whatA gas of particles that each align with their neighbours plus some noise. Sweep the noise and the gas crosses a real phase transition: jitter, then travelling bands, then everything moving one way. The chart at top right draws the transition as it is measured.

whyThe Vicsek model is the hydrogen atom of collective motion, the simplest system where flocking appears with a sharp threshold. The piece shows what a phase transition looks like when the particles are alive.

howEach step, every particle sums the headings of everyone within a radius, adds a random vector whose size is the noise, and takes the direction of the sum, then moves at constant speed in a box that wraps around. That is all. Near the threshold, dense travelling bands sweep across a dilute disordered gas. The chart accumulates the measured order at every noise level you visit. Left alone the piece sweeps the noise itself.