Effective and asymptotic scaling in a one-dimensional billiard problem

Abstract

The emergence of power laws that govern the large-time dynamics of a one-dimensional billiard of $N$ point particles is analysed. In the initial state, the resting particles are placed in the positive half-line $x\geqslant 0$ at equal distances. Their masses alternate between two distinct values. The dynamics is initialized by giving the leftmost particle a positive velocity. Due to elastic inter-particle collisions, the whole system gradually comes into motion, filling both right-hand and left-hand half-lines. As shown by [Chakraborti S., Dhar A., Krapivsky P., SciPost Phys., 2022, 13, 074], an inherent feature of such a billiard is the emergence of two different modes: the shock wave that propagates in $x\geqslant 0$ and the splash region in $x<0$. Moreover, the behaviour of the relevant observables is characterized by universal asymptotic power-law dependencies. In view of the finite size of the system and of finite observation times, these dependencies only start to acquire a universal character. To analyse them, we set up molecular dynamics simulations using the concept of effective scaling exponents, familiar in the theory of continuous phase transitions. We present results for the effective exponents that govern the large-time behaviour of the shock-wave front, the number of collisions, the energies and momentum of different modes and analyse their tendency to approach corresponding universal values.