggests if an object is spinning,
the waves can be amplified by extracting energy from the parts
of the wave that are scattered—a phenomenon called
superradiance. In this new effort, the researchers conducted
experiments designed to prove the theory correct.

The experiments consisted of placing water in a 3 x 1.5 meter
tank with a 4 cm hole at the center to serve as a drain—the
researchers took measurements of wave activity by sensors
mounted on the side of the tank (and by a high-speed,
three-dimensional air–fluid interface sensor) as pumped-in
water was drained, creating a . The researchers report that they observed
waves propagating on the surface and that measurements
confirmed the waves were amplified after scattering occurred.
They further report that the largest amplification recorded was
14 percent +/– 8 percent with waves of 3.70Hz in water that was
just 6.25 cm deep. They claim their findings agree with theory,
and therefore that their findings can be applied to research
surrounding black holes. This is because they believe the
scattering of shallow waves on water is analogous to the action
that occurs at the event horizon of a black hole. They also
note that new, more sensitive gravitational wave detectors
might someday be able to measure roughly the same behavior with
real .

Press
release
from University of Nottingham

Explore further:

Researchers uncover new gravitational wave characteristics

More information: Theo Torres et al. Rotational
superradiant scattering in a vortex flow, Nature Physics
(2017). DOI:
10.1038/nphys4151

© 2017 Phys.org

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