Time-Symmetry Breaking in Hamiltonian Mechanics

TL;DR

This paper explores how Hamiltonian systems, despite their time-reversible equations, exhibit time-symmetry breaking in their Lyapunov instability, revealing an intrinsic arrow of time through chaos analysis in various physical models.

Contribution

It demonstrates that Lyapunov instability can break time-symmetry in Hamiltonian trajectories, providing new insights into the origin of the arrow of time in chaotic systems.

Findings

Lyapunov spectra differ in forward and backward directions

Time-symmetry breaking observed in fluid shockwaves and molecular collisions

Lyapunov instability varies dramatically in opposite time directions

Abstract

Hamiltonian trajectories are strictly time-reversible. Any time series of Hamiltonian coordinates {q} satisfying Hamilton's motion equations will likewise satisfy them when played "backwards", with the corresponding momenta changing signs : {+p} --> {-p}. Here we adopt Levesque and Verlet's precisely bit-reversible motion algorithm to ensure that the trajectory reversibility is exact, with the forward and backward sets of coordinates identical. Nevertheless, the associated instantaneous Lyapunov instability, or "sensitive dependence on initial conditions" of "chaotic" (or "Lyapunov unstable") bit-reversible coordinate trajectories can still exhibit an exponentially growing time-symmetry-breaking irreversibility. Surprisingly, the positive and negative exponents, as well as the forward and backward Lyapunov spectra, are usually not closely related, and so give four differing topological measures of "local" chaos. We have demonstrated this symmetry breaking for fluid shockwaves, for free expansions, and for chaotic molecular collisions. Here we illustrate and discuss this time-symmetry breaking for three statistical-mechanical systems, [1] a minimal (but still chaotic) one-body "cell model" with a four-dimensional phase space; [2] relatively small colliding crystallites, for which the whole Lyapunov spectrum is accessible; [3] a near-continuum inelastic collision of two larger 400-particle balls. In the last two of these pedagogical problems the two colliding bodies coalesce. The particles most prone to Lyapunov instability are dramatically different in the two time directions. Thus this Lyapunov-based symmetry breaking furnishes an interesting Arrow of Time.