Zhong-Ying Fan

Water in a kettle, as it boils, clearly separates into liquid and steam. But under enormous pressure, the boundary vanishes: the substance enters a supercritical state — neither one nor the other, but something in between. Black holes can do this too.

A rotating black hole is like a whirlpool in a river — spacetime swirls around it, dragging along everything nearby. In some gravity theories, this whirlpool also includes an extra field — a vector field. At each point, it has a direction and a length, like a compass needle.