Black Holes with Less Entropy Than A/4

TL;DR

This paper demonstrates that by surrounding a black hole with a reflective shell and adiabatically squeezing it inward, the entropy can be made less than A/4, challenging traditional views on black hole entropy.

Contribution

It introduces a method to reduce black hole entropy below A/4 by using a reflective shell, suggesting entropy resides in the thermal atmosphere.

Findings

Black hole entropy can be less than A/4 with a reflective shell.

The entropy reduction factor can be of order unity.

Practical limitations restrict the extent of entropy reduction.

Abstract

One can increase one-quarter the area of a black hole, A/4, to exceed the total thermodynamic entropy, S, by surrounding the hole with a perfectly reflecting shell and adiabatically squeezing it inward. A/4 can be made to exceed S by a factor of order unity before the shell enters the Planck regime, though practical limitations are much more restrictive. One interpretation is that the black hole entropy resides in its thermal atmosphere, and the shell restricts the atmosphere so that its entropy is less than A/4.