Having discovered chaos, Lorenz explored deeper and found that a form of unpredictable order could spontaneously arise from it. Paul Rodgers on a remarkable scientist

Could the flap of a butterfly's wings in Brazil set off a tornado in Texas, asked mathematician and meteorologist Edward Lorenz in a 1972 paper? It was a small question, with an appropriately huge answer.

The weather, Professor Lorenz demonstrated, is extremely sensitive to minute changes in initial conditions. Forecasting a month in advance is all but impossible. Though Lorenz shied away from blaming storms around Houston on the Fluminense swallowtail Parides ascanius, the Butterfly Effect, as his insight came to be known, set off a rethink of classical physics to rival relativity and quantum – a rethink now known as Chaos theory.

The experiment that started this scientific revolution was conducted at the Massachusetts Institute of Technology in 1961. Lorenz - who died aged 90 last month - had written one of the first computer models of the way the atmosphere moves. His Eureka moment came after he fed interim results from the previous day’s work back into his model. He expected the model to repeat itself. Instead the new output quickly diverged from the previous day’s. The reason, he eventually determined, was that the model used figures precise to six decimal places, but those in the printout contained only three. That difference, less than a tenth of a percentage point, should have been trivial, instead it was enough to generate dramatically different results. Professor Lorenz had discovered Chaos.

The other two great revolutions in physics during the 20th century, relativity and quantum, applied to the very large and the very small. The deterministic universe built by Sir Isaac Newton and René Descartes was undermined in extreme cases, but remained true over a wide range of scales. We might know, in an abstract way, that the world is not deterministic, but our technology continued to work like, well, clockwork.

Determinism was perhaps best described by Pierre-Simon Laplace, the brilliant French astronomer and mathematician, who argued in 1820 that if some intellect (popularly known as Laplace’s Demon) could know all positions and forces in nature it would be able to read the future as easily as the past. Chaos tore the guts out of his demon. Even everyday functions, for example the size and timing of drips falling from a leaky tap, are so complex that that the only way to predict what they will be doing at some point in the future is to work them out, step by step, at an infinite level of precision.

This insight alone would have been enough to raise Professor Lorenz to the scientific pantheon. It won him numerous honours, including the Crafoord Prize, awarded by the Swedish Academy of Sciences for research in fields not eligible for Nobel Prizes. Yet Lorenz had more to offer. In 1963 he discovered the Lorenz Attractor, the first of a class of mathematical phenomena called Strange Attractors. In essence, having discovered chaos, Lorenz explored deeper and found that a form of unpredictable order could spontaneously arise from it. It is not clear whether the shape of his Strange Attractor, which when graphed looks like the wings of a butterfly, influenced his choice of metaphor for the 1972 paper. Had he stuck with his original image, climatologists would today be talking about the Seagull Effect. A small change, but small changes can have huge consequences.

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