If you're trying to catch a bus in Cuernavaca, Mexico, don't look for schedules: there aren't any. Yet there is a strange kind of order. Petr Seba, a physicist at the Czech Academy of Sciences, has discovered that Cuernavaca's buses move about like randomly interacting subatomic particles and that chaos serves the riders just fine.
In Cuernavaca, as in Mexico City and many other parts of the country, bus lines are not closely regulated, as they are in the United States. "There are no bus companies," says Seba. "Drivers own their buses, and this results in competition for passengers." To pick up as many riders as possible, the drivers are constantly speeding up or slowing down in response to the buses ahead and to the number of potential customers along the route. The resulting movements, Seba and his colleagues noticed, follow the equations that describe the distribution of subatomic particles in a state known as quantum chaos; the drivers' actions mimic the natural forces on the particles.
"The bus timetable emerges because the system is self-organizing," says Seba. The Mexican system actually gets passengers where they need to go more efficiently than buses that run on inflexible schedules, he argues.