Thermodynamic parameters of fluids on conformally connected spacetimes

Local thermal equilibrium generally implies the absence of heat flux within a fluid. We find the relations between a set of thermodynamic variables of a fluid on a general spacetime and those defined on a conformally connected spacetime, assuming both descriptions are at thermal equilibrium. The scaling relations appear to be consistent with Dicke's heuristic argument and the previous analysis done on the basis of various restrictions. Within the present framework, it is observed that the satisfaction of Klein's law on one of the spacetimes implies its validity on the other one. Moreover, our analysis bypasses some of the imposed restrictions and thereby reveals the generality of the earlier predictions. These relations are further shown to preserve the geometric structure of thermodynamics, known as geometrothermodynamics, such that the associated metrics on two conformally connected spacetimes are themselves conformally related. This provides an alternative consistency check as well as a distinct geometric interpretation of the relations.