Putting a Spin on Black Holes

Karl Schwarzschild worked out his equations for a hypothetical black hole that does not rotate on its axis. So scientists came to call a theoretical, non-spinning black hole a Schwarzschild black hole. This model worked well enough to calculate the distance from the singularity to the event horizon. But did it accurately describe the real state of a black hole? Most physicists felt that it did not. This is because they already knew that all of the bodies ever observed in outer space both rotate and possess a measurable property known as angular momentum.

Angular momentum is the tendency of a spinning object to keep on spinning. Even if the object gets larger or smaller, its original rotational energy will be preserved by altering the speed of rotation appropriately. For example, when a spinning ice skater extends his or her arms, in effect making the skater’s body larger, the rate of spin slows down; by contrast, when the skater draws his or her arms in tight to the body, the rate of spin increases.

This same effect can be seen when a large star collapses into a neutron star. The original star rotates at a certain rate, perhaps once every twenty or thirty days. After the collapse, its angular momentum is transferred into the much smaller neutron star, which now spins around many times in a second. It stands to reason that a black hole will follow this same scenario. Isaac Asimov summarizes it this way:

When a star collapses, to make up for that, its speed of rotation must increase. The more extreme the collapse, the greater the gain in speed of rotation. A brand-new neutron star can spin as much as a thousand times a second. Black holes must spin more rapidly still. There’s no way of avoiding that. We can say, then, that every black hole has mass and angular momentum.

Thus, what was needed after Schwarzschild introduced his calculations for non-spinning black holes was a mathematical solution that would describe the workings of black holes that rotate, as all black holes are believed to do. This goal was attained in 1963 by New Zealander astronomer Roy P. Kerr, who was then working at the University of Texas. Since that time, in Kerr’s honor, it has become common for scientists to refer to spinning black holes as Kerr black holes. The first definite confirmation of these objects came in August 2001, when scientists at NASA’s Goddard Space Flight Center (in Greenbelt, Maryland) detected the spin of a black hole lying about ten thousand light-years from Earth. (A light-year is the distance that light travels in a year, or about 6 trillion miles.)