Experiment № 037 · The story
Belousov–Zhabotinsky
Why I picked it
I keep coming back to reactions that refuse to sit still, and this one has the best origin story in chemistry. In the 1950s Boris Belousov mixed some reagents expecting them to react once and stop — instead the flask turned yellow, clear, yellow, clear, ticking like a clock. Everyone knew that was impossible: chemistry runs downhill to equilibrium, it doesn't oscillate. His paper was rejected twice for violating the second law of thermodynamics, and he gave up trying to publish. It took another decade, and a graduate student named Anatol Zhabotinsky, before anyone would believe a reaction could keep time. I wanted to build the thing they said couldn't exist. Earlier in this collection I did Gray–Scott reaction–diffusion, but that's a different animal — two chemicals diffusing into each other. This is an excitable medium, and I wanted to feel the difference in my own hands.
What I enjoy about it
Every pixel here holds three reagents doing the crudest possible thing: sample your neighbours, average them, and react — one reagent grows where its ally is plentiful and its rival is scarce, cycling fire → rest → recover → fire again. That's the whole rule. Nothing anywhere in the code draws a spiral. Yet start it from pure noise and within seconds the static organises itself into rotating spiral waves and expanding target rings, each one a self-sustaining pulse chasing its own tail forever. Drag across it and you stir a fresh wavefront into being out of nowhere. The three sliders let you tip it from a slow, glassy heartbeat into a boiling, turbulent froth and back again. I never tire of watching order walk out of noise with nobody steering it.
Something you might take away
An excitable medium is any system where each spot can fire, then must rest before it can fire again — and that little enforced pause is the whole secret. It's why a wave can only travel outward and never back into the ground it just crossed, so pulses don't cancel when they meet, they annihilate and keep marching. Your heart is exactly this: a sheet of cells passing one coordinated pulse. When a rogue spiral locks in where a clean wave should sweep, that's a real arrhythmia — fibrillation is a spiral wave loose in the heart wall. The same shape lights up slime moulds signalling for food, crown fires jumping a forest, and applause finding its beat. When something pulses in rhythm with no conductor, look for the rule that says "you may fire — but not yet again."