How Black Holes Spin Space Time

If there’s one thing cooler than a black hole it’s a rotating black hole. Why? Because we can use them as futuristic power generators, galactic-scale bombs, and portals to other universes.

Black holes are self-sustaining holes in the fabric of spacetime. Space at the event horizon cascades downwards, dragging more space behind it, sort of like how water drags itself near the edge of a waterfall. In a Kerr black hole, space above the event horizon is dragged around in a circle - so less waterfall and more whirlpool. Water spiraling down a drain in a flat sink doesn’t know about the hole - it only knows about the motion of the water around it. In fact it’s possible to construct a black hole in general relativity rotating or otherwise - without any mass. Warp spacetime so it looks like the exterior of a black hole, and that warping will persist. So what is rotating? Spacetime is rotating.