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A 50-year-old theoretical process for extracting energy from a rotating black hole finally has experimental verification.
Using an analogue of the components required, physicists have shown that the Penrose process is indeed a plausible mechanism to slurp out some of that rotational energy – if we could ever develop the means.
That’s not likely, but the work does show that peculiar theoretical ideas can be brilliantly used to explore the physical properties of some of the most extreme objects in the Universe.
Black holes are wild – the end stage of the life cycle of a star so massive that, once it’s gone supernova, the core can no longer withstand its own gravity and collapses totally into a singularity – a single one-dimensional point of infinite density.
This singularity sits inside a region called the event horizon – the point at which the gravity around the black hole is so strong, not even light-speed is sufficient to achieve escape velocity. And outside the event horizon, an extended region of space-time becomes twisted as it’s dragged along with the black hole’s rotation, an effect called frame-dragging.
This is where the Penrose process comes in. In 1969, mathematical physicist Roger Penrose proposed that a region just outside the event horizon called the ergosphere, where frame-dragging is at its strongest, could be exploited to extract energy.
According to Penrose’s calculations, if an object dropped into the ergosphere were to split in two, one part would be flung beyond the event horizon.
The other, however, would be accelerated outwards, with an additional kick from the black hole. If everything went just right, it would emerge from the ergosphere with around 21 percent more energy than it entered with.
Now, we can’t just nip over to a black hole to test this out. But in 1971, Soviet physicist Yakov Zel’dovich proposed a more practical experiment. You could replace the black hole with a rotating metal cylinder, and fire twisted beams of light at it. If the cylinder was rotating at just the right speed, the light would be reflected back with additional energy extracted from the cylinder’s rotation, due to a quirk in something called the rotational Doppler effect.
If you’re a regular reader, you might already be familiar with said effect: It can be seen when a rotating source emits waves, which shorten and lengthen depending on the direction of the rotation. The waves from the side that’s rotating towards you will appear to shorten; waves from the side that’s rotating away appear to lengthen. This is how astronomers can gauge the rotations of stars and galaxies.
There was just one problem with Zel’dovich’s proposal. The speed of the rotating cylinder would need to be at least 1 billion rotations per second – remember, there’s still a lot of room for impracticality in “more practical than a black hole”.
So there the matter sat – until a team of physicists from the University of Glasgow’s School of Physics and Astronomy in Scotland came along. They devised an experiment based on Zel’dovich’s work – but instead of using light waves, they used sound waves.
The experiment. (Cromb et al., Nature Physics, 2020)
The experiment consisted of a ring of speakers set up to introduce a twist in the sound waves, analogous to the twisted light in Zel’dovich’s experiment. The ‘black hole’ was a rotating sound absorber made out of a foam disc, the rotation of which would speed up as the sound waves hit it. An array of microphones on the other side of the disc would detect the sound waves after they had passed through the disc.
The smoking gun that would verify the Penrose process was a shift in pitch and amplitude in the sound waves that passed through the disc.
“The twisted sound waves change their pitch when measured from the point of view of the rotating surface,” explained physicist and astronomer Marion Cromb of the University of Glasgow, lead author on the team’s paper.
“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.”
The results were amazing. As the disc’s rotation accelerated, the pitch of the sound hitting the microphones lowered until it was inaudible. Then it began to rise again back to the original pitch – but 30 percent louder than the sound coming from the speakers. The sound waves were picking up additional energy from the rotating disc.
(University of Glasgow)
“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.”
The team plans to try to figure out how to extend this research to electromagnetic waves – light – but this research is a pretty awesome step forward in understanding black holes. It shows how their extreme properties can be probed in laboratory settings if you have the right tools – and they needn’t always be fancy, high-tech Bose-Einstein condensates.
Research such as this could also lead to new technologies, if a way can be devised to harness this fascinating phenomenon.
“We’re thrilled to have been able to experimentally verify some extremely odd physics a half-century after the theory was first proposed,” said physicist Daniel Faccio of the University of Glasgow.
“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.”
The research has been published in Nature Physics.