Today, a science-fiction question gets a scientific answer. 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.

Everyone who's ever spent time on a campus has eventually run into a marvelous piece of graffiti. It says, "Beam me up Scotty, there's no intelligent life down here." Students have always questioned the intelligence of life on their campuses. But we want to ask about intelligent life on other planets.

In 1960 we moved from speculation about life on other worlds to a scientific search for it. We began using radio telescopes to listen for signals from other planets. By now we've listened at 50 sites for well over a hundred thousand hours. Most of the work has been government-funded; but when the Harvard listening project ran low on funds, Steven Spielberg came to its rescue. No one's heard anything yet, and we might wonder if it's all a fool's game. Do we really have reason to expect a close encounter of any kind?

The rationale for expecting to find other intelligent civilizations was summarized by I.S. Shklovskii and Carl Sagan in 1966. It goes like this: We know the number of stars within radio telescope range. Some have habitable planets. Life will arise on some of those, and a few will reach technological intelligence. When such civilizations arise, they'll last for a certain time -- but how long? Shklovskii and Sagan estimated these numbers. Then they used the laws of probability to show that a million intelligent species should be within reach of our radar.

But the phone hasn't rung yet. Maybe Shklovskii and Sagan were overoptimistic. As we fail to hear signals, we revise their estimates downward, and we settle in for a long wait.

Scientists speculate on at least two reasons for silence from outer space. One's the so-called "Zoo Hypothesis." It says that our civilization is very young -- that others are way ahead of us. Far from racing to communicate, the more advanced races seal us off and watch us like animals in a zoo. The other idea is that evolution itself required far more luck -- or even miracle -- than we realize. Perhaps the evolution of intelligent life is such a remarkable miracle of nature that we are, in fact, a unique species after all.

In the end, the research hasn't given us independent answers to philosophical and religious questions about our uniqueness as a species. But it has led to improved radio telescopy. It's made us learn more about the nature of planetary radiation fields and about the nature of humankind. The search itself is both humbling and exciting. In the end, we're not just listening -- we're thinking at the same time, and that's what makes the search worthwhile.

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

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Shklovskii, I. S., and Sagan, C., Intelligent Life in the Universe. San Francisco, CA: Holden Day, 1966.

For further arguments on the question of extraterrestrial life, see:

Barrow, J. D. and Tipler, F. J., The Anthropic Cosmological Principle. New York: Oxford University Press, 1988.

Pollard, W. G., Transcendence and Providence: Reflections of a Physicist and Priest. Edinburgh: Scottish Academic Press, 1987.