One of the most notable aspects about our planet—if observed from the outside—is that it spins. Earth’s spin defines our days, setting the fundamental rhythm of life on our world.
The moon spins, too. So do the planets and all their moons. The sun spins as well, as do all stars. Even galaxies spin; the Milky Way rotates as stars swing around its center in multimillion-year orbits.
It seems obvious, then, that, cosmically speaking, everything spins—but this basic fact becomes downright bizarre in the head-spinning case of black holes. Spin, it turns out, is one of the most important characteristics for these gravitational monsters and has wide-ranging impacts from how they feast on matter to the ways they can shape the very structure of galaxies.
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Angular momentum is the core concept to understand when pondering spinning black holes. This is just like the momentum you’re familiar with from your everyday life (what we call linear momentum) but for a spinning object. It’s easiest to think about in terms of inertia—that is, how hard it is to stop an object from spinning. The faster something is spinning—and the more massive this something is—the more inertia it has and the harder its spinning is to stop.
Angular momentum is a special characteristic of an object in that it’s conserved. Absent some external force, something rotating on its own will continue to rotate forever. If you try to slow it down or speed it up by, say, grabbing it, some of its angular momentum will be transferred to you (or from you to it) so that the combined, total angular momentum between you and it doesn’t change.
An object’s angular momentum depends on its spin rate (of course!), its mass and, most importantly for our discussion, its size. Ice skaters provide a classic example: they throw their arms wide to start a spin, then, when they bring their arms in close to their body, their spin rate dizzyingly skyrockets. That’s how angular momentum is conserved; the size decreases, so the spin rate increases.
The same is true for stars, which are balanced on the cusp between exploding outward because of their radiance and collapsing inward on account of their internal gravity. When a high-mass star runs out of fuel, this balance is broken, and the core collapses, generating a gigantic explosion—a supernova—that blasts away the outer layers of the star. As the core shrinks down, it also spins up. And if its mass is more than about three times the sun’s, the core (which was formerly tens of thousands of kilometers wide) will become a black hole a mere 10 kilometers or so in diameter.
This dramatic downsizing can boost the black hole’s spin by a factor of several million over its more sedate stellar progenitor so that it spins hundreds of times per second. And because angular momentum is conserved, even though almost everything else about the star is annihilated in the black hole’s birth, the spin sticks around.
In fact, only three factors can be used to define a black hole: its mass, its angular momentum and its electric charge. In reality, that charge will be neutral or very close to it, so in practical terms, the first two factors are key.
Because of this, we expect that most, if not all, black holes will be spinning very fast.
It’s an odd concept because black holes don’t have a physical surface that can spin. But given that angular momentum can’t be destroyed, black holes must retain it when they form.
And this must be true for black holes born from stars, as well as the supermassive black holes we see in the centers of big galaxies, too, even though we don’t fully understand how these giants form. And remarkably, in some cases, we can actually measure these colossal cosmic spins.
The trick is to realize that, while a black hole’s angular momentum can’t simply disappear, it can certainly grow. Material falling in to a black hole adds its angular momentum to the system, increasing the black hole’s spin. There’s a theoretical limit to how fast a black hole can spin; it’s a complicated mathematical concept, but effectively that limit is when the black hole is rotating at the speed of light. It’s possible, though difficult, to measure the spin of a black hole by the way light is emitted from material just before it falls in, and, for example, the nearby galaxy NGC 1365 has a central supermassive black hole that has been measured to be spinning at very nearly this limit.
But of course it gets weirder. A bizarre aspect of Einstein’s general theory of relativity is that spacetime can act like a fabric, a substance in which masses are embedded. Einstein predicted that as massive objects rotate, they drag spacetime around them in what’s called the Lense-Thirring effect, or more commonly “frame dragging.” The effect is strongest very close to the black hole’s event horizon, its Point Of No Return, and gets weaker with distance. It’s like sticking a hand mixer into a big bowl of honey; the honey nearby will rotate along with the mixer, but is so viscous that a few centimeters out it will hardly move at all.
This relativistic frame dragging affects material just outside the black hole profoundly. Material close to the black hole gets dragged along with the space around it, accelerating by stealing energy from the black hole’s spin. This moving material generates a strong magnetic field, powered by the rotation. As the matter orbits the black hole, the magnetic field lines get wound up, creating twin vortices like tornadoes. These are so powerful they can draw matter away from the black hole and accelerate it away at nearly the speed of light! Astronomers call these beams “jets,” and with supermassive black holes they can be hundreds of thousands of light-years long.
Astronomers still aren’t sure how supermassive black holes form. Do they grow huge from material falling in from the still-forming host galaxy, or do many smaller black holes form in the center and merge to create a single huge one? The spin of the resulting black hole may tell us the answer. If it forms from a disk of infalling material, the spin will be near the limit, but if it formed from other black holes moving in random directions that merge, their spins can cancel out, leaving a final black hole with lower spin. It’s not quite that simple, of course, but it may be possible in principle to observe young supermassive black holes with something like the James Webb Space Telescope, to see if the spin can be measured and one of the formation methods supported or discounted.
We do know the supermassive black hole forms and grows along with its host galaxy. As protogalactic gas from a huge cloud collapses and coalesces into stars, the black hole in its center is already huge and spinning rapidly. If it forms jets, these plow through the matter falling in to form the galaxy itself, slamming into this material and even reversing its course, blasting it away. This can quench star formation, limiting how many stars the galaxy has. In this way, the spin of a black hole directly affects the size and structure of the galaxy around it.
No matter how you look at them, black holes are bizarre. The fact that they exist and we can understand them at all is, to me, exciting and profound. We live in a galaxy with a supermassive black hole at its center, and we may owe our existence to it. That alone is enough reason to try to understand them.
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