Fluctuation Dissipation Relations for Active Field Theories

Abstract: Breakdown of time-reversal symmetry is a defining property of non-equilibrium systems, such as active matter, which is composed of units that consume energy. We employ a formalism that allows us to derive a class of identities associated with the time-reversal transformation in non-equilibrium field theories, in the spirit of Ward-Takahashi identities. We present a generalization of the fluctuation-dissipation theorem valid for active systems as a particular realization of such an identity, and consider its implications and applications for a range of active field theories. The field theoretical toolbox developed here helps to quantify the degree of non-equilibrium activity of complex systems exhibiting collective behavior.