New definitions of thermodynamic temperature and entropy not based on the concepts of heat and thermal reservoir

From a new rigorous formulation of the general axiomatic foundations of thermodynamics we derive an operational definition of entropy that responds to the emergent need in many technological frameworks to understand and deploy thermodynamic entropy well beyond the traditional realm of equilibrium states of macroscopic systems. The new treatment starts from a previously developed set of carefully worded operational definitions for all the necessary basic concepts, and is not based on the traditional ones of “heat” and of “thermal reservoir”. It is achieved in three steps. First, a new definition of thermodynamic temperature is stated, for any stable equilibrium state. Then, by employing this definition, a measurement procedure is developed which defines uniquely the property entropy in a broad domain of states, which could include in principle even some non-equilibrium states of few-particle systems, provided they are separable and uncorrelated. Finally, the domain of validity of the definition is extended, possibly to every state of every system, by a different procedure, based on the preceding one, which associates a range of entropy values to any state not included in the previous domain. The principle of entropy non-decrease and the additivity of entropy are proved in both the domains considered.