Engines of Our Ingenuity

No. 841:
A TIDAL WAVE

by John H. Lienhard

Click here for audio of Episode 841.

Today, let's talk about technology, revolution, and tsunamis. 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.

Here's a thought for you. It's about technological revolution -- about any revolution, for that matter. It is a tidal wave analogy. Somewhere, under the ocean, the plate tectonics shift slightly. We suffer an undersea earthquake. It launches a wave.

The wave is only a few feet high, but it's hundreds of miles long -- much longer than the ocean is deep. It moves like a wave in a water-filled cookie sheet. It travels hundreds of miles an hour and carries enormous energy in complete quiet and repose.

The wave takes an hour to pass a ship at sea. You don't even know it's been there. You take an hour to rise a few feet, then be let back down. The tidal wave is quite invisible to you.

Only when it reaches the sloping coastal shore does it pile up into a great crashing wall of water. Only then is its energy released. It was a long time coming. Now it uncoils in seconds.

So it is with technological revolutions. The Medieval Church moved the earth in the 12th century when her scholars started the first universities -- when they aggressively started asking questions about the nature of things.

That set off a new craving for written material. For 300 years inventors worked on the problem of mass-producing books. They worked out the technologies of copying, organizing text, binding, paper and ink-making, and block printing. Finally Gutenberg put the pieces together and perfected the printed book.

People who lived through those years couldn't have known a tidal wave was passing. But now the wave reached shore. In the next 40 years we printed 20 million new books. Life on Earth was changed. Suddenly we knew a tsunami had broken over our heads.

Try another tsunami wave; Around 1880 we could buy a telephone. That was the first real medium of interactive electronic communication. When I used a phone in the 1930s, I had little sense that the whole fabric of human concourse was being rewoven.

Actually two plate tectonics shifted in 1880. The first commercial hand-cranked calculators also came on the market about then. Those two technologies, computers and electronic communication, finally married in the mid 1980s.

Now the tidal wave is just breaking on the shore. People are either buying into the new electronic networks or shrinking from them in horror. Now, at last, we see the cresting wave, and it is far larger than anything we were prepared for.

I've said before that our machines rise out of our animal nature and ride beyond our conscious control. So the tidal waves of human ingenuity gather energy invisibly and finally break upon us -- suddenly, irrevocably, and magnificently, as well.

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

(Theme music)


Bascom, W., Waves and Beaches. New York: Anchor Books, 1980.

For more on the print revolution see Episodes 696, 736, 753, and 756. For more on the electronic revolution see Episodes 680, 685, 708, 725, and more. This idea was the central theme of a joint lecture series developed for a sophomore design class by John Lienhard, Pat Bozeman, and Nancy Buchanan (the latter two from the UH Library.)

Students of fluid mechanics identify a tidal waves as a type of "shallow wave." That means the wave is much longer than the depth of fluid in which it moves. Consider a typical tidal wave:

The surface of the ocean might be sinusoidal with a wavelength of 400 miles and an amplitude of 3 feet. The local depth of the ocean, h, might be around 12,000 feet. This wave is 176 times as long as the depth of the ocean. Let us calculate the speed of the wave under those circumstances.

The "phase velocity," c, of a shallow wave is given by:

c = SQRT(gh)

where g is the acceleration of gravity, and where the phase velocity is the speed that shape of the wave travels. The actual liquid only moves slowly up and down as the horizontal shape moves horizontally. In the example at hand,
c = SQRT[(32.2 ft/sec/sec)*12,000 ft)]

= 622 ft/sec

= 424 mi/hr

This is over half the speed of sound at normal atmospheric conditions. That is an extremely high velocity, yet at this speed the wave still takes 400/424 hr, or 57 minutes, to pass.

The tidal wave presents us with a problem of perceiving relative scale. The wave on a cookie sheet is understandable to us. When we scale that up to oceanic dimensions, it surprises us in many ways.

And so, of course, do technological revolutions.


The Engines of Our Ingenuity is Copyright © 1988-1997 by John H. Lienhard.

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