The locusts have no king, and yet they all go forth in ranks, noted King Solomon some three thousand years ago. That a multitude of simple creatures could display coherent collective behavior without any leader caused his surprise and amazement, and it has continued to do so for much of our thinking over the following millennia. Caesar’s legions conquered Europe, Napoleon’s armies reached Moscow: We always think of a great commander telling the thoughtless multitudes what to do.
Statistical physics pioneered an opposite view. When a piece of iron is cooled down to a certain temperature (the Curie temperature), the majority of the atoms align their spins, thereby making it magnetic. No atomic general gives any commands; each atom communicates only with its neighbors, and yet there is an overall alignment. It shows us that local microscopic interactions as such can lead to dramatic global behavior, and this realization brought about a revolution in the understanding of swarm behavior.
Some hundred years ago, serious biologists still thought that the coordination of birds in a flock was reached by telepathy, and the synchronized light emission by fireflies in the Asiatic jungle was attributed to faulty observation by the observer. The introduction of physics concepts in biology has to a large extent resolved these puzzles. Flocks of birds are much more like the atoms in iron than they are like the armies of Napoleon, and the fireflies act much like a laser. Collective behavior in the world of living beings is after all not so different from that in the inanimate world.
