Down the (Gravitational) Drain

1963: Roy Kerr

In the early 1960s, not only was there no direct evidence of black holes, they didn't even have a name yet. But thanks to a solution of Albert Einstein's equations by a young University of Texas mathematician, physicists had a perfect model of what they would look like and how they would behave.

Roy Kerr

Roy Kerr was a New Zealand native who was not yet 30 years old. Yet he devised a solution to the field equations of Einstein's theory of gravity that described how massive stars "drag" the spacetime around them like water circling around a bathtub drain.

Kerr's solution required a mathematically pure body, which had only two characteristics: mass and spin. Ordinary stars don't fit that model because they have "hair" — lots of messy qualities like magnetic fields and winds that can have an effect on the space around them.

Other physicists quickly realized, however, that Kerr had devised the perfect description of a spinning black hole. And since every black hole, like every other object, is expected to spin, Kerr had described every black hole in the universe.

An earlier solution to Einstein's equations, for a perfectly spherical, non-spinning body, produced two components: a singularity, which contains all of the black hole's mass, and the event horizon, which is the "boundary" between the black hole and the outside universe.

But all real black holes should spin, either because the stars from which they were born were spinning or because the process of ingesting infalling matter makes them spin.

Kerr's equations, which added rotation to the mix, added a third component to a black hole, called an ergosphere. It's a region of spacetime outside the event horizon that is dragged by the gravity of the black hole. In profile it looks like an oval, with a wider bulge around the equator than through the poles. (A spinning black hole is wider through the equator than the poles as well.)

(Kerr's work also seemed to lead to a model in which the black hole's matter is compressed into a ring, not a singularity. In this model, the ring could serve as a portal to another region of spacetime; anything entering it would exit through a "white hole" somewhere else in space and time. Physicists have since rejected this idea in favor of a singularity.)

As seen by an outside observer, an object falling toward the black hole, such as a spacecraft, would be captured in the ergosphere and would appear to spin around the black hole. An astronaut aboard the ship, though, would see his path as a straight line into the black hole.

Kerr's work not only led to a perfect model of real black holes, it led physicist Roger Penrose to devise a way to "steal" energy from a black hole.

A spaceship would fly into the swirl of spacetime around the black hole and split apart. Half of the ship would fall into the black hole, while the other half would be thrown back into space. The escaping half would carry out much more energy than the whole spacecraft carried in. This "theft" of rotational energy would cause the black hole to slow down a tiny bit. Present-day spacecraft use a similar technique to steal orbital energy from a planet, getting a boost toward more-distant destinations.

With Kerr's equations and the follow-up work of several other researchers, scientists now had a perfect description of a black hole. Now all they needed was a name for these oddballs, and evidence that they really exist.