An illustration of a black hole tearing a star to pieces while firing powerful beams of energy into space. （*nrao/aui/nsf/nasa）
The gravitational pull of a black hole is so strong that nothing escapes its control. So, can we harness the immense power of black holes as energy**?
In a new study, scientists have proposed two ways to one day use black holes as an energy source. They took advantage of the rotational and gravitational properties of black holes to extract energy from black holes.
We know we can extract energy from black holes, and we know that we can inject energy into them, which almost sounds like batteries," lead author Fengmai Zhan, a postdoctoral researcher at Peking University's Kavli Institute of Astronomy and Astrophysics, told Live Science.
In the first hypothetical scenario, scientists would "recharge" the black hole by injecting it with huge charged particles. In their study, published Nov. 29 in the journal Physical Review D, the scientists explained that these charges will continue to be sucked in until the black hole itself has an electric field that begins to repel any additional charges they are trying to inject.
When this electromagnetic repulsion is greater than the gravitational pull of a black hole, scientists consider it to be "fully charged." According to Einstein's theory of general relativity, the theory suggests that mass can be considered equivalent to energy, and that the available energy of a black hole will come from the charges injected into it and the combination of masses of those charges.
Black hole batteries are converting the energy of the particle mass into charge energy," Mai said.
The researchers calculated that the charging process is 25% efficient, which means that a black hole battery can convert about a quarter of its mass into usable energy in the form of an electric field. The team calculated that this would make the battery about 250 times more efficient than an atomic bomb.
To extract the energy, the researchers will utilize a process called hyperradiation, which is based on the theory that space-time is actually dragged around the rotation of a spinning black hole due to its strong gravitational field.
Gravitational waves or electromagnetic waves that enter this region of rotation will also be dragged, but assuming they haven't passed through the black hole's event horizon — a boundary from which nothing, not even light, can escape — some waves may be deflected with more energy than they originally carried, the researchers wrote. This process converts the black hole's rotational energy (determined by its mass) into a deflected wave.
Another way to harness the energy of a black hole is to extract it in the form of so-called Schwinger pairs, or pairs of particles that form spontaneously in the presence of an electric field.
If we start with a black hole full of charge, the electric field near the event horizon can be so strong that it spontaneously produces an electron and positron, it's like an electron, but the charge is reversed, Mai explains. If a black hole is positively charged, positrons are ejected from the black hole due to repulsion. Theoretically, this runaway particle could be harvested as energy.
Mai said he didn't know if we would see such a battery, but the theoretical Xi was inspired by scientists' previous attempts to theoretically extract energy from black holes.
We think black holes are where quantum mechanics and gravity have to come together somehow," Daniele Faccio, a physicist at the University of Glasgow who was not involved in the study, told Live Science. "By looking at them from the point of view of energy extraction, we can learn more about what is happening.