A black hole as an energy source?
We know black holes as powerful singularities, regions in space-time where gravity is so overwhelming that nothing, not even light itself, can escape.
About 50 years ago, British physicist Roger Penrose proposed that black holes could be a source of energy. Now researchers at the University of Glasgow in Scotland have shown that it may be possible.
Marion Cromb is the main author of this new study. They are PhD students at the Faculty of Physics and Astronomy at the University of Glasgow. The article is titled “Amplification of Rotatable Body Waves.” It is published in the journal Nature Physics.
People interested in space and science know that black holes have a singularity at the center and an event horizon, the limit over which nothing can return once it passes. But black holes have other elements in their complex structure. This new research revolves around the ergosphere of the black hole.
The ergosphere is the outer region of the event horizon. In 1969, Penrose theorized that if you lowered an object into the ergosphere, it could generate energy.
In the ergosphere, it is impossible for an object to stand still due to dragging of the frame. General relativity predicts that a rotating mass, like the black hole, will drag adjacent spacetime along with it. Then, any object placed in the ergosphere will start moving, and there is no way to stop it.
Penrose said that if an object were thrown into the ergosphere, it would gain negative energy. If an object is dropped and then split in two, half would be swallowed by the black hole, and the other half would not. If that half were to recover from the ergosphere, the recoil action means that the recovered half would lose negative energy. Since a minus sign of a negative sign makes a plus sign, that object would gain some energy from the rotation of the black hole.
Clearly, this is not something that human civilization will attempt in the short term. Penrose said that only a highly advanced civilization would come close to something like that. And even then …
But after Penrose came up with the idea, another physicist thought about it a little more. Yakov Zel’dovich proposed that the idea could be tested by sending twisted light waves to the surface of a rotating metal cylinder. If sent at the correct speed, these waves would bounce off the cylinder after acquiring additional energy from the cylinder’s rotation. It is all due to a strange property of the Doppler effect.
When people talk about the Doppler effect, they generally mean the linear Doppler effect. The frequently used example is an ambulance siren. When an ambulance approaches the listener, the sound waves are compressed at a higher frequency in front of the ambulance, and the listener hears it as an increase in pitch. In contrast, after the ambulance passes the listener, the sound waves are no longer compressed by the ambulance’s forward motion, and the listener hears the low frequency as the lowest pitch.
But this idea implies the rotational Doppler effect.
Lead author Cromb describes it this way in a press release: “The rotational Doppler effect is similar, but the effect is limited to a circular space. Twisted sound waves change their pitch when measured from the point of view of the rotating surface. If the surface spins fast enough, the frequency of sound can do something very strange: It can go from a positive to a negative frequency, and in doing so, steal some energy from the surface’s rotation. “
In any case, Zel’dovich’s idea was never tested. The problem is that the cylinder would have to be spinning at an unreachable speed of billions of times per second, because light itself travels so fast. That is outside the scope of our technology.
The University of Glasgow team found a way to test this. They reasoned that everything could be tested with sound waves, which travel much slower than light. That means the cylinder would only need to rotate at a much slower and achievable speed as well.
In their study, the authors wrote: “Although wave amplification due to a rotating absorber is very difficult to verify with optical or electromagnetic waves, direct measurements with acoustic waves are possible.”
In their lab, the team built a speaker ring that could create a twist in the sound waves, similar to the twist required to light in Zel’dovich’s proposal.
The device starts with a speaker ring to produce the twisted sound waves. Those waves are directed towards a rotating foam disk that absorbs sound. Behind the foam disk is a microphone to measure sound. When the experiment begins, the rotation speed of the foam disk increases.
The team was looking for a different change in both the frequency and amplitude of the sound as the sound waves passed through the foam disk. At first, as the speed of the rotating disc increased, the pitch of the sound became so low that it is inaudible to human ears. Then the pitch, or frequency, went up again. It reached its original pitch again, but this time the amplitude, or volume, increased 30% higher than the original. The sound waves had acquired energy from the spinning disk.
“What we heard during our experiment was extraordinary,” said Cromb. “What is happening is that the frequency of the sound waves shifts Doppler to zero as the spin speed increases. When the sound starts again, it is because the waves have shifted from a positive frequency to a negative frequency Those negative frequency waves are able to take part of the energy from the rotating foam disk, becoming stronger in the process, as proposed by Zel’dovich in 1971.
It just shows us that some ideas may seem outrageous and unanswerable at any given time. But as time goes by, they can be tested. Like relativity, for example, and the curvature of light using gravitational lenses.
Professor Daniele Faccio is co-author of the article and is also from the Faculty of Physics and Astronomy at the University of Glasgow. In the press release, Faccio said: “We are delighted to have been able to experimentally verify some extremely strange physics half a century after the theory was first proposed. It is strange to think that we have been able to confirm a half-century theory with cosmic origins here in our laboratory in the west of Scotland, but we think it will open up many new avenues for scientific exploration. We are looking forward to seeing how we can investigate the effect on different sources, such as electromagnetic waves in the near future. “