The second-order correlations and linear responses of various physical systems have a fundamental relation, formulated as fluctuation-dissipation theorem.
— Professor Renbao Liu
While the fluctuation-dissipation theorem provides a general relation between the response of equilibrium systems to small external perturbations and their spontaneous fluctuations, no general relation is known for systems out of equilibrium.
— Wikipedia on Fluctuation-dissipation theorem
2013.07.01 Monday ACHK