Time-Symmetry Breaking in Hamiltonian Mechanics. III. A Memoir for Douglas James Henderson [1934-2020]

TL;DR

This paper investigates time-symmetry breaking in Hamiltonian systems through molecular dynamics simulations of shockwaves and rarefaction waves, revealing how microscopic reversibility conflicts with macroscopic irreversibility due to instabilities.

Contribution

It extends the understanding of time-symmetry breaking by analyzing shock and rarefaction waves using molecular dynamics and fluid mechanics, highlighting the role of Lyapunov and Navier-Stokes instabilities.

Findings

Reversed shock simulations lead to unsteady rarefaction fans.

Microscopic time-reversibility conflicts with macroscopic irreversibility.

Instabilities explain the breaking of time symmetry in simulations.

Abstract

Following Berni Alder [1] and Francis Ree [2], Douglas Henderson was the third of Bill's California coworkers from the 1960s to die in 2020. Motivated by Doug's death we undertook better to understand Lyapunov instability and the breaking of time symmetry in continuum and atomistic simulations. Here we have chosen to extend our explorations of an interesting pair of nonequilibrium systems, the steady shockwave and the unsteady rarefaction wave. We eliminate the need for boundary potentials by simulating the collisions of pairs of mirror-images projectiles. The resulting shock and rarefaction structures are respectively the results of the compression and the expansion of simple fluids. Shockwaves resulting from compression have a steady structure while the rarefaction fans resulting from free expansions continually broaden. We model these processes using classical molecular dynamics and Eulerian fluid mechanics in two dimensions. Although molecular dynamics is time-reversible the reversed simulation of a steady shockwave compression soon results in an unsteady rarefaction fan, violating the microscopic time symmetry of the motion equations but in agreement with the predictions of macroscopic Navier-Stokes fluid mechanics. The explanations for these results are an interesting combination of two (irreversible) instabilities, Lyapunov and Navier-Stokes.