Black Hole Configurations with Total Entropy Less than A/4

TL;DR

This paper explores how surrounding a black hole with a reflective shell and adiabatically compressing it can reduce the total entropy below the standard Bekenstein-Hawking limit, suggesting entropy resides in the thermal atmosphere.

Contribution

It demonstrates that black hole entropy can be less than A/4 when considering the thermal atmosphere constrained by a shell, challenging traditional entropy bounds.

Findings

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

The thermal atmosphere accounts for the entropy residing outside the horizon.

Entropy reduction is achievable before quantum gravity effects dominate.

Abstract

If one surrounds a black hole with a perfectly reflecting shell and adiabatically squeezes the shell inward, one can increase the black hole area A to exceed four times the total entropy S, which stays fixed during the process. A can be made to exceed 4S by a factor of order unity before the one enters the Planck regime where the semiclassical approximation breaks down. 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.