Kate Becker
Kate Becker

Black holes are boring.

Galaxies are a menagerie of glittering whirlpools and luminous eggs; they appear as glowing bulges, blobs and contortions of every shape and variety, each one unique.

Stars shine in a rainbow of colors, and they host flocks of planets that are diverse beyond astronomers' imaginations. No two are alike.

But black holes? If you've seen one, you've seen them all. That's because, no matter what goes into the black hole -- whether it was Elvis or Bigfoot or, more likely, cosmic dust and gas -- the black hole "looks" the same, which is to say, like nothing at all. Since nothing can escape from the black hole, not even light, if you could look straight into the eye of a black hole, you would be staring into a blank, black void. (Whether information about the objects devoured by black holes is truly lost forever is a story for another day.)

You can picture a black hole, then, as an enormous, black cue ball, utterly featureless and completely identical to every other cue ball in the universe. But black holes do have a few distinguishing properties: mass, spin and electric charge.

Measuring the mass of a black hole isn't too hard. Astrophysicists have worked out the mass of the black hole at the center of the Milky Way, for example, by carefully clocking the motion of stars orbiting around its gravitational vortex. From there, they can infer the mass of the black hole itself: about four million times the mass of our sun.


But determining a black hole's spin is much trickier -- and potentially more interesting. That's because the rate at which the black hole spins tells you something about how the black hole evolved and grew. For the supermassive black holes that live at the centers of galaxies, that can help reveal the history of the entire galaxy.

A black hole's spin can also be used to test Einstein's theory of gravity, general relativity. Because black holes are so dense, they warp space and time dramatically, and a spinning black hole will actually drag space along with it as it rotates. To return to our cue ball analogy, picture that black cue ball hitting a snag on the felt of a pool table and pulling and twisting the green fabric as it whirls.

Astronomers can't observe the spinning "cue ball" -- the black hole -- directly, and they can't look at the "felt" -- empty space -- either. Luckily, there are other balls in play on this cosmic pool table: a swirling disk of dust and gas, called an accretion disk, that encircles the black hole. As X-rays bounce off this accretion disk, they are distorted by the disk's rotation. So by measuring X-rays coming from the vicinity of the black hole, it should be possible to infer whether the black hole is really spinning and, if it is, how fast and in what direction.

At least, that's how it works in principle. In practice, astronomers feared that clouds around the black hole were muddying their measurements, making it impossible to be sure that the distortions they were seeing were really due to the effects of relativity. But now, observations from an X-ray satellite called the Nuclear Spectroscopic Telescope Array, or NuSTAR, are being combined with data from the older XMM-Newton mission to resolve this puzzle.

NuSTAR looks at extremely high-energy X-ray light, where it is possible to disentangle the effect of obscuring clouds from the real prize: gravitational distortion due to the black hole's spin.

When astronomers used this technique to look at the supermassive black hole at the center of a galaxy called NGC 1365, they found that it was spinning so fast that it is butting up against the very limits of relativity. And now that they've shown it can be done, astronomers are looking forward to measuring how fast black holes all around the universe are spinning.

Of course, you probably knew that I never thought black holes were boring. But now that astronomers are learning how to make black holes tell their stories, things are really going to get interesting.
