“In 1971, Zel’dovich predicted that quantum fluctuations and classical waves reflected from a rotating absorbing cylinder will gain energy and be amplified. This concept, which is a key step towards the understanding that black holes may amplify quantum fluctuations, has not been verified experimentally owing to the challenging experimental requirement that the cylinder rotation rate must be larger than the incoming wave frequency.”
Until now. This opening paragraph of a new study published in the journal Nature Physics is referring to the idea that black holes are not just matter-sucking bodies consuming all energy in their path but may actually be able to be mined by energy by a brave alien civilization or even humans – provided they can figure out a way to get close enough to plug their battery into the black hole charger. But how?
The idea came from British physicist Roger Penrose. In 1969, he theorized that rotational energy could be harvested from a black hole by lowering an object into the outer edge of its event horizon – the ergosphere – and then collecting the energy the black hole gave off as it dined. The first and biggest challenge was that the collector had to be stationary with the black hole, which meant it had to be moving at faster than the speed of light. This energy-harvesting idea became known as the Penrose Process.
While Sir Roger went on to share the 1988 Wolf Prize for physics with Stephen Hawking for the Penrose–Hawking singularity theorems, renowned Soviet physicist Yakov Zel’dovich picked up the Penrose Process in 1971 and developed an experiment that could be conducted on Earth to test the theory. It involved shooting twisted light waves (used in optical tweezers and ultra-powerful microscopes) at a rotating cylinder which reflect them back with additional energy taken from the cylinder’s rotation due to something called the rotational Doppler Shift. Zel’dovich was certain it would work – he just needed a cylinder that rotated at least a billion times a second. Sigh.
Fast forward to the present. Marion Cromb, a Ph.D. student in the University of Glasgow’s School of Physics and Astronomy and lead author of the paper, came up with an alternative that would allow her and her research team to successfully test the Penrose Process using sound instead of light. Sound obviously travels far slower than light but performs the same way in a rotational Doppler Shift. They used a small ring of speakers to create a twist in the sound waves, then directed them at a rotating sound absorber made from a foam disc. Microphones on the other side picked up the sound as it passed through the disc. And?
“What we heard during our experiment was extraordinary. 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.”
Eureka! … or whatever researchers yell these days — Cromb preferred to comment quietly to Phys.org. Penrose and Zel’dovich’s theories on the possibility of harvesting energy from a black hole were proven in a lab experiment using the equivalent of your Wifi speaker and a fast-spinning Nerf bat. Yes, that is a long way from charging your spaceship at a roadside black hole for the ride home, but it’s still a proof.
One more point to ponder. Penrose came up with the process as a way for alien civilizations to harness black hole power. The experiment proves this is possible. Does this mean it may already be happening? What would that mean?
Akerue! (That’s a reverse Eureka!)