Tuesday , March 9 2021

Black Hole jets could be fueled by a strange "negative energy", say astronomers



When a black hole is actively feeding itself, something strange can be observed: extremely powerful plasma jets shoot from their poles, at speeds approaching the speed of light.

Given the intense gravitational interactions at play, just as these jets form is a mystery. But now, using computer simulations, a team of physicists has found an answer – particles that appear to have "negative" energy draw energy from the black hole and redirect to the jets.

And this theory united, for the first time, two different and seemingly irreconcilable theories on how energy can be extracted from a black hole.

The first is called the Blandford-Znajek process and describes how the magnetic field of a black hole can be exploited to draw energy from its rotation.

As the material in the accretion disk approaches more and more of the event horizon, the theory states that it grows more and more magnetized, producing a magnetic field. Within this field, the black hole acts as a rotating conductor, inducing tension between the poles and the equator; this voltage is discharged from the poles as jets.

The second is called the Penrose process, and depends on the conservation of the moment, instead of magnetism. The rotational energy of a black hole is located not within the event horizon, but in a region outside of it called the ergosphere, which comes into contact with the event horizon at the poles.

According to the Penrose process, if an object within this region were separated, with one piece running toward the black hole and the other cast outward against the black hole, the outward-facing piece would emerge with more energy, extracted from the rotation. This produces a kind of "negative energy."

Both scenarios are compelling, but so far we were not sure of the correct answer.

"How can the energy in the rotation of a black hole be extracted to make jets?" said theoretical physicist Kyle Parfrey of the Lawrence Berkeley National Laboratory. "This has been a question for a long time."

The team designed a non-collision plasma simulation (in which particle collisions do not play a major role) in the presence of the strong gravitational field of a black hole. They were also responsible for creating pairs of electrons and positrons in the electric fields, which allowed for more realistic plasma densities.

The resulting simulation naturally produced the Blandford-Znajek process – electrons and positrons moving in opposite directions around the black hole, producing energy in the electromagnetic field that shoots out of the poles like jets.

But that Besides that produced a variation of the Penrose process. Because of relativistic effects, some particles appeared to have "negative energy" as they disappeared into the black hole – which delayed the rotation of the black hole, only a small fraction.

"If you were standing next to a particle, you would not see anything strange about it. But to a distant observer, it looks like it has negative energy," Parfrey said. New scientist.

"You are left with this strange case where falling into the black hole will cause mass and spin to subside."

The effect did not contribute much to the general extraction of energy, Parfrey noted, but it is possible that it is somehow connected to the electric currents that twist the magnetic fields.

The simulation is also missing some components, such as the accretion disk, and the physics of positron-electron creation is not as detailed as it could be. The team will work to develop an even more realistic simulation to study the process in more detail.

"We hope to provide a more consistent picture of the whole problem," Parfrey said.

The team's research was published in the journal Physical Review Lettersand can be read in full in arXiv.


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