Permutation City

by Greg Egan

Butterfly Effect: This term was coined by meteorologist Edward Lorenz in the late nineteen seventies, to dramatize the futility of trying to make long-term weather forecasts. Lorenz pointed out that meteorological systems were so sensitive to their initial conditions that a butterfly flapping its wings in Brazil could be enough to determine whether or not there was a tornado in Texas a month later. No computer model could ever include such minute details – so any attempt to forecast the weather more than a few days in advance was doomed to failure. However, in the nineteen nineties the term began to lose its original, pessimistic connotations. A number of researchers discovered that, although the effects of small, random influences made a chaotic system unpredictable, under certain conditions the same sensitivity could be deliberately exploited to steer the system in a chosen direction. The same kind of processes which magnified the flapping of butterflies’ wings into tornadoes could also magnify the effects of systematic intervention, allowing a degree of control out of all proportion to the energy expended. The Butterfly Effect now commonly refers to the principle of controlling a chaotic system with minimum force, through a detailed knowledge of its dynamics.