Where is the ergosphere located relative to the event horizon, and what energy process does it enable?

• A. The ergosphere lies inside the event horizon and energy extraction is not possible.
• B. The ergosphere lies outside the event horizon and energy extraction via frame-dragging is possible.
• C. The ergosphere lies exactly on the event horizon and cannot affect energy processes.
• D. The ergosphere does not exist for rotating black holes.

Answer: (C)

In a rotating black hole, spacetime is dragged around by the hole’s spin. This creates an ergosphere, a region outside the event horizon where no object can remain at rest relative to distant stars because spacetime itself is being pulled along. The event horizon sits inside this ergosphere, not on its boundary. Inside the ergosphere, the dragging is so strong that every worldline must rotate with the hole, which makes it possible to extract energy from the rotation through frame-dragging effects.

This energy extraction is tied to the Penrose process: a particle entering the ergosphere can split, with one fragment falling into the black hole with negative energy (relative to infinity) and the other escaping to infinity with more energy than the original particle had. So the correct understanding is that the ergosphere lies outside the event horizon and energy extraction via frame-dragging is possible.