When a potential (energy) is added to a system (this is a perturbation), the expectation values of observables change. This change is called the response. In equilibrium systems the ‘fluctuation-dissipation theorem’ tells us that this response is equal to the correlation (in the unperturbed system) between the observable and the added potential. par For systems that are not in equilibrium this no longer holds. Our contribution has been to express the response as the sum of two correlation functions in the unperturbed system: one is a correlation function between the observable and the heat dissipation into the environment, the other is a correlation between the observable and a quantity we call traffic, which measures in a sense the dynamical activity of the system.

IPhT