Kepler's Orrery Lite is a gravity simulator that composes and plays
ambient music. In other words, it's a generative music system that uses gravity
equations as its driving force.

Kepler's Orrery3 (Lite) is the iPad version.

The physics: using basic Newtonian gravity equations and a 1st-order Euler
simulation, Kepler's Orrery solves n-body problems in a stepwise
fashion. Along the way it plays music while you fiddle with the
physics, build new worlds, and throw planets around to watch them bounce!

Each world, or planetary system, starts with its own initial
conditions, and then the planets and gravity wells attract each
other, swirl around, and collide according to the laws of
physics. The worlds are all different -- changing the initial
conditions a tiny bit makes the simulation come out differently.

Then there's the music: think of a system like a "song," which
changes with every different simulation. Whenever a body hits
something it plays the next note on its melody. Each system has a
different set of melodies that play on different instruments. Some
worlds make slow and ambient music; others are random, sharp, and
surprising.

You can play with the physics: change the gravitational constant; add
"antigravity" or friction; visualize the force fields and velocities.

Tilt the device, and the planets slide around in a virtual space
that's linked to our own "real" gravitational field.

You can also change the initial conditions of a world: with the world
editor, you can change the mass, positions and
velocities of the bodies and then restart the world to see how it
comes out different.