After Lorenz, we came to see that determinism might give you short-term predictability, but in the long run, things could be unpredictable. “Determinism was equated with predictability before Lorenz. “It was philosophically very shocking,” says Steven Strogatz, a professor of applied mathematics at Cornell and author of Nonlinear Dynamics and Chaos. That the tiny change in his simulation mattered so much showed, by extension, that the imprecision inherent in any human measurement could become magnified into wildly incorrect forecasts. Yet Lorenz’s own deterministic equations demonstrated how easily the dream of perfect knowledge founders in reality. Unpredictability plays no role in the universe of Newton and Laplace in a deterministic sequence, as Lorenz once wrote, “only one thing can happen next.” All future events are determined by initial conditions.

“It became a wonderful instance of a seemingly esoteric piece of mathematics that had experimentally verifiable applications in the real world,” says Daniel Rothman, a professor of geophysics at MIT. Yet his insight turned into the founding principle of chaos theory, which expanded rapidly during the 1970s and 1980s into fields as diverse as meteorology, geology, and biology. In 1963, Lorenz condensed his findings into a paper, “Deterministic Nonperiodic Flow,” which was cited exactly three times by researchers outside meteorology in the next decade. Like the results of a wing’s flutter, the influence of Lorenz’s work was nearly imperceptible at first but would resonate widely. And the butterfly effect, also known as “sensitive dependence on initial conditions,” has a profound corollary: forecasting the future can be nearly impossible. The idea came to be known as the “butterfly effect” after Lorenz suggested that the flap of a butterfly’s wings might ultimately cause a tornado.

The unexpected result led Lorenz to a powerful insight about the way nature works: small changes can have large consequences.

Chaos theory james gleick