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E Pluribus Unum

Brian Hayes

Flocks of starlings, herds of zebra, schools of mackerel, traffic jams, a nuclear chain reaction, an ant colony, fairy rings of fungi, the spinning electrons of a ferromagnet, a forest fire spreading from tree to tree, the spiral arms of a galaxy, epidemic disease. What golden thread ties together the items on this motley list? They are all phenomena that emerge from the interactions of many individual things. Multiplicity is essential here: One zebra does not make a herd, nor one car a traffic jam. But there is unity too: The herd, the traffic jam, the magnet and the galaxy have an existence of their own, with properties different from those of their constituents. After all, a flock can't flap its wings, and a bird can't split itself in two to fly around an obstacle.

Figure 1. Aggregation of slime-mold cellsClick to Enlarge Image

Computer simulation offers an attractive way to explore these curious, one-from-many systems. A simulation can be built from the bottom up, starting with the rules that govern the individual actors—birds, stars, cars, ants and so on. If these "microscopic" laws are correct, the "macroscopic" behavior of the higher-level system should emerge automatically, just as it does in nature. You don't have to teach a herd of zebra how to stay together; you only have to tell the individual zebras how to get along with their nearest neighbors.

A programming system called StarLogo is designed for building simulations of just this kind. StarLogo is the creation of Mitchel Resnick of the Epistemology and Learning Group in the Media Laboratory at the Massachusetts Institute of Technology. Resnick describes the system and explains the ideas behind it in his 1994 book Turtles, Termites and Traffic Jams: Explorations in Massively Parallel Microworlds. StarLogo software is freely distributed over the Internet; for more information see the note at the end of this article.

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