The study of the linear response is one of the most topical issues in statistical physics. For equilibrium systems, time reversal symmetry ensures the existence of a steady state distribution and it uniquely relates the response of the system to its spontaneous
fluctuations. Such universality is lost in nonequilibrium steady states, where entropy is being produced already before the perturbation. Here I revise the theory of nonequilibrium response and derive a generalized fluctuation relation for a markovian dynamics in full phase space. Then, I apply this general formalism to the case of an active system, revealing how the response of the system is affected by activity.
Andrea Puglisi