Details

Snapshot 1: This is the initial state. The particles are at rest. Due to gravity they will fall downwards. This is also the case if "dt" is given a negative value! For each of the three particles in the right half of the container there is a mirror-image twin, shown in a corresponding color of lower saturation.

Snapshot 2: After many reflections of particles with the container wall and with other particles, the arrangement of particles and their mirror twins is no longer strictly symmetric.

Snapshot 3: Even less symmetric now!

Snapshot 4: Further reflections and collisions destroy the symmetry completely.

The destruction of mirror symmetry by numerical inaccuracies was studied for more complex particles in three dimensions in the following references.

References

[1] U. Mutze. "Precision-Dependent Symmetry Breaking in Simulated Motion of Polyspherical Grains." (Jul 7, 2009) www.ma.utexas.edu/mp_arc/c/09/09-113.pdf.

[2] U. Mutze. "Comparing Motion of Polyspherical Particles Computed with High and with Low Numerical Precision, UM 2009-03-03." (Mar 3, 2009) www.ulrichmutze.de/symmetry_lost_movie/symmetry_lost.html.