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Hard disks moveaboutina box much like billiard balls.The rules for wall
and pair collisions are quickly programmed ona computer, allowing usto
follow thetimeevolution of the hard-disk system (see Fig. 2.1). Given
the initial positions and velocities at time t =0,a simplealgorithm
allows usto determine the state ofthe system at t =10.37, butthe
unavoidablenumerical imprecision quickly explodes.This manifestation
ofchaosis closely related to the statistical description of hard disks and
other systems, as weshall discuss in this chapter.

wall collision pair collision

t = 0 t = 1.25 t = 2.18 t = 3.12 t = 3.25 t = 4.03

t = 4.04 t = 5.16 t = 5.84 t = 8.66 t = 9.33 t = 10.37

Fig. 2.1 Event-driven molecular dynamics simulation with four hard
disks in a square box.