A shortcut through macroscopic fluctuation theory: a generalised Fick law (opens in new tab)

Macroscopic fluctuation theory is a powerful tool to characterise the large-scale dynamical properties of diffusive systems, both in and out of equilibrium. It relies on an action formalism in which, at large scales, the dynamics are fully determined by the minimum of the action. Within this formalism, the analysis of the statistical properties of a given observable reduces to solving the Euler–Lagrange equations with appropriate boundary conditions. One must then compute the action at its mi...