What is a Kerr black hole?

A Kerr black hole is a rotating black hole, described by the solution Roy Kerr found in 1963. Since all stars rotate and collapse conserves angular momentum, essentially every black hole in the universe is a Kerr black hole.

What is frame dragging?

Frame dragging is the twisting of spacetime around a rotating mass. Near a spinning black hole it is strong enough that orbits, light paths and even the notion of standing still are pulled around in the direction of rotation.

What is the ergosphere?

The ergosphere is the region outside the event horizon where frame dragging is total: nothing can remain stationary relative to distant stars, because doing so would require travelling faster than light. Objects can still escape from it, unlike from inside the horizon.

Can you extract energy from a black hole?

From a spinning one, yes. The Penrose process uses the ergosphere to extract rotational energy, up to about 29% of the black hole's mass for maximal spin. The same physics, applied to magnetic fields, is thought to power relativistic jets.

What is a Spinning Black Hole?

A Kerr black hole is a rotating one — which means very nearly all of them. Stars rotate, collapse conserves angular momentum, and there is no way to shed it on the way down. Spin is not an exotic special case; a non-rotating black hole is the special case, and almost certainly does not exist anywhere.

Rotation does something no other property of a black hole does: it drags spacetime itself around with it. Close in, the dragging becomes total. There is a region outside the horizon — the ergosphere, bounded by an oblate surface that touches the horizon at the poles and bulges at the equator — in which standing still is not merely difficult but impossible, because holding position against the rotation of space would require moving faster than light. Anything that enters is compelled to orbit.

That has a consequence Roger Penrose spotted in 1969, and it is remarkable. Send an object into the ergosphere and split it in two, letting one half fall through the horizon on a retrograde orbit; the other half comes back out with more energy than the whole thing went in with. The difference is paid by the black hole, which slows down slightly. Up to 29% of a maximally spinning black hole's mass is stored as rotational energy and can, in principle, be taken out. It is the most efficient energy source general relativity permits, and it is what powers the jets of quasars and blazars.

The scene makes spin visible in three ways, all emerging from the marcher rather than drawn on. The shadow is not a circle: co-rotating light is carried further round than light fighting the rotation, so one side flattens and the whole silhouette shifts off the optical axis by about a fifth of its own radius — the amount Kerr theory predicts for this spin and viewing angle. The innermost stable orbit has collapsed almost onto the horizon, so the disc starts far closer in and runs blue-white instead of orange. And the ergosphere is drawn as a faint teal skin, so the region where nothing can be still has an edge you can see.