A black hole isn’t the energy sink you might think it is. By hurling matter towards a black hole, it might be possible to get energy out of it. Learn how a spinning black hole could be an energy turbine for an entire civilization.

#### Spin and the Ergosphere

Like nearly every other thing in the universe, black holes spin. Their spin makes a difference in the space around them. It makes a black hole, practically speaking, no longer perfectly symmetrical.

The event horizon of a black hole is the point at which no light or matter can escape. It’s where the black hole turns truly black. It looks like a perfect sphere around the central point of the black hole. No matter which direction you approach the black hole from, your experience with the event horizon will be the same.

But then there’s the ergosphere. The ergosphere is the region of spacetime that gets dragged around by the spinning black hole, the way water in a lake is dragged around by a central whirlpool. It’s more of an ellipse than a sphere—or perhaps a better way to think about it would be the white of an egg and the event horizon being the yoke, since the ergosphere exists only outside of the event horizon, not inside. The ergosphere is thinnest at the north or south “pole” of the spinning black hole. Right at the poles, the boundary of the event horizon and the ergosphere are the same. As you move towards the “equator” of a black hole, the ergosphere gets thicker and thicker.

So your experience with the ergosphere will vary a great deal, depending on the direction from which you approach the black hole. Approach it from a pole, and you won’t feel a trace of spin before you hit the event horizon and disappear forever. Approach it from the midline, and you’ll get caught up in the spin long before you’re even close to the event horizon.

#### Divide and Conquer

So something, let’s say the spaceship you’re riding in, is caught in the ergosphere. At that point, it’s no longer possible for the spaceship to stay still with respect to the rest of the universe. It gets dragged into a spin, along with the rest of spacetime around it. You can even feel your space ship breaking apart.

Don’t worry, all is not lost.

According to physicist Roger Penrose, not only could you live through this, you could fly away faster than you came in. When an object with just the right trajectory and the right velocity gets into the ergosphere, it snaps in two. The unfortunate half of the of the object gets pulled, or falls, down into the event horizon and disappears from the known universe forever. The other half of the object comes zooming out of the ergosphere again.

And the half of the object that escapes has more energy than the entire object did in the first place. Where does that energy come from? It comes from the spin of the black hole. If you are on a merry-go-round spinning one direction, and you hurl a fellow rider off in the direction of the spin, the rider goes flying and the merry-go-round spins a little more slowly than it had been before. Its energy has been given to the ejected rider. Black holes do the same thing; a tiny part of their spin goes into hurling the top half of your space ship away with more energy than the entire ship had when it first entered the ergosphere. Some people think that a space civilization may be able, with careful calculations and a lot of power, to spin down a black hole by throwing things towards it, and harvesting the energy they have when they’re ejected again.