A 50-year theory, which began as speculation about how an alien civilization could use a black hole to generate power, has been confirmed at the University of Glasgow.
Back in 1969, a British physicist called Roger Penrose proposed that energy could be created by dropping an object into the so-called ergosphere of the black hole, the outer layer of the black hole’s event horizon, where an object would have to travel faster than the speed of light in order to stay still.
Penrose predicted that the object would obtain negative energy in this particular area of space. By releasing the object and dividing it in two so that one half falls into the black hole while the other recovers, the recoil action would measure a loss of negative energy; indeed, the recovered half would gain energy drawn from the rotation of the black hole.
However, the scale of the engineering challenge that the process would require is so great that Penrose suggested that only a highly advanced, perhaps bizarre, civilization would be capable of the task.
Two years later, another physicist named Yakov Zel’dovich suggested that the theory could be tested with a more practical and terrestrial experiment.
He proposed that “twisted” light waves, which hit the surface of a rotating metal cylinder rotating at the correct speed, would eventually reflect off additional energy drawn from the cylinder’s rotation thanks to a peculiarity of the rotational Doppler effect.
Zel’dovich’s idea has only remained 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 per second, another insurmountable challenge for current limits for human engineering.
Now, researchers at the University of Glasgow School of Physics and Astronomy have finally found a way to experimentally demonstrate the effect Penrose and Zel’dovich proposed by twisting sound rather than light, a much lower frequency source, and therefore much more practical to demonstrate in the laboratory.
In a new paper published in Nature Physics, the team describes how they built a system that uses a small ring of loudspeakers to create a twist in sound waves analogous to the twist in light waves proposed by Zel’dovich.
Those twisted sound waves were directed at a spinning sound absorber made of a foam disk.
A set of microphones behind the disc picked up the sound from the speakers as it passed through the disc, constantly increasing the speed of its spin.
What the team sought to hear to find out 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 peculiarity of the Doppler effect.
Marion Cromb, student of the Faculty of Physics and Astronomy of the University, and main author of the study explained:
“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 disk increases during the researchers’ experiment, the pitch of the sound from the speakers decreases until it becomes too low to hear. The tone then rises again until it reaches its previous, but louder, tone, with amplitude up to 30% greater than the original sound coming from the speakers.