When a black hole actively feeds, something strange can be observed: incredibly powerful plasma jets fly out of its poles at a speed approaching the speed of light.
Given the intense gravitational interaction in the game, how exactly these jets are formed remains a mystery. But now, using computer simulation, the team of physicists has found the answer – particles that seem to have "negative energy", extract energy from a black hole and redirect it to jets.
And this theory for the first time combined two different and seemingly irreconcilable theories about how energy can be extracted from a black hole.
The first is called the Blandford-Znak process and describes how a black hole’s magnetic field can be used to extract energy from its rotation.
According to the theory, when the material in an accretion disk rotates closer and closer to the event horizon, it is increasingly magnetized, creating a magnetic field. Inside this field, the black hole acts as a rotating conductor, causing tension between the poles and the equator; this voltage is discharged from the poles in the form of jets.
The second is called the Penrose process, and it relies on the conservation of momentum, and not on magnetism. The energy of rotation of a black hole is not inside the event horizon, but in an area outside of it, called the ergosphere, which comes into contact with the event horizon at the poles.
According to the Penrose process, if the object inside this area fell apart, one piece flew toward the black hole, and the other broke out, against the rotation of the black hole, the piece going out would have appeared with more energy extracted from the rotation. It produces a kind of "negative energy."
Both of these scenarios are convincing, but so far we have not been sure of the right answer.
"How can you extract energy from the rotation of a black hole to create jets?" Said theoretical physicist Kyle Parfrey of the National Laboratory. Lawrence Berkeley. "This has been a question for a long time."
The team developed a simulation of a collisionless plasma (where particle collisions do not play a major role) in the presence of a strong gravitational field of a black hole. They also explained the creation of electron-positron pairs in electric fields, which made it possible to obtain more realistic plasma densities.
The resulting simulation naturally led to the Blandford-Znaek process – electrons and positrons move in opposite directions around the black hole, producing energy in an electromagnetic field that flies out of the poles in the form of jets.
But this also made a change to the Penrose process. Due to the relativistic effects, some particles appear to have “negative energy” when they disappeared into the black hole, which slowed the rotation of the black hole, only a small fraction.
“If you were right next to the particle, you would not see anything strange in this. But to an outside observer, it looks like it has negative energy, ”said Parfri. New Scientist,
"You are left with this strange case when, if it falls into a black hole, it will lead to a decrease in mass and rotation."
Parfri remarked that this effect doesn’t really affect the total energy extraction very much, but it is quite possible that it is somehow connected with the electric currents that twist the magnetic fields.
Some components are also missing from the simulation, such as an accretion disk, and the positron-electron creation physics is not as detailed as it could be. The team will work to create an even more realistic simulation for a more detailed study of the process.
“We hope to provide a more consistent picture of the whole problem,” said Parfri.
Team research was published in the journal Physical review letters, and can be read completely on arXiv.