A black hole as a source of energy?
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 demonstrated that it may be possible.
Marion Cromb is the lead author of this new study. They’re a PhD student at the University of Glasgow’s School of Physics and Astronomy. The paper is titled “Amplification of waves from a rotating body.” It’s published in the journal Nature Physics.
Space-interested and science-interested people know that black holes have a singularity at the very center, and an event horizon, the boundary over which nothing can return once it passes. But black holes have other elements to their complex structure. This new research revolves around the black hole’s ergosphere.
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 frame-dragging. General relativity predicts that a rotating mass, like the black hole, will drag adjacent space-time along with it. So any object put into the ergosphere will start to move, and there’s no way to stop it.
Penrose said that if an object were dropped into the ergosphere, it would gain negative energy. If an object were dropped in and then split in two, one half would be swallowed up by the black hole, and one half wouldn’t. If that half were recovered from the ergosphere, recoil action means that the recovered half would lose negative energy. Since a minus of a minus makes a plus, that object would gain some energy from the black hole’s rotation.
Clearly, this is not something that human civilization will be attempting any time soon. Penrose said that only a highly advanced civilization would even come close to something like that. And even then…
But after Penrose floated the idea, another physicist thought about it some more. Yakov Zel’dovich proposed that the idea could be tested by sending twisted light waves towards the surface of a rotating metal cylinder. If sent at the right speed, these waves would bounce off the cylinder after acquiring additional energy from the cylinder’s rotation. It’s all due to a strange property of the Doppler effect.
When people talk of the Doppler effect, they’re usually referring to the linear Doppler effect. The often-used example is an ambulance siren. As an ambulance approaches the listener, the sound waves are compressed to a higher frequence in front of the ambulance, and the listener hears that as an increase in pitch. Conversely, after the ambulance passes the listener, the sound waves are not compressed by the forward motion of the ambulance anymore, and the listener hears the lowered frequency as lower pitch.
But this idea involves the rotational Doppler effect.
Lead author Cromb describes it like this in a press release: “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.”
In any case, Zel’dovich’s idea was never tested. The problem is that the cylinder would have to be rotating at an unattainable rate of billions of times per second, because light itself travels so fast. That’s well out of reach for our technology.
The team at the University of Glasgow came up with a way of testing this. They reasoned that the whole thing could be tested with sound waves, which travel much slower than light. That means that the cylinder would only need to rotate at a much slower, attainable rate too.
In their study, the authors wrote, “Although amplification of waves due to a rotating absorber is very hard to verify with optical or electromagnetic waves, direct measurements of it are possible using acoustic waves.”
In their lab, the team built a ring of speakers that could create a twist in the sound waves, similar to the twist required in the light in Zel’dovich’s proposal.
The device begins with a ring of speakers to produce the twisted sound waves. Those waves are directed toward a rotating foam disc that absorbs the sound. Behind the foam disc is a microphone to measure the sound. When the experiment starts, the rotational speed of the foam disc rises.
The team was looking for a distinct change in both the frequency and the amplitude of the sound as the sound waves passed through the foam disc. At first, as the speed of the rotating disc increased, the pitch of the sound became so low that it’s inaudible to human ears. Then the pitch, or frequency, rose again. It reached its original pitch again, but this time the amplitude, or loudness, was increased to 30% louder than the original. The sound waves had acquired energy from the rotating disc.
“What we heard during our experiment was extraordinary,” Cromb said. “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.”
It just goes to show us that some ideas can seem outlandish and untestable at one point in time. But as time goes on, they can be tested. Just like relativity, for example, and the bending of light by gravitational lensing.
Professor Daniele Faccio is a co-author on the paper, and is also from the University of Glasgow’s School of Physics and Astronomy. In the press release, Faccio said “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.”