Entropy bounds for uncollapsed rotating bodies

TL;DR

This paper extends entropy bounds from static to rotating uncollapsed bodies using classical relativity, thermodynamics, and the Unruh effect, establishing an upper entropy limit proportional to surface area.

Contribution

It provides a novel extension of entropy bounds to rotating systems, incorporating quantum effects via the Unruh effect within classical general relativity.

Findings

Entropy of uncollapsed matter bounded by surface area A.

Upper bound on entropy: S <= (1/2) A.

Extension from static to rotating configurations.

Abstract

Entropy bounds in black hole physics, based on a wide variety of different approaches, have had a long and distinguished history. Recently the current authors have turned attention to uncollapsed systems and obtained a robust entropy bound for uncollapsed static spherically symmetric configurations. In the current article we extend this bound to rotating systems. This extension is less simple than one might at first suppose. Purely classically, (using only classical general relativity and basic thermodynamics), it is possible to show that the entropy of uncollapsed matter inside a region enclosed by a surface of area A is bounded from above by S <= kappa(surface) A / (4 pi T). Here kappa(surface) is a suitably defined surface gravity. By appealing to the Unruh effect, which is our only invocation of quantum physics, we argue that for a suitable class of fiducial observers there is a lower bound on the temperature (as measured at spatial infinity): T >= max kappa(FIDOs) / (2 pi). Thus, using only classical general relativity, basic thermodynamics, and the Unruh effect, we are able to argue that for uncollapsed matter S <= {1/2} A.