Experiment confirms 50-year-old
theory describing how an alien civilization could exploit a black hole
by University of Glasgow
A 50-year-old theory that began
as speculation about how an alien civilization could use a black hole to
generate energy has been experimentally verified for the first time in a
Glasgow research lab.
In 1969, British physicist Roger
Penrose suggested that energy could be generated by lowering an object into the
black hole's ergosphere—the outer layer of the black hole's event horizon,
where an object would have to move faster than the speed of light in order to
remain still.
Penrose predicted that the object
would acquire a negative energy in this unusual area of space. By dropping the
object and splitting it in two so that one half falls into the black hole while
the other is recovered, the recoil action would measure a loss of negative
energy—effectively, the recovered half would gain energy extracted from the
black hole's rotation. The scale of the engineering challenge the process would
require is so great, however, that Penrose suggested only a very advanced,
perhaps alien, civilisation would be equal to the task.
Two years later, another
physicist named Yakov Zel'dovich suggested the theory could be tested with a
more practical, earthbound experiment. He proposed that "twisted"
light waves, hitting the surface of a rotating metal cylinder turning at just the
right speed, would end up being reflected with additional energy extracted from
the cylinder's rotation thanks to a quirk of the rotational doppler effect.
But Zel'dovich's idea has
remained solely in the realm of theory since 1971 because, for the experiment
to work, his proposed metal cylinder would need to rotate at least a billion
times a second—another insurmountable challenge for the current limits of human
engineering.
Now, researchers from the
University of Glasgow's School of Physics and Astronomy have finally found a
way to experimentally demonstrate the effect that Penrose and Zel'dovich
proposed by twisting sound instead of light—a much lower frequency source, and
thus much more practical to demonstrate in the lab.
In a new paper published today in
Nature Physics, the team describe how they built a system which uses small ring
of speakers to create a twist in the sound waves analogous to the twist in the
light waves proposed by Zel'dovich.
Those twisted sound waves were
directed towards a rotating sound absorber made from a foam disc. A set of
microphones behind the disc picked up the sound from the speakers as it passed
through the disc, which steadily increased the speed of its spin.
What the team were looking to
hear in order to know that Penrose and Zel'dovich's theories were correct was a
distinctive change in the frequency and amplitude of the sound waves as they
traveled through the disc, caused by that quirk of the doppler effect.
Marion Cromb, a Ph.D. student in
the University's School of Physics and Astronomy, is the paper's lead author.
Marion said: "The linear version of the doppler effect is familiar to most
people as the phenomenon that occurs as the pitch of an ambulance siren appears
to rise as it approaches the listener but drops as it heads away. It appears to
rise because the sound waves are reaching the listener more frequently as the
ambulance nears, then less frequently as it passes.
"The rotational doppler
effect is similar, but the effect is confined to a circular space. The twisted
sound waves change their pitch when measured from the point of view of the
rotating surface. If the surface rotates fast enough then the sound frequency
can do something very strange—it can go from a positive frequency to a negative
one, and in doing so steal some energy from the rotation of the surface."
As the speed of the spinning disc
increases during the researchers' experiment, the pitch of the sound from the
speakers drops until it becomes too low to hear. Then, the pitch rises back up
again until it reaches its previous pitch—but louder, with amplitude of up to
30% greater than the original sound coming from the speakers.
Marion added: "What we heard
during our experiment was extraordinary. What's happening is that the frequency
of the sound waves is being doppler-shifted to zero as the spin speed
increases. When the sound starts back up again, it's because the waves have
been shifted from a positive frequency to a negative frequency. Those
negative-frequency waves are capable of taking some of the energy from the
spinning foam disc, becoming louder in the process—just as Zel'dovich proposed
in 1971."
Professor Daniele Faccio, also of
the University of Glasgow's School of Physics and Astronomy, is a co-author on
the paper. Prof Faccio added: "We're thrilled to have been able to
experimentally verify some extremely odd physics a half-century after the theory
was first proposed. It's strange to think that we've been able to confirm a
half-century-old theory with cosmic origins here in our lab in the west of
Scotland, but we think it will open up a lot of new avenues of scientific
exploration. We're keen to see how we can investigate the effect on different
sources such as electromagnetic waves in the near future."
The research team's paper, titled
"Amplification of waves from a rotating body," is published in Nature
Physics.