Accretion Disks and Shooting Gas Jets

The Kerr solution recognizes the singularity, the event horizon, the Schwarzschild radius, and other black hole properties found in the Schwarzschild solution. However, the rapid spin inherent in a Kerr black hole creates a considerably more complex and dynamic situation.

First, the singularity is not a single point, but a warped area of space shaped like a ring. Second, the event horizon, marking the boundary between the black hole and ordinary space beyond it, is moving in the same direction that the singularity is spinning. And as it moves, it drags part of the nearby region of space along with it. As Kip Thorne explains:

The hole’s spin grabs hold of its surrounding space [shaped like the bell of a trumpet] and forces it to rotate in a tornado-like manner. . . . Far from a tornado’s core, the air rotates slowly, and similarly, far from the hole’s [event] horizon, space rotates slowly. Near the tornado’s core the air rotates fast, and similarly, near the horizon space rotates fast. At the horizon, space is locked tightly onto the horizon. It rotates at precisely the same rate as the horizon spins.

At the same time, any matter that happens to lie in that area of rotating space is carried along in the moving current. The matter spins around the outside of the black hole and forms a flattened disk of material in a manner similar to that in which small pieces of rock and ice form a flattened disk of rings around the planet Saturn. Scientists call this spinning disk around a black hole an accretion disk.

The material in the accretion disk plays an important role in another effect of a black hole’s spin the creation of two narrow but powerful jets of gas that appear to be shooting out of some black holes. (In reality, the jets do not come from inside the hole; instead, they originate in the disk, outside of the event horizon.)

Astronomers have advanced a number of convincing explanations to explain how these jets might form. In one, the tremendous pressures produced by the rapidly rotating gases in the accretion disk create two vortexes, whirlpools similar to the kind formed by water swirling down a drain. These vortexes shoot jets of hot gases outward at high speeds in opposite directions.

Thorne summarizes another possible scenario for the creation of these jets. This one involves a powerful magnetic field generated by the black hole’s spin:

Magnetic field lines [invisible strands of magnetism] anchored in the [accretion] disk and

This artist’s conception shows the accretion disk and gas jets of a black hole. The jets may be caused by powerful magnetic and electrical forces.

sticking out of it will be forced, by the disk’s orbital motion, to spin around and around. . . . Electrical forces should anchor hot gas (plasma) onto the spinning field lines. . . . As the field lines spin, centrifugal forces [forces pushing outward] should fling the plasma outward along them to form two magnetized jets, one shooting outward and upward, the other outward and downward.