When scientists talk about the rotation of a black hole, what are they referring to? If we can’t see a black hole, how can we tell if it rotates?

This artist’s concept depicts a supermassive black hole at the center of a galaxy. Illustration: NASA/JPL-Caltech

A spinning black hole has a different gravitational field than a non-spinning one. Spin is an intrinsic aspect of the black hole. A spinning black hole will radiate more energy than a non-spinning one. The accretion disk’s temperature also increases the faster the black hole spins. We can observe these differences in the X-ray spectra of accreting black holes.

Another method involves measuring the speed that matter rotates around the black hole.