Rotating black holes (a.k.a. Kerr black holes) have a unique feature: a region outside their outer event horizon called the ergosphere. A black hole is formed when a really massive star runs out of fuel to fuse, blows up, leaving its core to implode under its weight to form a black hole.
The centre of a black hole is a gravitational singularity, a point where the general theory of relativity breaks down, i.e. where its predictions don’t apply. A black hole’s great gravitational pull emerges as if from the singularity.
The event horizon describes a sphere around the singularity: when anything enters this sphere, it can’t escape unless it travels faster than light (which is impossible). Just beyond this sphere, a rotating black hole will also have an ergosphere – a bigger sphere that an object can enter and then leave if it’s moving fast enough, but still less than the speed of light.
The label ‘ergosphere’ comes from ‘ergon’, the Greek word for ‘work’. It is so named because it is possible to extract matter and energy from the ergosphere, but not from beyond the event horizon.
Some scientists have suggested using this possibility to send an object into the ergosphere and allow it to accelerate there along the black hole’s direction of rotation, so that it comes out moving faster. This energy ‘gain’ will translate to the black hole losing some angular momentum.
