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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?

Mike Rhodes, San Juan Capistrano, California
black-hole
This artist's concept depicts a supermassive black hole at the center of a galaxy. Illustration: NASA/JPL-Caltech

Any celestial body — such as a planet, star, or black hole — can rotate. It’s just that the physics of black holes are a bit trickier.

This is where the conservation of angular momentum — what makes a spinning figure skater speed up as she pulls her arms toward her body — comes in. The same notion affects a dying star that’s collapsing to form a black hole. If the star is spinning (meaning it has angular momentum), then the black hole that forms from it must also have the same amount of angular momentum, or spin. The spin rate, however, has a limit. If the star rotates too fast, instead of collapsing into a black hole, it would fling itself apart.

The theory describing black holes, Einstein’s general theory of relativity, tells us that only two properties, its mass and spin, describe its structure. These two numbers direct how the black hole warps space-time around it, and thus determine the surrounding gravitational field. A spinning black hole has a different gravitational field than a non-spinning one. Spin is an intrinsic aspect of the black hole.

These differences lead to a number of theoretically observable effects with ways to measure the black hole’s spin. For example, near a black hole lies a distance inside of which matter can’t follow stable orbits. This is called the innermost stable circular orbit (ISCO), and it’s closer to the event horizon for a faster-spinning black hole. (The “event horizon” is the distance where nothing — not even light — can escape out of.) This means that a black hole’s surrounding accretion disk can come nearer to the event horizon if the black hole is spinning.

There’s more available gravitational energy closer to the event horizon. So, other things being equal, 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. Recent efforts suggest that one of the Milky Way Galaxy’s most powerful accreting black holes, GRS 1915+105, may be spinning close to the maximum rate that a black hole can spin.

Another method involves measuring the speed that matter rotates around the black hole. Matter closer to the event horizon will orbit at a higher speed; this “orbital frequency” will be greater around a spinning black hole than one that doesn’t spin. 

— Tod Strohmayer, NASA’s Goddard Space Flight Center, Greenbelt, Maryland

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