Non-Equilibrium Currents in Stochastic Field Theories: a Geometric Insight

Abstract: We introduce a new formalism to study nonequilibrium steady-state currents in stochastic field theories. We show that generalizing the exterior derivative to functional spaces allows identifying the subspaces in which the system undergoes local rotations. In turn, this allows predicting the counterparts in the real, physical space of these abstract probability currents. The results are presented for the case of the Active Model B undergoing motility-induced phase separation, which is known to be out of equilibrium but whose steady-state currents have not yet been observed, as well as for the KPZ equation. We locate and measure these currents and show that they manifest in real space as propagating modes localized in regions with non-vanishing gradients of the fields.