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