Magnetic Black Holes: from Thomson Dipoles to the Penrose Process and Cosmic Censorship

TL;DR

This paper explores how magnetic black holes interact with charged matter, revealing mechanisms for angular momentum loss, superradiant energy extraction, and the stability of extremal black holes against overcharging or overspinning.

Contribution

It introduces new insights into the dynamics of magnetic black holes, including superradiant accretion processes and the proof of their stability under certain energy conditions.

Findings

Black holes tend to lose angular momentum by accreting charges while remaining neutral.

Superradiant accretion can extract energy and angular momentum, involving floating bubbles.

Extremal dyonic black holes cannot be overcharged or overspun if the null-energy condition is satisfied.

Abstract

We consider accretion of charged test matter by rotating, magnetic black holes and discuss a number of aspects in which the interaction of the angular momentum contained in the electromagnetic field and the spin of the hole plays a fundamental role. First, we argue that such a black hole tends to lose its angular momentum by accreting charges while remaining globally neutral. Then, we show that accretion can happen in a superradiant manner via an enhanced Penrose process. In particular, we find that the regions from which energy and angular momentum can be extracted contain the axis of rotation and, in some cases, consist of floating bubbles disconnected from the black hole itself. Finally, we address the question of whether extremal dyonic rotating black holes can be overcharged or overspun via accretion of arbitrary matter, and prove that this can not happen if the null-energy condition holds. We conclude by discussing some future research directions.