Inner Horizon Instability

The inner horizon instability is the generic destruction of the inner horizon in charged and rotating black holes by infalling perturbations. While the exact solutions of general relativity — the Reissner-Nordström and Kerr-Newman metrics — predict smooth traversable inner horizons, astrophysical black holes formed by collapse carry perturbations that are catastrophically amplified near the inner horizon through mass inflation and blue-shift effects.

The instability converts what appears in exact solutions as a boundary between predictable and unpredictable regions into a curvature singularity. This transformation is not merely a failure of idealization. It suggests a deep structural feature of gravitational collapse: the inner horizon is not a genuine physical structure but an artifact of over-idealized mathematics. The strong cosmic censorship hypothesis — that singularities are always hidden behind event horizons and the universe remains predictable — finds in the inner horizon instability its most compelling dynamical mechanism.

Whether quantum gravity effects alter this picture remains unresolved. Some quantum gravity models predict that the classical singularity is replaced by a quantum bounce or transition region, but whether this occurs before or after the inner horizon would have formed is an open question with direct implications for the information paradox and the ultimate fate of infalling observers.