Isentropic processes for axisymmetric Black Holes

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 analyze within the semiclassical WKB approximation, the conditions under which isentropic absorption may become allowed. Broader implications for the second law and extremality bounds are discussed.