NichtGleichgewichtszuständen

Nichtgleichgewicht, often translated as nonequilibrium, denotes physical conditions in which a system’s macroscopic properties evolve in time or vary spatially due to external driving or internal gradients. In thermodynamics, equilibrium states are characterized by uniform intensive properties and no net flows; nonequilibrium states sustain fluxes of energy, matter, or momentum and typically produce entropy over time.

Key concepts include relaxation toward equilibrium, steady nonequilibrium states maintained by constant driving, and distinctions between classical (macroscopic) and stochastic (microscopic) descriptions. The evolution is usually described by kinetic equations (such as the Boltzmann equation for dilute gases or master equations for discrete states) or continuum theories (diffusion, heat conduction, reaction–diffusion systems). Linear nonequilibrium thermodynamics treats processes near equilibrium, with fluxes proportional to forces and with Onsager reciprocity; more general nonlinear and stochastic frameworks address stronger driving and fluctuations, including stochastic thermodynamics and fluctuation theorems.

Examples include heat flow in a rod connected to reservoirs at different temperatures, diffusion in concentration gradients, driven chemical reactions, and active matter systems. Nonequilibrium quantum dynamics study how quantum systems evolve under external driving or after disturbances, such as quantum quenches.

Significance lies in its relevance to real-world processes, where most natural and engineered systems operate away from equilibrium. Understanding nonequilibrium behavior is essential for energy transport, chemical processing, materials science, and biological function.