  Tolman's relation for the temperature gradient in an equilibrium
self-gravitating general relativistic fluid is broadly accepted within the
general relativity community. However, the concept of temperature gradients in
thermal equilibrium continues to cause confusion in other branches of physics,
since it contradicts naive versions of the laws of classical thermodynamics. In
this paper we discuss the crucial role of the universality of free fall, and
how thermodynamics emphasises the great distinction between gravity and other
forces. To do so we will present an argument given by Maxwell and apply it to
an electro-thermal system, concluding with an reductio ad absurdum. Among other
issues we shall show that Tolman temperature gradients could also (in
principle) have been derived circa 1905 - a decade before the development of
full general relativity.

General Relativity and Quantum Cosmology;Statistical Mechanics