Physicists have long been intrigued by the possibility of tapping one of the most powerful energy sources in all the cosmos: a rotating black hole. But could this really be done, and if so, what would be required of any civilization advanced enough to be up for the task?
The idea has its roots in Einstein’s theory of general relativity, where the famous physicist predicted that rotating black holes would have to contain enormous amounts of energy. Decades later, in 1969, Nobel laureate Roger Penrose put forward the idea that an advanced alien civilization might be able to extract this energy by dropping objects into a region of space-time dragged along by a spinning black hole known as its ergosphere.
Since that time, several physicists have introduced additional ideas that build on this theory, involving how magnetic fields interacting with plasmas around a rotating black hole might be leveraged to draw out powerful energy jets.
This idea was the basis of a 2021 study published in Physical Review D by researchers Luca Comisso of Columbia University and Felipe Asenjo of Universidad Adolfo Ibáñez, who argued that the breaking and reconnecting of magnetic field lines within a black hole’s ergosphere could accelerate plasma in opposite directions, propelling it almost at the speed of light.
In theory, one stream of this plasma could escape while carrying energy away, while negative energy particles fall into the black hole. The effect is that the black hole’s rotational energy would effectively be reduced. Since some of the escaping energy comes from the black hole itself, the researchers calculated that the process could achieve an apparent energy extraction efficiency of as much as 150 percent.
In theory, therefore, the mechanism described here could potentially be something an advanced civilization might be able to harness—if one hasn’t done so already—offering an incredible energy source.
Capturing the Power of a Black Hole
The Debrief recently reached out to Felipe Asenjo about his work with Luca Comisso on this intriguing physics question, who said that when it comes to black holes, although these cosmic objects remain deeply mysterious in many ways, they essentially would have to behave according to the same governing laws of physics as any rotating body.
“Every body that rotates has rotational energy,” Asenjo told The Debrief in an email. “This is the energy needed to rotate, and it is different from other forms of energy, such as inertial-mass energy or kinetic energy.”
“A black hole is not different from other bodies in that sense,” Asenjo explained. “Therefore, any black hole that rotates has an intrinsic energy associated with its rotation. All that energy is stored in the rotation itself, and if the black hole has no interaction with anything else, the conservation of energy allows it to keep rotating indefinitely.”
“If you ‘steal’ its rotational energy,” Asenjo added, “then the black hole starts rotating slower.”
The problem is that, to achieve this, Asenjo says one would have to be positioned outside the black hole’s event horizon.
“You need to interact with the gravitational field produced by the black hole, but without entering the black hole,” Asenjo told The Debrief. “The interaction with a black hole occurs through the spacetime curvature that it produces, there is no need to enter it.”
Similar to earlier musings about tapping the energy from black holes by Roger Penrose and others, Asenjo and Comisso determined that this could be achieved in the black hole’s ergosphere, which exists just beyond the event horizon.
“In such a zone, any observer is forced to co-rotate with the black hole,” Asenjo said. “This is the key, because from the perspective of an observer very far from the black hole, any phenomenon in the ergosphere is trapped in the rotation of the black hole (although it is still outside of it).”
According to Asenjo, any gravitationally “trapped” event will also possess negative energy. As an example, Asenjo points to the energy of Earth as it moves around the Sun, which is considered negative energy because its elliptical orbit is also a trapped orbit.
“Loosely speaking, negative energies mean that you are trapped by a force,” Asenjo says. “The Penrose process takes advantage of this, proposing that when a positive energy particle… breaks in two pieces in the ergosphere, one piece acquires negative energy (the part that is trapped) while the other piece can escape with a larger energy compared to the initial one.”
“Thus, the escaping piece steals part of the rotational energy of the black hole,” Asenjo said. “In our work, we extend the Penrose process to a plasma phenomenon, called magnetic reconnection.”
Magnetic Reconnection in the Ergosphere of a Black Hole
In their 2021 study, Asenjo and Comisso argued that magnetic reconnection within the ergosphere of a rotating black hole can send negative-energy particles toward the event horizon, while other accelerated particles escape, taking with them some of the black hole’s rotational energy.
“In the work that we did with Luca Comisso, we extend the Penrose process (that only applies to particles) to plasmas,” Asenjo explained. “We showed that the same principle behind the Penrose process can be used when magnetic fields in a plasma experience magnetic reconnection.”
“When this occurs, magnetic fields can change their form and, in the process, they generate two plasma jets of flow that can move very fast in opposite directions,” Asenjo said. “We demonstrate that when magnetic reconnection happens in the ergosphere, one of these jets of plasma flow can have negative energy (being trapped by gravity and falling to the black hole) while the other jet can escape. The jet that escapes can steal part of the rotational energy of the black hole.”
Intriguingly, Asenjo notes that rather than just a single particle breaking under these conditions, “now we are dealing with a bunch of particles forming a huge fast flow.”
“Thus, the amount of energy that can be extracted is much more,” he adds.
Alien Civilizations Powered by a Rotating Black Hole?
Taking this idea to its most speculative technological conclusion, could an extraordinarily advanced civilization use a rotating black hole as a source of energy? Asenjo thinks it’s possible, with a few caveats.
“Yes, in principle, this is a source of energy that can be used. When this occurs, the black hole starts to slow down its rotation. So, any advanced civilization can use a black hole as an engine,” Asenjo told The Debrief, adding that doing so essentially only requires “advanced technological engineering to capture those very energetic plasma flows that escape from the black hole.”
“There is no fundamental scientific limitation to that,” Asenjo says, with the sole exception of the conditions of the black hole rotation itself.
As far as the kind of technology that might be capable of capturing the energy from the black hole’s powerful jets, Asenjo concedes that he doesn’t know exactly what kinds of technological advancements would be required.
“However,” he adds, “I am pretty sure that such a civilization must live and grow around a black hole, using it in a way [similar to how] we use our Sun to grow as a civilization.”
Asenjo and Comisso’s paper, “Magnetic Reconnection as a Mechanism for Energy Extraction from Rotating Black Holes,” was funded by the National Science Foundation’s Windows on the Universe initiative, and can be read in its entirety on the preprint arXiv.org server.
Micah Hanks is the Editor-in-Chief and Co-Founder of The Debrief. A longtime reporter on science, defense, and technology with a focus on space and astronomy, he can be reached at micah@thedebrief.org. Follow him on X @MicahHanks, and at micahhanks.com.