Radiation entropy bound from the second law of thermodynamics

TL;DR

This paper demonstrates that the entropy bound for thermal radiation, suggested by previous heuristic arguments, can be derived from the second law of thermodynamics assuming CPT symmetry and general matter conditions, implying a universal density of states limit.

Contribution

It provides a thermodynamic derivation of the radiation entropy bound based on fundamental symmetries and conditions, strengthening the theoretical foundation of the bound.

Findings

The entropy bound follows from the second law with CPT symmetry.

A wide class of Lorentz invariant local quantum field theories obey this bound.

The bound constrains the density of states in quantum field theories.

Abstract

It has been suggested heuristically by Unruh and Wald, and independently by Page, that among systems with given energy and volume, thermal radiation has the largest entropy. The suggestion leads to the corresponding universal bound on entropy of physical systems. Using a gedanken experiment we show that the bound follows from the second law of thermodynamics if the CPT symmetry is assumed and a certain general condition on matter holds. The experiment suggests that a wide class of Lorentz invariant local quantum field theories obeys a bound on the density of states.