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arXiv · 2506.06527

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

Nicolas Sanchis-Gual·Alejandro Belchí·Carlos Herdeiro·José A. Font
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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-Nordström 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.

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Nicolas Sanchis-Gual, Alejandro Belchí, Carlos Herdeiro, José A. Font. 2025-06-06. Reducing the irreducible: the charged black hole bomb in a moving cavity. https://arxiv.org/abs/2506.06527

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