Engines of Our Ingenuity

No. 652:
THE BUTTERFLY EFFECT

by John H. Lienhard

Click here for audio of Episode 652.

Today, our notion of cause and effect changes forever. The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

Author James Gleick tells about MIT meteorologist Edward Lorenz. In 1960 Lorenz tried to model the weather. He wrote simplified equations and solved them on a primitive computer. Sure enough, his output did behave a lot like real weather. His colleagues watched over his shoulder. They were fascinated.

One day, Lorenz tried to continue a run he'd done the day before. He restarted it halfway through. He put in a number from the first run. The output started out just the way it had the day before. Then it began to diverge, crazily.

The equations were the same. The starting point was the same. But the results diverged. Lorenz checked his computer. He checked his arithmetic. Nothing had changed. Same equations, but on subsequent days the results diverged.

There was one difference, but how could it matter? Lorenz rounded off the fourth decimal place of the starting number on the second day. So he stopped to consider. All weather predictions do what his program just did. You can predict the weather for the day after tomorrow. Stretch that to a week, and your prediction always departs from reality.

The implication was staggering. We've always presumed that if you barely change a cause, you'll barely change the effect. Suddenly, Lorenz saw that the weather would change utterly if you started things out just a little differently.

No wonder real weather is so unpredictable! Weather obeys physical laws. But if you change one breath of air, those laws will spin out in a wholly different story.

Meteorologists began talking about something they called the Butterfly Effect. The idea was that if a butterfly chances to flap his wings in Beijing in March, then, by August, hurricane patterns in the Atlantic will be completely different.

Not long after that day in 1960, the scientific world began changing. Perhaps all kinds of nasty problems we can't solve are nasty just because we can never state them accurately enough.

Lorenz had taken the first step on the road to showing that our world is far more chaotic than we dreamed. For generations engineers and scientists have been predicting things. But we've only predicted those things that are predictable -- the breaking load on beams -- the thrust of a rocket.

And weather, of course, is just one face of the larger thing we all want to know, but which we never shall predict. Somewhere in the world, a butterfly will always flap its wings and thwart our age-old craving to predict -- our own future.

I'm John Lienhard, at the University of Houston, where we're interested in the way inventive minds work.

(Theme music)


Gleick, J., CHAOS: Making a New Science. New York: Penguin Books, 1987.



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Copyright © 1988-1997 by John H. Lienhard. All Rights Reserved.
University Libraries, University of Houston, Houston, TX 77204-2091.


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