Kepler's Orrery is a generative music system that uses gravity equations to "compose" and play music.
Start with planets (or stars or particles) that each have mass, position, and velocity, then run a n-body gravity simulator to make them move. They attract each other, accelerate, swirl around, and slingshot off each other. Sometimes they collide, and that's what plays the music.
Each body can have a melody attached to it, and plays its next note when it collides.
Composing for this system is tricky, but fun. You set up the initial world by specifying where the planets and the rocks (represented by squares; they have mass and position, but don't move) are, give them initial velocities, and see how they move. Then you have to make melodies that work when played randomly against the other melodies, with no control of when the notes get played or for how long -- just the order.
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