Rotating black holes are the most powerful phenomenon in the known Universe. Their powerful gravity radically alters the curvature of spacetime around them, leading to relativistic effects like time dilation, light distortion, and for matter and photons to infall around them, forming accretion disks. In the case of supermassive black holes (SMBHs) at the center of galaxies, these disks are accelerated to close to the speed of light and emit powerful radiation that temporarily outshines all the stars in the disks.
In 1969, Sir Roger Penrose theorized that advanced civilizations could harness the rotational energy of black holes as a power source. Jorge Pinochet, a professor of physics at the Metropolitan University of Educational Sciences (UMCE), recently analyzed how Penrose’s theory could be realized. In simple terms, his paper explores this potential source of energy, the efficiency of the energy extraction process, and some of the mechanisms that could allow an advanced civilization to harness it – which might include humanity someday!
According to the Kerr and Kerr–Newman metric, two proposed resolutions to Einstein’s field equations for GR, all black holes are believed to be rotating. This rotation causes infalling matter directly beyond their event horizon (known as the ergosphere) to be “dragged” around by them. This effect even applies to spacetime itself, a phenomenon known as “frame dragging” (aka. the Lense-Thirring Effect), which some astronomers believe is responsible for the massive jets sometimes observed emanating from their poles.
As Pinochet told Universe Today via email, the rotation of a black hole can produce energy with an efficiency tens of times greater than the thermonuclear reactions that power the Sun:
“A particle in the vicinity of a rotating black hole will orbit it at a speed close to the maximum allowed by the laws of physics, the speed of light in a vacuum, which has a value of 300,000 km/s. But motion is the most basic form of energy. As we learn in school, the energy associated with speed is called kinetic energy. A black hole then transfers rotational kinetic energy to objects in its vicinity.
“This energy can be harnessed in many ways, as long as the objects being swept up in the rotation do not cross a black hole’s point of no return, the so-called Event Horizon. Once an object penetrates the Event Horizon, it is forever trapped by the black hole’s powerful gravity, and its energy cannot be used.”
In his original paper, “Gravitational Collapse: The Role of General Relativity,” Penrose proposed a mechanism that could, in theory, accelerate a spacecraft to relativistic speeds (a fraction of the speed of light), allowing it to traverse between star systems in a matter of years instead of centuries (or longer). However, the same process could also be used as a power source by a sufficiently advanced civilization.
This is in keeping with prevailing theories about advanced civilizations and the level of technical development they may have reached. The best-known example is the Kardashev Scale, the classification scheme proposed by Nikolai Kardashev in 1960. In accordance with this scheme, a civilization capable of harnessing the power of black holes would be somewhere between a Type II and Type III Civilization. Similarly, the ability to harness the energy of rotating black holes is predicted by the Transcension Hypothesis, proposed by John M. Smart in 2012.
However, as Pinochet explored in his paper, the original theory is impractical since it would require extremely high velocity, precision, and timing. A far more promising method, he notes, is to harness the energy released by the accretion disks that orbit black holes just beyond their Event Horizon:
“Nature has found a more practical and viable way to extract energy from rotating black holes, using so-called quasars and microquasars. We speak of a quasar when the phenomenon occurs around a supermassive black hole, whose typical mass is between approximately one million and ten billion solar masses. We speak of a microquasar when the phenomenon occurs around a stellar black hole, whose mass varies between approximately 3 and 100 solar masses. In addition to the accretion disk, these phenomena are accompanied by relativistic jets, which are jets of particles that travel at speeds close to the speed of light.”
This process, where quasars and microquasars are harnessed for their energy, was proposed in 1977 by physicists Roger Blandford and Roman Znajek. “In honor of its authors, this process is known as the BZ mechanism and is more complex than the Penrose process.” This process involves modifying the black hole’s ergosphere’s electromagnetic field, perhaps using a magnetic harness. As the ergosphere causes the magnetosphere inside it to rotate, the outgoing flux of angular momentum can be used to extract energy from the black hole.
This represents a potential technosignature that SETI surveys could be on the lookout for in the coming years. And as Pinochet summarized, it is not beyond the realm of possibility for humanity someday.
“The first to propose the use of the energy of rotating black holes for practical purposes was Penrose, but in general, this is a subject that has not been explored much, perhaps because it is too far removed from our current technological capabilities,” he said. “In any case, it is not unreasonable to think that perhaps, in the very distant future, our civilization may find in rotating black holes a clean and efficient solution to the complex energy problems that we will surely have to face as a society, provided that we can survive our current technological infancy.”
Further Reading: arXiv