Second Law Violations in Lovelock Gravity for Black Hole Mergers

TL;DR

This paper demonstrates that in Lovelock gravity theories, black hole mergers can lead to instantaneous decreases in entropy, challenging the classical second law of thermodynamics.

Contribution

It constructs explicit scenarios in Lovelock theories where the second law of black hole thermodynamics is violated during mergers.

Findings

Entropy decreases during certain black hole mergers in Lovelock gravity.

Lovelock theories can violate the classical second law of thermodynamics.

Open issues in defining the second law in these theories are discussed.

Abstract

We study the classical second law of black hole thermodynamics, for Lovelock theories (other than General Relativity), in arbitrary dimensions. Using the standard formula for black hole entropy, we construct scenarios involving the merger of two black holes in which the entropy instantaneously decreases. Our construction involves a Kaluza-Klein compactification down to a dimension in which one of the Lovelock terms is topological. We discuss some open issues in the definition of the second law which might be used to compensate this entropy decrease.