Outer and Inner Horizons
Unlike the single horizon in the Schwarzschild solution, the Kerr metric possesses two horizons. The outer horizon \(r_+\) acts as the standard point of no return. The inner horizon \(r_-\), known as the Cauchy horizon, marks the boundary beyond which predictability breaks down. Crossing the outer horizon does not immediately lead to destruction; you can pass through the ergosphere and enter the region between the horizons.
The Ring Singularity
At the center of a Kerr black hole, the singularity is not a point but a ring lying in the equatorial plane. The radius of this ring is equal to the specific angular momentum \(a\). This topology allows for theoretical trajectories that pass through the ring into regions of negative radial coordinate, although physical stability near the Cauchy horizon remains a subject of intense debate.
Horizon Radii
The radii of the horizons are given by \(r_{\pm} = \frac{GM}{c^2} \pm \sqrt{(\frac{GM}{c^2})^2 - a^2}\). For a horizon to exist, we must have \(a \leq GM/c\). If \(a\) exceeds this limit, the singularity becomes 'naked,' violating the Cosmic Censorship Hypothesis.