Which statement best distinguishes a Schwarzschild black hole from a Kerr black hole in terms of horizon structure and rotational effects?

• A. A Schwarzschild black hole is non-rotating with a single event horizon and no ergosphere; a Kerr black hole is rotating, has an outer event horizon and an inner Cauchy horizon, and possesses an ergosphere where frame dragging allows energy extraction.
• B. A Schwarzschild black hole has an ergosphere, while Kerr has no event horizon.
• C. Both Schwarzschild and Kerr black holes have identical horizon structures, differing only in accretion disk temperature.
• D. The Schwarzschild solution includes both event and Cauchy horizons, while Kerr has only one horizon.

Answer: (A)

Horizon structure and rotational effects distinguish Schwarzschild and Kerr black holes. A Schwarzschild black hole is non-rotating, so it has a single event horizon at the Schwarzschild radius and no ergosphere; spacetime outside the horizon is static and there’s no frame dragging. In contrast, a Kerr black hole is rotating, which changes the geometry: there are two horizons—the outer event horizon and an inner Cauchy horizon—and there is an ergosphere outside the outer horizon where frame dragging is so strong that no observer can remain at rest relative to distant stars. In this ergosphere, energy extraction from the black hole’s rotation is, in principle, possible.

That combination—rotation with two horizons plus an ergosphere—best distinguishes the two cases. The other statements either claim an ergosphere for the non-rotating case, deny the extra horizon in rotation, or suggest identical horizon structures, which doesn’t match how rotation changes the spacetime.