Reversible simulation of hard disk fluid

The Letter describes a 2D simulation of a system of regular polygons moving within the unit square. The polygons, having 4n sides, are not allowed to rotate and their interactions are of the hard body type. The requirement that the simulation be reversible necessitates the use of rational values for positions and velocities. The latter then enable to introduce the notion of absolute age which gives a novel look at the irreversibility. It is observed that with growing n the dynamics approaches the hard disk one with the Maxwell-Boltzmann distribution of speeds. The simulation also enables to demonstrate the gradual appearance of hydrodynamic behavior with growing n, retaining at the same time the reversibility of the dynamics.