Onsager Reciprocal Relations

The Onsager Reciprocal Relations state that in a physical system near thermodynamic equilibrium undergoing several coupled irreversible processes, such as heat and matter simultaneously diffusing under combined temperature and concentration gradients, the matrix of coefficients relating each thermodynamic flux to every thermodynamic force is symmetric, so the effect of one force on another flux equals the effect of that other force on the first flux. Named for Lars Onsager, they are a foundational result of nonequilibrium statistical mechanics derived from the microscopic reversibility of the underlying physical laws.