Gravity's universality: The physics underlying Tolman temperature gradients

TL;DR

This paper explains why temperature gradients are necessary in gravitational fields at equilibrium, using a kinematic approach based on light red-shift and the universality of free-fall, ensuring thermodynamic consistency.

Contribution

It provides a simple, relativistic, and kinematic verification of the necessity of temperature gradients in gravitational equilibrium states, emphasizing the role of universality of gravity.

Findings

Temperature gradients are required for equilibrium in gravitational fields.

The universality of free-fall enables Tolman's temperature gradients without violating thermodynamics.

The approach is based on gravitational red-shift and a relativistic extension of Maxwell's argument.

Abstract

We provide a simple and clear verification of the physical need for temperature gradients in equilibrium states when gravitational fields are present. Our argument will be built in a completely kinematic manner, in terms of the gravitational red-shift/blue-shift of light, together with a relativistic extension of Maxwell's two column argument. We conclude by showing that it is the universality of the gravitational interaction (the uniqueness of free-fall) that ultimately permits Tolman's equilibrium temperature gradients without any violation of the laws of thermodynamics.