###### From the Archives

# Making Sense of Nature's Mess

## SCIENCE: OUTPOSTS

WHERE CHAOS begins, classical science stops. For as long as the world has had physicists inquiring into the laws of nature, it has suffered a special ignorance about disorder: in the atmosphere, in the turbulent sea, in the fluctuations of wildlife populations, in the oscillations of the heart and brain. The irregular side of nature, the discontinuous and erratic side -- these have been puzzles to science, or worse, monstrosities.

As recently as two decades ago, most practicing scientists shared these beliefs:

Now all that has changed. In the intervening 20 years, scientists have created an alternative set of ideas: Simple systems give rise to complex behavior. {See box.} Complex systems give rise to simple behavior. More important, the laws of complexity hold universally, caring not at all for the details of a system's constituent atoms. And "chaos" -- the obstinate element of disorder within order, of variation where predicability was expected -- has become a shorthand name for a fast-growing movement that is reshaping the fabric of the scientific establishment.

Its advocates contend that it has become the century's third great revolution in the physical sciences. As one physicist put it: "Relativity eliminated the Newtonian illusion of absolute space and time; quantum theory eliminated the Newtonian dream of a controllable measurement process; and chaos eliminates the Laplacian fantasy of deterministic predictability."

**Clouds of Knowing **

The modern study of chaos began with the creeping realization in the 1960s that quite simple mathematical equations could model systems every bit as violent as a waterfall. Tiny differences in input could quickly become overwhelming differences in output -- a phenomenon given the name "sensitive dependence on initial conditions." In weather, for example, this translates into what is known as the Butterfly Effect -- the notion that a butterfly stirring the air today in Beijing can transform storm systems next month in New York.

In 1960, MIT research meteorologist Edward Lorenz had developed a simulated weather model in his new electronic computer, based on 12 numerical rules -- equations that expressed the relationships between temperature and pressure, pressure and wind speed, and so forth. Lorenz understood that he was putting into practice the laws of Newton. Thanks to the determinism of physical law, futher intervention would then be unnecessary. Those who made such models took for granted that, from present to future, the laws of motion provide a bridge of mathematical certainty. Understand the laws and you understand the universe.

But there was always one small compromise, so small that working scientists usually forgot it was there, lurking in a corner of their philosophies like an unpaid bill: Measurement could never be perfect. Scientists marching under Newton's banner actually waved another flag that said something like this: Given an approximate knowledge of a system's initial conditions and an understanding of natural law, one can calculate the approximate behavior of the system. This assumption lay at the heart of science. As one theoretician liked to tell his students: "There's a convergence in the way things work, and arbitrarily small influences don't blow up to have arbitrarily large effects."

At first, Lorenz's printouts seemed to behave in those recognizable ways. They matched his cherished intuition about the weather, his sense that it repeated itself, displaying familiar patterns over time, pressure rising and falling, the airstream swinging north and south. But the repetitions were never quite exact. There was pattern, with disturbances. An orderly disorder.

One day in the winter of 1961, wanting to examine one sequence at greater length, Lorenz took a shortcut. Instead of starting the whole run over, he started midway through. To give the machine its initial conditions, he typed the numbers straight from the earlier printout. Then he walked down the hall to get away from the noise and drink a cup of coffee.