Reducing the irreducible: the charged black hole bomb in a moving cavity

TL;DR

This paper explores how a moving cavity affects superradiant phenomena around charged black holes, revealing new equilibrium states and mass reduction mechanisms that challenge traditional thermodynamic limits.

Contribution

It introduces a fully non-linear numerical analysis of charged black holes with a moving mirror, uncovering novel equilibrium states and mass dynamics influenced by cavity size variations.

Findings

Expanding the mirror enhances superradiant charge extraction.

Shrinking the cavity allows the black hole to reduce its irreducible mass.

In the limit of no cavity, the system reverts to Reissner-Nordström black hole.

Abstract

We revisit the charged black hole bomb by numerically solving the fully non-linear Einstein-Maxwell-(charged, complex) Klein-Gordon system with a moving mirror. By dynamically varying the cavity size, we find that the system evolves toward new hairy black hole equilibria. Expanding the mirror radius enhances superradiant extraction, increasing both the scalar field charge and the black hole's irreducible mass. Remarkably, on the other hand, shrinking the cavity size has the opposite effect: the black hole is able to reduce its irreducible mass as more charge than energy flows back from the field, without violating charge conservation or energy conditions. As a consistency check, in the limit of a vanishing cavity, we find that the system returns to the original Reissner-Nordstr\"om configuration. We discuss the implications of these findings for black hole thermodynamics in confined configurations where superradiant modes exist and the limitations of this setup, particularly in relation to Hawking's black hole area theorem.