Thermodynamics and Irreversibility as Emergent Properties of Open Physical Systems: A Mechanism-First, Interpretation-Neutral Perspective

Thermodynamics and statistical mechanics are among the most successful frameworks in physics, yet the conceptual status of irreversibility, entropy increase, and coarse-grained state variables remains subtle. This paper presents a mechanism-first, interpretation-neutral analysis of thermodynamics as an effective description of open physical systems. The central argument is that thermodynamic irreversibility need not arise from a fundamental time-asymmetric law. Instead, entropy increase and the macroscopic arrow of time emerge when microscopic correlations disperse into degrees of freedom that become dynamically inaccessible under realistic open-system conditions. On this view, the second law remains valid within its proper regime, while its arrow is understood as an emergent property of coarse-grained dynamics, restricted control, and loss of microscopic accessibility. The paper distinguishes thermodynamic principles that are preserved as robust constraints, such as energy bookkeeping and equilibrium consistency, from assumptions introduced through coarse-graining, ensemble reasoning, and regime-limited access to microscopic information. It also examines the breakdown of effective descriptions, the limits of geometric reasoning, and the interpretation of the Planck scale as a boundary of applicability rather than direct evidence for fundamental spacetime discreteness. The aim is not to replace thermodynamics or statistical mechanics, but to clarify what these frameworks are doing, when they work, and why irreversibility becomes reliable in real physical systems.