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 spiralling 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.
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