Active processes in one dimension

TL;DR

This paper studies one-dimensional active particles with internal spin degrees of freedom, revealing how activity affects irreversibility, correlations, and escape dynamics, and deriving a generalized equation for their spatial density.

Contribution

It introduces a generalized telegraph equation for active particles with multiple spin states and analyzes how activity influences irreversibility and escape laws in 1D systems.

Findings

Irreversibility appears in higher-order correlations.

Activity modifies the Arrhenius escape law, emphasizing potential slope.

Internal currents can transmit activity effects to translational motion.

Abstract

We consider the thermal and athermal overdamped motion of particles in 1D geometries where discrete internal degrees of freedom (spin) are coupled with the translational motion. Adding a driving velocity that depends on the time-dependent spin constitutes the simplest model of active particles (run-and-tumble processes) where the violation of the equipartition principle and of the Sutherland-Einstein relation can be studied in detail even when there is generalized reversibility. We give an example (with four spin values) where the irreversibility of the translational motion manifests itself only in higher-order (than two) time correlations. We derive a generalized telegraph equation as the Smoluchowski equation for the spatial density for an arbitrary number of spin values. We also investigate the Arrhenius exponential law for run-and-tumble particles, due to their activity the slope of the potential becomes important in contrast to the passive diffusion case and activity enhances the escape from a potential well (if that slope is high enough). Finally, in the absence of a driving velocity, the presence of internal currents such as in the chemistry of molecular motors may be transmitted to the translational motion and the internal activity is crucial for the direction of the emerging spatial current.