Isentropic processes for axisymmetric Black Holes

TL;DR

This paper shows that classical isentropic absorption of charged particles is forbidden for certain black holes, based on near-horizon geometry and thermodynamics, with quantum considerations potentially allowing it under specific conditions.

Contribution

It proves a universal classical restriction on isentropic absorption for axisymmetric black holes, independent of the gravitational theory, and explores quantum effects in Kerr-Newman black holes.

Findings

Classical isentropic absorption is forbidden for all 3+1D stationary, non-extremal, axisymmetric black holes.

Quantum tunnelling can permit isentropic absorption under certain conditions.

Implications for the second law and extremality bounds are discussed.

Abstract

We demonstrate that the isentropic absorption of a classical charged test particle is classically forbidden for all 3+1 dimensional stationary, non-extremal, axisymmetric black holes in any diffeomorphism invariant theory of gravity. This result is derived purely from the near-horizon geometry and thermodynamic properties of the black hole spacetime, independent of the specific gravitational theory. We further consider the Kerr-Newman black hole in general relativity and analyse, using the quantum tunnelling approach, the conditions under which isentropic absorption may become allowed. Broader implications for the second law and extremality bounds are discussed.