black hole spin

Almost everything in the universe spins — planets, stars, galaxies — because they inherit rotation from the matter that formed them. Black holes are no exception. When a spinning star collapses, its rotation is not lost; it is concentrated, often to an astonishing degree, into the black hole that remains. Black hole spin is one of just two numbers (along with mass) needed to fully describe a real black hole, and a rapidly spinning black hole behaves quite differently from a still one.

A rotating black hole literally drags space around with it, a relativistic effect called frame-dragging — nearby space itself is swept into rotation, like honey stirred by a spoon, so that nothing close by can stay still even if it tries. Spin is measured on a scale from 0 (not rotating) up to a maximum of 1, beyond which the math forbids a black hole to spin; the fastest measured black holes whirl at well over 90 percent of this limit, their horizons turning at a sizeable fraction of the speed of light. Spin also shrinks the innermost stable orbit, letting matter fall in from closer and release more energy.

Spin matters enormously in practice. A black hole's rotation is the likely power source for some of the most energetic jets in the universe, as twisted magnetic fields tap the spin energy and fling matter outward at nearly light speed. Spin also stores extractable energy — in principle up to about 29 percent of a maximally spinning black hole's mass could be harvested. And measuring spin in merging black holes, through gravitational waves, gives clues to how the original stars lived and died. A note: spin does not make a black hole 'suck harder' — it changes the geometry, not the long-range pull.