Magnetic Field Shapes Spinning Black Hole
On May 19, Jonathan McKinney sent an email to his supervisor and a colleague at Stanford University. "Let's try to keep this quiet for now," he wrote, and then launched into an explanation about electromagnetic force, super hot plasma and black hole spin. "I know usually new results are always exciting and feel more worthy than they really are," he concluded. "But in this case, objectively, it seems no one has even thought of this possibility."
The astrophysicist, who was finishing up a five-year stint at Stanford and about to start a tenure-track position at the University of Maryland, had made a discovery that he suspected could change the way we understand the behavior of black holes.
A black hole is Einstein's solution for what happens when a star dies. Some massive stars will burn off all of their fuel and then collapse into what's called a neutron star, a fantastically tiny, tremendously dense object. (Imagine a billion tons of weight condensed into a teaspoon.)
And some of these neutron stars become so massive that gravity will cause them, too, to collapse under their own weight to a point -- a black hole. Black holes are thought to live at the center of most galaxies. They are voracious eaters that gulp up everything they can -- dust clouds, stars and other space debris -- and they're shrouded by what's known as an event horizon. That's the area where the gravitational pull is so strong that nothing -- not even light -- can escape.
Some black holes spin, dragging matter and space-time around them as they rotate. That material takes the form of a disk. The turbulence and mixing in this disk creates a balloon of magnetic field. And the charged particles falling through the magnetic field into the black hole create a powerful current, which is flung out in a jet of superheated plasma.
A black hole is a very messy eater, explained Shep Doeleman, an astronomer with MIT's Haystack Observatory and the Harvard-Smithsonian Center for Astrophysics. Focusing all of the matter, gas, dust and gravitational field surrounding a black hole into an incomprehensibly tiny region creates a sort of "cosmic traffic jam."
"Think about trying to suck an elephant through a straw," Doeleman said.
Near the black hole, that traffic jam causs matter to break down into its most basic parts and particles to rub up against each other, creating friction. The temperature of the gas heats to billions of degrees, which causes the above-mentioned jet to flame out of the black hole's poles and spray across the galaxy close to the speed of light. The jet can be hundreds of times more powerful than all of the stars in the galaxy combined, said Roger Blandford, director of the Kavli Institute for Particle Physics and Cosmology, and also an author on the paper.
It's that jet that allows scientists to observe black holes. For example, the black hole at the center of the galaxy M87, which is 53 million light years away and nearly 7 billion times as massive as our sun.
"This jet stretches hundreds of thousands of light years," Doeleman said. "If I was giving you direction to M87, I'd probably say, "Turn left at the giant jet."
But back to the research. At the time of our interview, McKinney was working in a small office on the fourth floor of the physics building of the University of Maryland. He had just moved and his office had boxes everywhere, a bicycle shoved into a corner and two tables awkwardly positioned in the middle of the room. The tables were coated with a layer of black soot that had dropped through a ceiling air vent.
"My own jet of grossness," he called it.
It's not surprising he'd use that metaphor - jets are very much on his mind. What McKinney found was that the jet of a black hole plays a much greater role in the black hole’s behavior than scientists had realized.
He discovered this by writing advanced computers models that simulated black holes. In the past, Blandford said, scientists lacked the computer programs and computing power to study the physical laws that govern the behavior of magnetism and gas around black holes.
"And obviously," Blanford joked, "one can't do a real experiment because we're talking about massive black holes here. We don't have one available in the laboratory at the moment."
The simulations showed that magnetic fields that thread through the orbiting disk of gas in a black hole create the current that becomes the jet.
They also showed that if the axis of the disc is misaligned with the axis of the spinning black hole, it still continues to spin, and the jet is able to twist in space and find a stable configuration.
McKinney simulated a range of black hole spins and found that no matter how he oriented the jets that were launched by the black hole, those jets eventually became aligned with the black hole's rotational axis. But before that happened, the jet put up a fierce battle, pushing against the disk.
You can watch a video of the simulation here. The first half shows the normal behavior of a black hole feeding on matter. Halfway through, the black hole gets tilted and the jet disappears, reappears on the new spin axis and realigns:
He knew the magnetic fields were strong near the black hole. By studying the jet's behavior, he realized that magnetic fields were actually moving the disc around. The jet, he suspected, was dominating the black hole.
"Normally you think the disk determines everything about the jet's behavior," he said. "The orientation, the way it points, how much energy it contains. This result is so amazing, because it shows that it's the other way around in fact. It shows that in most cases, actually the jet is probably the dominant force. The jet is probably the most powerful part of the black hole-disc jet system."
"I think the simulations that he's done are very complete, very thorough," Doeleman said. "I think it's an important new mechanism for how thick disks interact with black holes. It looks very compelling to me."
Doeleman's team is already using a telescope called Event Horizon, which links radio dishes throughout the world, to peer at the base of the M87 jet.
He added that he hopes to observe evidence of the mechanisms that McKinney describes in an actual black hole. "We may be able to do it with M87," he said.