Gravity's universality: The physics underlying Tolman temperature gradients

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 redshift/blueshift 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.