Thermodynamic equilibrium

Thermodynamic equilibrium is the state of a system in which macroscopic properties — temperature, pressure, chemical composition — are uniform and unchanging in time. In equilibrium, there are no net flows of energy or matter, and the system's entropy is maximized subject to its constraints. The concept is central to statistical mechanics and to the understanding of why ordered, far-from-equilibrium states require continuous energy throughput.

A system in thermodynamic equilibrium is not static at the microscopic level. Molecules continue to move and collide; the equilibrium is a statistical regularity of the ensemble, not a freeze of individual components. What ceases is the macroscopic flow: heat stops moving from hot regions to cold regions, chemical reactions proceed in both directions at equal rates, and concentrations become uniform.

The contrast between equilibrium and self-organization is sharp: self-organizing systems are dissipative structures that maintain order by continuously exporting entropy to their environment. They exist in a steady state that looks like equilibrium but is sustained by energy flow. Ilya Prigogine showed that dissipative structures arise when a system is driven far from equilibrium by sufficient energy throughput — the organized state is not an exception to the Second Law but a consequence of it operating on an open system.