Thermodynamics Meets Special Relativity – or What Is Real in Physics?

In this paper we carefully reexamine the various framworks existing in the field of relativistic thermodynamics. We scrutinize in particular the different conceptual foundations of notions like the relativistic work, heat force, moving heat and relativistic temperature. As to the latter notion we argue that, as in ordinary thermodynamics, relativistic absolute temperature should be introduced operationally via relativistic Carnot processes. We exhibit the more implicit or even hidden tacit preassumptions being made and point to a couple of gaps, errors and inconclusive statements in some of the existing literature. We show in particular that there is a wide-spread habit to draw general conclusions from the analysis of too restricted and special thermodynamic processes, e.g. processes with constant pressure, which is dangerous and sometimes leads to wrong results. Furthermore, we give a detailed analysis of the so-called zeroth law of relativistic thermodynamics with the help of a relativistic Carnot process. We rigorously show that, contrary to certain statements in the literature, thermodynamic systems at different relativistic temperatures, moving relative to each other, can thermally stably coexist provided that their respective temperatures obey a certain functional relation (given by the Lorentz factor). This implies however that their respective rest temperatures are the same.