In a Kerr black hole, there is an outer event horizon and an inner Cauchy horizon, whereas a Schwarzschild black hole has only one horizon.

• A. Kerr black holes have no event horizon; Schwarzschild black holes have multiple horizons.
• B. Both black holes have the same horizon structure; rotation does not affect horizons.
• C. Kerr black holes have an outer event horizon and an inner Cauchy horizon; Schwarzschild black holes have a single horizon.
• D. Schwarzschild black holes have two horizons; Kerr black holes have one.

Answer: (C)

The important idea is how rotation changes the way light and cause-and-effect travel near a black hole. In a rotating (Kerr) black hole, the spacetime structure admits two distinct null surfaces where the radial light cones close: an outer boundary called the event horizon, and an inner boundary known as the Cauchy horizon. This comes from the Kerr solution, where the horizon locations come from a function that can have two positive roots. The outer root is the one that traps everything outside, while the inner root marks a second boundary inside the black hole, beyond which predictability from initial data breaks down due to the complicated inner geometry and mass-energy effects.

In contrast, a non-rotating (Schwarzschild) black hole has no such inner boundary. Its horizon is a single null surface at the Schwarzschild radius, so there is only one horizon.

So the statement that Kerr black holes have an outer event horizon and an inner Cauchy horizon, whereas Schwarzschild black holes have a single horizon, matches how rotation splits the horizon structure into two, while no rotation yields just one. (If the rotation parameter is extreme, the two horizons coincide; if rotation were hypothetical beyond that limit, horizons would disappear, but that’s beyond typical physical black holes.)