This animation is of one of Dimension's test scenes, physically integrated with collision detection and gravity. Each collision is perfectly elastic. The 1000-sphere scene was integrated and collision-detected 100 times per frame. The integration and rendering took 30 minutes on 12 cores, or 3 hours of CPU time. You should watch in HD.
Collisions were detected using a naïve \(O(n^2)\) algorithm, but rendering time (23 minutes wall-clock, 173 minutes CPU time) still dwarfed integration time (7 minutes).