Kerr Black Holes

Most real black holes are not static Schwarzschild black holes — they spin. A rotating black hole is described by the Kerr metric, and its properties are dramatically different. Its event horizon is smaller than a non-spinning hole of the same mass, and it is surrounded by a region called the ergosphere, where spacetime itself is dragged along with the spin. Inside the ergosphere, nothing can stay still; everything is forced to co-rotate with the black hole. Roger Penrose showed that an object entering this region can steal some of that rotational energy and be flung back out faster than it came in — the Penrose process.