In a Glasgow research lab, a 50-year-old idea that started out as
conjecture about how an extraterrestrial society may harness a black hole to
generate energy has been empirically proven for the first time.
Roger Penrose, a British scientist, proposed in 1969 that energy may be
produced by lowering an item into the ergosphere, the outermost region of
the black hole's event horizon, where an object would have to travel faster
than light to stay still.
Penrose anticipated that in this peculiar region of space, the item would
pick up negative energy. The recoil action would measure a loss of negative
energy if the object was dropped and divided in half, with one half falling
into the black hole and the other being retrieved. In other words, the
recovered half would acquire energy from the black hole's spin. Penrose
proposed that only a highly developed, maybe extraterrestrial civilization
might be capable of completing the procedure due to the immense engineering
difficulty it would entail.
Two years later, Yakov Zel'dovich, a different scientist, proposed that a
more realistic, on-Earth experiment might test the idea. He postulated that
a peculiarity of the rotational doppler effect would cause "twisted" light
waves to be reflected with extra energy taken from the cylinder's rotation
when they struck the surface of a revolving metal cylinder spinning at
precisely the proper speed.
Zel'dovich's experiment would need a metal cylinder to revolve at least a
billion times per second, which is beyond the existing capabilities of human
engineering. As a result, Zel'dovich's notion has remained only theoretical
since 1971.
The effect that Penrose and Zel'dovich proposed has now finally been
experimentally demonstrated by researchers from the University of Glasgow's
School of Physics and Astronomy by twisting sound instead of light, a much
lower frequency source and therefore much more practical to demonstrate in
the lab.
The team describes how they created a system that employs a tiny ring of
speakers to generate a twist in the sound waves that is similar to the twist
in the light waves that Zel'dovich envisioned in a new publication that was
published today in Nature Physics.
The revolving foam disc sound absorber was the target of the bent sound
waves. The sound from the speakers was picked up by a series of microphones
behind the disc, which caused the disc to spin faster and faster as it went
past.
The researchers needed to detect a noticeable shift in the frequency and
amplitude of the sound waves as they passed through the disc, which was
brought on by that peculiarity of the doppler effect, to confirm that
Penrose and Zel'dovich's hypotheses were accurate.
The principal author of the article is Marion Cromb, a Ph.D. candidate in
the University's School of Physics and Astronomy. According to Marion, "The
majority of people are familiar with the linear form of the doppler effect,
which is the phenomena where the pitch of an ambulance siren appears to
raise as it approaches the listener and drop as it moves away. It seems to
climb because as the ambulance gets closer, the listener hears the sound
waves more often, and as it passes, the listener hears them less
often.
"The effect is limited to a circular region, but the rotating doppler
effect is comparable. When seen from the perspective of the revolving
surface, the twisted sound waves exhibit a shift in pitch. The sound
frequency can perform an extremely odd thing if the surface spins quickly
enough: it can go from a positive to a negative frequency and take some
energy from the surface's rotation in the process."
The pitch of the sound coming from the speakers lowers until it is
inaudible during the researchers' experiment as the speed of the rotating
disc rises. After after, the pitch increases once again until it reaches its
initial pitch, but this time it sounds louder and has an amplitude that can
be up to 30% higher than what was initially audible from the speakers.
"What we heard during our experiment was extraordinary," Marion said. What
is actually occurring is that when the spin speed rises, the sound waves'
frequency is doppler-shifted to zero. The reason the sound comes back is
that the waves have changed from having a positive frequency to a negative
frequency. As Zel'dovich suggested in 1971, the negative-frequency waves are
able to absorb part of the energy from the rotating foam disc and increase
in volume as a result."
Co-author of the article is Professor Daniele Faccio of the University of
Glasgow's School of Physics and Astronomy. A half-century after the
hypothesis was initially put forward, Prof. Faccio continued, "We're
thrilled to have been able to experimentally verify some extremely odd
physics." It's surreal to consider that in our lab in the west of Scotland,
we have been able to prove the cosmic roots of a fifty-year-old idea, but we
believe this will pave the way for many new scientific investigations. In
the near future, we're interested in learning how we might look at the
impact on other sources, such electromagnetic waves."
"Amplification of waves from a rotating body," the study team's work, is
published in Nature Physics.
