The unreasonable effectiveness of equilibrium theory for interpreting nonequilibrium experiments

There has been much interest in applying the results of statistical mechanics to single molecule experiments. Recent work has highlighted nonequilibrium work-energy relations and fluctuation theorems that have an equilibriumlike (time independent) form. I give a simple heuristic example where an equilibrium result (the barometric law for colloidal particles in water) can be derived using the thermodynamically nonequilibrium behavior of a single colloidal particle falling through the water due to gravity. This description is possible because the particle, even while falling, is in mechanical equilibrium (the gravitational force equals the viscous drag force) at every instant. The results are generalized using Onsager's thermodynamic action approach for stochastic processes to derive time independent equations that hold for thermodynamically nonequilibrium (and even nonstationary) systems. These relations offer great possibilities for the rapid determination of thermodynamic parameters from single molecule experiments. (C) 2006 American Association of Physics Teachers.