Functional Renormalisation Group for Brownian Motion I: The Effective Equations of Motion

Abstract

We use the functional Renormalisation Group (fRG) to describe the in and out of equilibrium dynamics of stochastic processes, governed by an overdamped Langevin equation. Exploiting the connection between Langevin dynamics and supersymmetric quantum mechanics in imaginary time, we write down renormalisation flow equations for the effective action, approximated in terms of the Local Potential Approximation and Wavefunction Renormalisation. We derive \textit{effective equations of motion} (EEOM) from the effective action (EA) $\Gamma$ for the average position $⟨x⟩$, variance $⟨{\left(x-⟨x⟩\right)}^2⟩$ and covariance. The fRG flow equations outlined here provide a concrete way to compute the EA and thus solve the derived EEOM. The obtained effective potential should determine directly the exact equilibrium statistics, name the position, the variance, as well as all higher order cumulants of the equilibrium Boltzmann distribution. This first paper of a two part series is mostly concerned with setting up the necessary formalism while in part two we will numerically solve the equations derived her and assess their validity both in and out of equilibrium.

11 + 4 pages, 2 Appendices. Part one of two part series. Significant changes, added discussion on non-equilibrium correlation functions, moved numerical results on equilibrium to part two

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