Spinning black holes store rotational energy that can be extracted; when a black hole is immersed in an externally supplied magnetic field, reconnection of magnetic field lines within the ergosphere can generate negative energy particles that fall into the black hole event horizon while the other accelerated particles escape stealing energy from the black hole. In a paper published in the journal Physical Review D, a duo of researchers from the United States and Chile shows analytically that energy extraction via magnetic reconnection is possible when the black hole spin is high and the plasma is strongly magnetized.
Black holes are believed to play a key role in a number of highly energetic astrophysical phenomena, from active galactic nuclei to gamma-ray bursts to ultraluminous X-ray binaries.
The extraordinary amounts of energy released during such events may have two different origins. It can be the gravitational potential energy of the matter falling toward an existing or forming black hole during accretion or a gravitational collapse. Or it can also be the energy of the black hole itself.
Indeed, a remarkable prediction of Einstein’s theory of general relativity is that a spinning black hole has enormous amounts of free energy available to be tapped.
For the last 50 years, theoretical physicists have tried to come up with methods to unleash this power.
Nobel physicist Roger Penrose theorized that particle disintegration could draw energy from a black hole; Stephen Hawking proposed that black holes could release energy through quantum mechanical emission; while Roger Blandford and Roman Znajek suggested electromagnetic torque as a main agent of energy extraction.
“Black holes are commonly surrounded by a hot soup of plasma particles that carry a magnetic field,” said first author Dr. Luca Comisso, a researcher in the Department of Astronomy and the Columbia Astrophysics Laboratory at Columbia University.
“Our theory shows that when magnetic field lines disconnect and reconnect, in just the right way, they can accelerate plasma particles to negative energies and large amounts of black hole energy can be extracted.”
“This finding could allow astronomers to better estimate the spin of black holes, drive black hole energy emissions, and might even provide a source of energy for the needs of an advanced civilization.”
Dr. Comisso and his colleague, Professor Felipe Asenjo from the Facultad de Ingeniería y Ciencias at Universidad Adolfo Ibanez, built their theory on the premise that reconnecting magnetic fields accelerates plasma particles in two different directions.
One plasma flow is pushed against the black hole’s spin, while the other is propelled in the spin’s direction and can escape the clutches of the black hole, which releases power if the plasma swallowed by the black hole has negative energy.
“A black hole loses energy by eating negative-energy particles. This might sound weird, but it can happen in a region called the ergosphere, where the spacetime continuum rotates so fast that every object spins in the same direction as the black hole,” Dr. Comisso said.
Inside the ergosphere, magnetic reconnection is so extreme that the plasma particles are accelerated to velocities approaching the speed of light.
“The high relative velocity between captured and escaping plasma streams is what allows the proposed process to extract massive amounts of energy from the black hole,” Professor Asenjo said.
“We calculated that the process of plasma energization can reach an efficiency of 150%, much higher than any power plant operating on Earth. Achieving an efficiency greater than 100% is possible because black holes leak energy, which is given away for free to the plasma escaping from the black hole.”
Luca Comisso & Felipe A. Asenjo. 2021. Magnetic reconnection as a mechanism for energy extraction from rotating black holes. Phys. Rev. D 103 (2): 023014; doi: 10.1103/PhysRevD.103.023014