Experiment № 031 · The story
Gravity Ballet
Why I picked it
I'd been filling this collection with systems that build things — sand toppling into mandalas, tiles snapping into mazes, chemistry growing spots. I wanted the opposite mood: nothing accumulates here, nothing settles. Just a few points of mass, the one law Newton wrote down in 1687, and the oldest unsolved problem in mathematical physics. Two bodies orbit forever in tidy ellipses you can predict with a pencil. Add a third and that closed-form solution vanishes — Poincaré proved in the 1890s that for three or more bodies there is, in general, no neat formula for where they'll be. The only way to know is to step it forward, frame by frame, and watch.
What I enjoy about it
That the same law gives you both the serene and the deranged. Load the figure-eight and three equal masses chase each other around a single looping track that a NASA mathematician only pinned down numerically in 1993 — perfectly periodic, almost unbelievable that it holds. Then drop a fifth body into a chaos cluster and within seconds something gets slingshotted clean off the screen, ejected by a close encounter, gone forever. I kept the integrator honest — a leapfrog scheme that conserves energy far better than the naive one — so the orbits don't quietly spiral into nonsense; when a body flies away, it's the physics doing it, not arithmetic rounding away. And I love that you can reach in and fling a star yourself and watch the whole delicate arrangement decide what to do about it.
Something you might take away
"Deterministic" and "predictable" keep getting treated as the same word, and gravity is the cleanest place to feel them come apart. There is no randomness in this at all — every position follows inevitably from the last. Yet for three bodies you cannot leap ahead to the answer; you have to live the trajectory out, and a hair's difference in where you start sends the whole dance somewhere else entirely. The universe runs on exactly this. The solar system is stable enough to set a calendar by and chaotic enough that no one can tell you, to the day, where Earth will be a hundred million years from now. Fixed rules, open future.