Two universes begin almost perfectly alike. In one, two velocities receive almost imperceptible, balanced nudges. Watch what gravity does to their futures. Sometimes the difference stays small. Sometimes it becomes the whole story.
In the left universe, three equal stars begin at the corners of an equilateral triangle and move in a perfect rotating formation. The right universe starts almost identically. Look closely as time passes: will symmetry survive an exceedingly small disagreement?
The positions are numerical solutions of Newtonian mutual attraction with a short-distance softening term, using a velocity-Verlet integrator and extra substeps during close approaches. In the triangle and figure-eight presets the softening is very small; the initial conditions approximate classical exact solutions. The reference and comparison calculations use identical numerical settings.
The difference graph measures the root-mean-square separation between corresponding stars. Divergence of this particular pair of trajectories illustrates sensitivity; it is not, by itself, a formal proof of chaos. In the triangle experiment, equal-mass rotating Lagrange configurations are known to be linearly unstable.
Every rise of one horizontal grid interval represents a hundredfold increase in the difference. The two universes never exchange information.