Dynamical fluctuations of a tracer coupled to active and passive particles

TL;DR

This study derives exact Langevin equations for a tracer coupled to active or passive particles, revealing how fluctuation-dissipation relations are violated at finite times and how an effective equilibrium emerges at small activity.

Contribution

The paper provides an exact analytical framework for tracer dynamics in active and passive baths, highlighting differences in fluctuation-dissipation relations and effective equilibrium behavior.

Findings

Exact form of dissipation kernel and noise for tracer dynamics

Violation of second fluctuation-dissipation relation at early times

Emergence of effective equilibrium in small activity limit

Abstract

We study the induced dynamics of an inertial tracer particle elastically coupled to passive or active Brownian particles. We integrate out the environment degrees of freedom to obtain generalized Langevin equation for the tracer dynamics in both cases. In particular, we find the exact form of the dissipation kernel and effective noise experienced by the tracer and compare it with the phenomenological modeling of active baths used in previous studies. We show that the second fluctuation-dissipation relation (FDR) does not hold at early times for both cases. However, at finite times, the tracer dynamics violate (obeys) the FDR for the active (passive) environment. We calculate the linear response formulas in this regime for both cases and show that the passive medium satisfies an equilibrium fluctuation response relation (FRR), while the active medium does not -- we quantify the extent of this violation explicitly. We show that though the active medium generally renders a nonequilibrium description of the tracer, an effective equilibrium picture emerges asymptotically in the small activity limit of the medium. We also calculate the mean squared velocity and mean squared displacement of the tracer and report how they vary with time.