How Could Humanity (Or Aliens) Use A Black Hole To Harvest Energy?

A number of power-supply options for advanced civilizations have been dreamt up by sci-fi writers and scientists alike, including Dyson spheres and swarms. These are hypothetical megastructures placed around a star to harvest most of their energy, before directing it where it is needed. 

While these sound cool as hell, they may come at a great cost for any civilization that wants to make one. Besides that, looking out into the cosmos, we have not yet found any signs of such a megastructure, suggesting there are no hyper-advanced civilizations out there, or that Dyson spheres are something that advanced civilizations deem unnecessary or not practical (or, of course, that we haven’t looked hard enough yet).

Another idea, first proposed by respected physicist and mathematician Roger Penrose, is that we could extract energy from rotating black holes, also known as Kerr black holes, after Roy Kerr, who described their geometry.

In 1969, Penrose suggested that energy could be extracted from black holes by lowering an object into the ergosphere (the region just outside of the event horizon) and allowing it to accelerate the object, stealing some of the black hole’s energy. The idea, known as the Penrose process, requires negative energy to be acquired by the object in order for it to be recovered from the black hole – otherwise, all you’d be doing is feeding the black hole.

“Let’s imagine that we launch a particle from very far away into the ergosphere of a Kerr black hole, following a retrograde orbit, that is, a trajectory directed against the black hole’s rotational direction,” Jorge Pinochet, professor in the physics department of the Universidad Metropolitana de Ciencias de la Educación, explains of the process in a new preprint paper. “Suppose we calculate the trajectory so that upon entering the ergosphere, the particle fragments into two pieces, one of which is absorbed by the black hole, and the other escapes outward, moving an arbitrarily large distance away.”

“Due to the extreme intensity of gravity inside a black hole, general relativity allows the absorbed fragment to have negative energy.”

Under these circumstances, Penrose showed that the escaping fragment would have more energy than the fragment absorbed by the black hole. That may sound like conservation of energy laws are being broken, but according to what we know of general relativity, that is not the case.

“The trick to obtain this result is that the black hole absorbs negative energy, which leads to a reduction in its mass-energy, which translates into a decrease in its rotational speed,” Pinochet continues. “In other words, we have extracted rotational energy from the black hole.”

Another process outlined in the new paper is known as the Blandford-Znajek mechanism, which is partly an attempt to describe how huge jets are formed by rotating black holes.

“When the gas comprising the accretion disk falls toward the black hole, it describes a spiral trajectory. As it does so, its different parts rub against each other and heat up to temperatures of several million kelvins,” Pinochet explains. “These high temperatures ionize the gas, turning it into a plasma composed of a sea of positive ions and negative electrons. These churning charged particles generate turbulent magnetic fields, which channel relativistic plasma jets into two jets pointing in opposite directions, in the direction of the rotation axis.”

Another idea, proposed much more recently, is known as the “halo drive“. An advantage of this hypothetical system is that no particles are fired into the black hole, and no negative energy is required.

As light passes through gravitational wells, we know that it gains energy, as spaceships do during gravitational assists. As light is traveling at the speed limit of the universe – the speed at which all particles without mass must travel – it cannot gain or lose speed from falling into or out of a gravity well. Instead, as light falls into a gravity well, its frequency becomes higher and is blue-shifted, while light coming out of a gravity well becomes red-shifted. It is this that is exploited by the halo drive.

The basic idea is that you send a beam of light around a pair of black holes spinning around each other prior to a merger, or a single black hole spinning quickly, and use the higher-energy blue-shifted light to accelerate your spacecraft.

“Using a moving black hole as a gravitational mirror, kinetic energy from the black hole is transferred to the beam of light as a blueshift and upon return the recycled photons not only accelerate, but also add energy to, the spacecraft,” David Kipping, assistant professor of astronomy at Columbia University, wrote in a paper proposing the drive. “It is shown here that this gained energy can be later expended to reach a terminal velocity of approximately 133% the velocity of the black hole.”

Of course, these are all pretty hypothetical. As much as we’d like to believe we have come to understand black holes in the last century, there is still much more work to be done before we could attempt such a project. And then there’s the matter of reaching such a black hole in the first place.

The new paper is posted to the preprint server arXiv and has not yet been peer reviewed.