Onsager Reciprocal Relations

The validity of the Onsager relations rests on the principle of microscopic reversibility — the time-reversal symmetry of the Hamiltonian equations governing molecular motion. This principle is stronger than the second law: it constrains not only the direction of time but the detailed statistics of fluctuations, ensuring that forward and backward trajectories are equally probable at equilibrium. Without this microscopic symmetry, the macroscopic reciprocal relations would not hold.