Time Reversal of the Overdamped Langevin Equation and Fixman's Law

We discuss how the first order Langevin equation for the overdamped dynamics of an interacting system has a natural time reversal of simple but surprising form, with consequences for correlation functions. This leads to the correlation of interactions as a strictly restraining term in the time-dependent diffusion tensor of the system, deriving the relation first suggested by Fixman. Applying this to the time-dependent diffusion of dilute polymer coils leads to the quantitative calibration of Kirkwood’s approximation for their hydrodynamic radius. We find the generalized ``Fixman Law" for dissipation with a memory kernel, which has revealing causal structure, and we also discuss the case of the second order Langevin Equation.