Fluctuation-dissipation theorem

The fluctuation-dissipation theorem (FDT) is a principle in statistical mechanics that relates the response of a system in thermal equilibrium to small perturbations (dissipation) and the spontaneous fluctuations occurring in the system (fluctuations). In essence, it provides a way to understand how the equilibrium properties of a system influence its dynamics when it is perturbed. The theorem states that the way a system responds to an external force (i.e.