Gedanken experiments on nearly extremal black holes and the Third Law

TL;DR

This paper explores thought experiments involving nearly extremal black holes, examining their thermodynamics and the Third Law, and argues that apparent violations are due to subtleties in the absorption process rather than actual physical violations.

Contribution

It introduces a gedanken experiment pushing black holes to extremality and clarifies why this does not violate the Third Law of black hole thermodynamics.

Findings

Black holes can be spun up to near extremality with reasonable matter.

Apparent violations of the Third Law are due to absorption process subtleties.

No actual violation of the Third Law occurs in realistic scenarios.

Abstract

A gedanken experiment in which a black hole is pushed to spin at its maximal rate by tossing into it a test body is considered. After demonstrating that this is kinematically possible for a test body made of reasonable matter, we focus on its implications for black hole thermodynamics and the apparent violation of the third law (unattainability of the extremal black hole). We argue that this is not an actual violation, due to subtleties in the absorption process of the test body by the black hole, which are not captured by the purely kinematic considerations.