Relativistic kinematic approach to the classical ideal gas

Abstract

Abstract The necessary and sufficient conditions for a unit time-like vector field to be the unit velocity of a classical ideal gas (CIG) are obtained. In a recent paper (Coll et al 2019 Phys. Rev. D 99 084035) we have offered a purely hydrodynamic description of a CIG. Here we take one more step in reducing the number of variables necessary to characterize these media by showing that a plainly kinematic description can be obtained. We apply the results to obtain test solutions to the hydrodynamic equation that model the evolution in local thermal equilibrium of a CIG.