The universe is packed with countless bizarre phenomena, yet black holes easily out-weird them all. These cosmic gobblers are formed when large stars explode, a process so violent that it punctures spacetime and creates a gravity well strong enough to prevent anything, even light, from escaping its grasp. In addition to their immense gravitational force, black holes are also capable of spinning at mind-boggling clips that can approach the speed of light.
What if we could harness that power to generate energy?
Back in the 1960s, physicist Roger Penrose proposed just that: the tremendous rotational energy of black holes could be mined by an advanced civilization, be it an alien species or a far-future iteration of humanity. The Penrose process, as it is now known, would be a kind of cosmic quid-pro-quo: A civilization could mooch some energy from the black hole’s spin in exchange for giving it a massive object to devour.
Now, scientists led by Marion Cromb, a PhD student at the University of Glasgow, have experimentally demonstrated a key part of this process using sound waves (specifically, the rotational Doppler effect), according to a study published on Monday in Nature Physics. The new work is the result of a collaborative effort between two research groups headed by University of Glasgow physicists Daniele Faccio and Miles Padgett, who are co-authors on the study.
The goal of the experiment was to test an analog of the Penrose process on Earth. Penrose suggested that energy could be extracted from the ergosphere, a spinning region of spacetime surrounding the event horizon of a black hole. Once an object has slipped beyond the event horizon, it can never come back, but an object could potentially dip into the ergosphere and get a power boost from the rotational energy within this warped region of spacetime. Physicist Yakov Zel’dovich suggested this could be tested on Earth by shooting light waves at a rotating cylinder, but it would have to rotate at an unachievable high speed: one billion times per second. Enter sound waves and the Doppler effect.
“My own group has been interested in rotational Doppler Shifts for many years,” Padgett said in an email. These rotational shifts are similar to the linear Doppler effect that is famously expressed in the changing pitch of an ambulance siren from the perspective of an observer on the street. Rotational Doppler Shifts exhibit a somewhat similar change in frequency, except they occur from the perspective of an observer on a rapidly rotating object.
Padgett visualized these rotational shifts as a phenomenon in which “the hands on a clock, that is itself rotating, appear to go around at a different speed than for a stationary clock.”
Meanwhile, Faccio’s group has spent years studying ways to reproduce gravitational effects using optical tests, such as the experiment proposed by Zel'dovich.
Sound waves in a lab seem incredibly different from black holes in the cosmos, but Padgett said that they are a workable Earthbound analog for the Penrose process.
“Both light and sound waves can carry angular momentum,” Padgett explained. “In waves that carry angular momentum, the field (electric for light, pressure for sound) appears to rotate just like the hands on a clock.”
“If one rotates fast enough then one ‘overtakes’ the rotating wave and the field appears to rotate in the other direction,” he continued. For instance, imagine that you were to spin a clock in a counter-clockwise direction. “If one does it fast enough, then then hands all appear to go backward!” Padgett said.
With this principle in mind, the team blasted twisted sound waves—meaning waves with angular momentum—toward a rotating foam disk to see if it would exhibit this “overtaking” process by switching from a positive to a negative frequency. To their delight, the waves were amplified about 30 percent by the disk, causing them to slip into a negative frequency range.
This is comparable to how Penrose imagined energy extraction from a black hole: An object would enter the ergosphere and get caught up in the breakneck spin of spacetime outside the event horizon. In order to escape, the object would have to be split in two, perhaps by a triggered explosion, so that half of it gets consumed by the black hole.
This sacrifice would allow the conservation of momentum to be maintained, even though the escaping object would leave the ergosphere with far more energy than it had when it entered. In simpler terms, the loss of the sacrificial object becomes an energy gain for the escaping object.
It’s certainly a trippy idea, but the new study shows it can be validated on small scales in laboratories. Speculatively speaking, an advanced civilization might be able to exploit the Penrose process to sap limitless energy from black holes to power complex technological societies or accelerate spacecraft to enormous speeds.
This vision of a black hole power plant would be an engineering challenge far beyond what we can manage with modern technology. Still, the new study reveals that rotational Doppler Shifts are a promising way to probe some of the most exotic processes in physics.
“We have shown here that in the ‘overtaking’/’negative frequency’ regime, strange things happen,” Padgett said. So, he concluded: “What else changes?”