Why do black holes rotate?
Black holes are rotating. So what's rotating about them?
So in order to understand why black holes rotate, we have to understand how they form. Black holes essentially form when you have a star that collapses under its own weight. There's two main processes happening inside a star. You have gravity that's pulled all of this hydrogen and helium together, and then you have a nuclear fusion at the core that's essentially forcing all of these hydrogen atoms together to form helium and then three heliums together to form a carbon. And that's providing a lot of energy that's stopping the collapse. But eventually stars are going to run out of this fuel at the centre and you no longer have a big force outward due to nuclear fusion. So you just have gravity forcing itself inward, and that star starts to collapse on itself. Now, all of the stars that we have observed have had some sort of spin, they've all rotated. And, in physics, just like there's conservation of energy, you have this thing called conservation of angular momentum. You have to keep the same momentum over time, over everything. You have to maintain it. So when the black hole is collapsing, it's going to keep that momentum of the star that existed before it. So you have a star that's spinning much like you have an ice skater that's spinning: they bring their arms in, they spin faster. That's the same thing that's happening. You have a star that's spinning, it collapses in on itself. It still has to spin. The question of what exactly is spinning, well, you have a singularity at the centre of a black hole, and we usually say it's this infinite density thing. You have a lot of mass in this infinitesimally small space, but in reality it does have some volume. It's just so small that we can just kind of say that it doesn't. And so it is that tiny, tiny volume of mass at the centre of a black hole that's spinning. Now, we don't exactly know what is happening at the centre of a black hole and we may never know. So it's important to say that, but that's what we've theorised to have happened based off of equations and physics and all of that. But physics gets super wonky on these small scales.