Mori–Zwanzig formalism

Summary

The Mori–Zwanzig formalism is an exact way to rewrite the dynamics of a large system as an equation for a few chosen variables. One projects the dynamics onto the chosen variables; the rest of the system then appears as a memory term and a noise term. The resulting equation has the form of a generalized Langevin equation. It is the starting point for deriving Brownian motion, hydrodynamics and other coarse-grained descriptions from microscopic laws.