Juicing power from the abyss: How black hole batteries could be the energy of the (distant) future

Black holes, despite their name, are far from being an empty space in the dark abyss. These condensed areas of space have mesmerized scientists for decades because gravity pulls so much that even light can’t escape them. Black holes are mountains of matter.

While we usually think of black holes as being inescapable, something humans should stay far away from, a recent paper published in the peer-reviewed journal Physics Review D resurfaces an ongoing idea in astronomy that there could be a use for black holes for us on Earth in the future  — as an energy source.

The paper comes from researchers at Tianjin University in China who proposed that spatially tiny black holes that have no spin, called primordial black holes, could essentially be turned into batteries and nuclear reactors. According to their calculations, a primordial black hole would have a 25% efficiency rate, which is pretty impressive. Residential solar panels range from 13% to 22.8% efficiency. Wind turbines range from 20% to 40% efficient at converting wind into energy.

Clearly, there is a need on Earth to generate energy without greenhouse gas emissions and negative consequences to the environment. Fossil fuels are slowly being phased out in many places around the world. Could black holes be a futuristic, and clean, energy source for us on Earth — maybe in hundreds or even thousands of years? 

Getting close to a black hole, spin or no spin, is a very difficult task, especially considering how whatever gets close enough to it could possibly be lost forever. But the researchers posit this could be possible. Particles moving around a spinning black hole are tossed and hurled away from the black hole with more energy than they go in, under certain conditions.

In fact, another recent study, this one in The Astrophysical Journal, presents some of the first ever proof that black holes can actually lose energy, which lends to the idea that it can be harvested via a mechanism known as the Blandford–Znajek process. First proposed in 1977 and named for astrophysicists Roger Blandford and Roman Znajek, it draws on a theory that rotating supermassive black holes will cause magnetic fields to twist into a helix that creates a voltage and brings energy out of the hole.

Previously, researchers suggested that these particles being thrown around outside of the primordial black hole could be harvested for their energy. But without a spin, researchers of the Physics Review D paper suggest feeding the tiny black hole with charged alpha particles. The gravitational pull of the black hole would outweigh the electric charge allowing the tiny particles to get sucked in and not spat out. This would allow the black hole to be able to be recharged several times, but it would have a limit. To extract the energy, they would surround the black hole with an electrically-charged field. Basically, an electrical grid for black holes. 

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The researchers also concluded that a primordial black hole could have the similar efficiency as a nuclear reactor. However, there is one big caveat to this study, which is that there is no definitive proof that primordial black holes actually exist. 

In a phone interview, Janna Levin, a professor of physics and astronomy at Barnard College, Columbia University, said the researchers cast their results in a clever way, and create enthusiasm around black holes in their purest form, which might be primordial ones.

“It is kind of an irony that the darkest phenomena in the universe figures out a way to become the brightest,” she said. “It engineers a way to become a particle accelerator.”

As mentioned, the idea of extracting energy from black holes isn’t new. Other physicists, such as Sir Roger Penrose, have theorized other ways to extract energy from a rotating black hole with the “Penrose Process." It hinges on the idea that particles falling into a black hole's ergosphere — a region outside the event horizon where space-time is dragged around by the spin of the black hole — are sped up so fast they break in two. One part goes into the black hole and the other one shoots out with more energy than before. However, Penrose himself has said this method is incredibly inefficient for energy harvesting.

More recently, a study published in 2020 in Nature Physics tested the physics of this possibility and verified it was possible. In an article published in The Conversation about their paper, the scientists didn’t discredit the possibility of using black holes for energy in the far away future. 

“While we are not anywhere close to extracting energy from a rotating black hole, this doesn’t mean it couldn’t be done by a very advanced alien civilisation – or indeed our own civilisation in the distant future,” the researchers wrote. “Such a civilization could build a structure around the black hole that rotates with it and then drop asteroids or even electromagnetic waves into it what would be reflected with more energy.”

In 2021, the U.S. National Science Foundation funded research that concluded the possibility could become a reality with spinning black holes for an advanced civilization as well. Luca Comisso, research scientist at Columbia University and first author on the study, said at the time that the biggest barrier is a “technological problem.”

"Thousands or millions of years from now, humanity might be able to survive around a black hole without harnessing energy from stars," Cosmisso said. “If we look at the physics, there is nothing that prevents it." 

Levin told Salon gravitational energy is “surprisingly rich.”

“If you can convert gravitational energy into light energy, or electric energy, like circuit energy, the stuff that power supplies, you should be able to do pretty well,” she said. “But they're not going to be replacing coal or fossil fuels.”

At least, not yet. 

“Who knows what somebody will be able to make in a laboratory,” she said.