Simulations of systems where large-scale behavior comes from simple local rules.
Each starling in a flock responds to a few nearby birds, and the whole flock turns together. A traffic jam moves backward along the road while the cars in it move forward.
These fourteen essays are grouped by mechanism rather than subject, because many famous examples come down to a few processes: local copying and alignment, frustrated couplings, a density threshold, a slowly driven system that relaxes in bursts, growth fed by a field that piles up at the tips, agents that write into their surroundings and read it back, and oscillators or chemicals coupled across space. Each essay follows one mechanism through a few models, with simulations you can run in the browser.
The Ising ferromagnet, Conway's Game of Life and lattice Boltzmann flow past a cylinder are in the companion series Simulating on a Lattice.
Five ideas that recur across the series.