Visualization of a general-relativistic collisionless plasma simulation. Image: Parfrey/LBNL
Researchers used one of the world’s most powerful supercomputers to better understand how jets of high energy plasma escapes the intense gravity of a black hole, which swallows everything else in its path—including light.Before stars and other matter cross a black hole’s point of no return—a boundary known as the “event horizon”—and get consumed by the black hole, they get swept up in the black hole’s rotation. A question that has vexed physicists for decades was how some energy managed to escape the process and get channeled into streams of plasma that travel through space near the speed of light.
As detailed in a paper published last week in Physical Review Letters, researchers affiliated with the Department of Energy and the University of California Berkeley used a supercomputer at the DoE’s Lawrence Berkeley National Laboratory to simulate the jets of plasma, an electrically charged gas-like substance.The simulations ultimately reconciled two decades-old theories that attempt to explain how energy can be extracted from a rotating black hole.The first theory describes how electric currents around a black hole twist its magnetic field to create a jet, which is known as the Blandford-Znajek mechanism. This theory posits that material caught in the gravity of a rotating black hole will become increasingly magnetized the closer it gets to the event horizon. The black hole acts like a massive conductor spinning in a huge magnetic field, which will cause an energy difference (voltage) between the poles of the black hole and its equator. This energy difference is then diffused as jets at the poles of the black hole.The other theory described the Penrose process, in which particles nearing a black hole’s event horizon split apart. In this scenario, one half of the particle shoots away from the black hole and the other half of the particle carries negative energy and falls into the black hole.“There is a region around a rotating black hole, called the ergosphere, inside of which all particles are forced to rotate in the same direction as the black hole,” Kyle Parfrey, the lead author of the paper and a theoretical astrophysicist at NASA, told me in an email. “In this region it’s possible for a particle to effectively have negative energy in some sense, if it tries to orbit against the hole’s rotation.”
In other words, if one half of the split particle is launched against the spin of the black hole, it will reduce the black hole’s angular momentum or rotation. But that rotational energy has to go somewhere. In this case, it’s converted into energy that propels the other half of the particle away from the black hole.According to Parfrey, the Penrose process observed in their simulations was a bit different from the classical situation of a particle splitting that was described above, however. Rather than particles splitting, charged particles in the plasma are acted on by electromagnetic forces, some of which are propelled against the rotation of the black hole on a negative energy trajectory. It is in this sense, Parfrey told me, that they are still considered a type of Penrose process.
The surprising part of the simulation, Parfrey told me, was that it appeared to establish a link between the Penford process and Blandford-Znajek mechanism, which had never been seen before.To create the twisting magnetic fields that extract energy from the black hole in the Blandford-Znajek mechanism requires the electric current carried by particles inside the plasma and a substantial number of these particles had the negative energy property characteristic of the Penrose process.“So it appears that, at least in some cases, the two mechanisms are linked,” Parfrey said.Parfrey and his colleagues hope that their models will provide much needed context for photos from the Event Horizon Telescope, an array of telescopes that aim to directly image the event horizon where these plasma jets form. Until that first image is produced, however, Parfery said he and his colleagues want to refine these simulations so that they conform even better to existing observations.