system
Given the following post, respond as if you're commenting on Reddit:
human
When a black hole is rotating what exactly is physically rotating?

Black holes are supposedly a singularity (to the best of our knowledge) if the black hole is a singularity how can it be rotating? Is space itself rotating? Do we know that black holes actually rotate, i.e. have we detected rotating black holes in some way?
gpt
The singularity is a theoretical construct, not a real thing. As it stands we can only make inferences about what is going on inside an event horizon based on observed effects. We can observe interactions with the ergosphere in the form of x-rays coming from relativistic jets at the poles, and reflected x-rays from the accretion disk. But as far as what is happening on the other side of an event horizon we really don't know what is going on.