Is entropy objective? Rethinking the foundations of irreversibility and the second law.

The standard formulations of statistical mechanics and the explanations of irreversibility rely on the way we describe physical systems. They use concepts such as coarse-graining or macroscopic states, which depend on the observables used to describe the system and their precision. In this Perspective, we examine the extent to which entropy, irreversibility, and the second law of thermodynamics can be regarded as observer-independent. To this end, we use Einstein's observable-dependent entropy and discuss the objectivity of several concepts and phenomena separately: irreversibility, equilibrium, entropy, temperature, and the second law. We also discuss the role of symmetry-breaking transitions in generating low-entropy states and in the arrow of time, as manifested through traces and footprints. Finally, we discuss the mechanical rationale of the second law based on ergotropy and passive states.