A Unified Nonequilibrium Framework: Thermodynamic Distance, Dissipation, and Stationary Laws via Effective State Count, Variational Stationarity, and Thermodynamic Bounds

Abstract

We propose a variational framework for nonequilibrium thermodynamics built around the effective number of accessible state, a multiplicative count that ranges from for a uniform distribution to one under complete localization, and whose logarithm coincides with the Gibbs Shannon entropy. This gives a natural thermodynamic distance to equipartition that bounds statistical distinguishability and grows monotonically under doubly stochastic relaxation. The construction extends to arbitrary nonequilibrium steady states with a chosen reference distribution, where the Kullback Leibler divergence splits into an entropy deficit and a reference weight coupling, acts as a Lyapunov functional when the reference is fixed, and reduces to excess free energy in canonical settings. We connect these static notions to dynamics by decomposing entropy production into adiabatic (housekeeping) and nonadiabatic parts, identify the latter with the rate of decay of the divergence to the reference, and complement this with trajectory and activity-based potentials that yield fluctuation symmetries, thermodynamic uncertainty bounds, and activity-limited speed constraints on steady currents