Diffusing Up the Hill: Dynamics and Equipartition in Highly Unstable Systems

Abstract

Stochastic motion of particles in a highly unstable potential generates a number of diverging trajectories leading to undefined statistical moments of the particle position. This makes experiments challenging and breaks down a standard statistical analysis of unstable mechanical processes and their applications. A newly proposed approach takes advantage of the local characteristics of the most probable particle motion instead of the divergent averages. We experimentally verify its theoretical predictions for a Brownian particle moving near an inflection in a highly unstable cubic optical potential. The most-likely position of the particle atypically shifts against the force despite the trajectories diverge in the opposite direction. The local uncertainty around the most-likely position saturates even for strong diffusion and enables well-resolved position detection. Remarkably, the measured particle distribution quickly converges to the quasi-stationary one with the same atypical shift for different initial particle positions. The demonstrated experimental confirmation of the theoretical predictions approves the utility of local characteristics for highly unstable systems which can be exploited in thermodynamic processes to uncover energetics of unstable systems.

arXiv commentSM as ancillary file

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• Regularized Boltzmann-Gibbs statistics for a Brownian particle in a non-confining field
• Living on the edge of instability
• Estimation of squeezing in a nonlinear quadrature of a mechanical oscillator
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• Local equilibrium properties of ultraslow diffusion in the Sinai model
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• Fluctuation Theorem of Information Exchange within an Ensemble of Paths Conditioned on Correlated-Microstates