Unusual Properties of Adiabatic Invariance in a Billiard Model Related to the Adiabatic Piston Problem

TL;DR

This paper investigates a simplified billiard model related to the adiabatic piston problem, revealing that certain adiabatic invariants are exactly conserved at wall collisions but decay at particle collisions, highlighting unusual properties.

Contribution

It demonstrates that the adiabatic invariant at wall collisions is exactly conserved, while it decays at particle collisions, providing new insights into adiabatic invariance in simplified models.

Findings

Adiabatic invariant is exactly conserved at wall collisions.

Adiabatic invariant decays linearly at particle collisions.

Model offers insights into classical adiabatic piston behavior.

Abstract

We consider the motion of two massive particles along a straight line. A lighter particle bounces back and forth between a heavier particle and a stationary wall, with all collisions being ideally elastic. It is known that if the lighter particle moves much faster than the heavier one, and the kinetic energies of the particles are of the same order, then the product of the speed of the lighter particle and the distance between the heavier particle and the wall is an adiabatic invariant: its value remains approximately constant over a long period. We show that the value of this adiabatic invariant, calculated at the collisions of the lighter particle with the wall, is a constant of motion (i.e., {an exact adiabatic invariant}). On the other hand, the value of this adiabatic invariant at the collisions between the particles slowly linearly in time decays with each collision. The model we consider is a highly simplified version of the classical adiabatic piston problem, where the lighter particle represents a gas particle, and the heavier particle represents the piston.