Searching for extraterrestrial intelligence has long been a hot topic among astronomers, biologists, and the general public. But not many recall how the subject was jump-started more than 50 years ago.

In September 1959, physicists Giuseppe Cocconi and Philip Morrison published a landmark article in the British weekly journal Nature with the provocative title, "Searching for Interstellar Communications." Cocconi and Morrison argued that radio telescopes had become sensitive enough to pick up transmissions that might be broadcast into space by civilizations orbiting other stars. Such messages, they suggested, might be transmitted at a wavelength of 21 centimeters (1,420.4 megahertz). This is the wavelength of radio emission by neutral hydrogen, the most common element in the universe. Other intelligences might see this as a logical landmark in the radio spectrum where searchers like us would think to look.

Seven months later, radio astronomer Frank Drake became the first person to start a systematic search for intelligent signals from the cosmos. Using the 25-meter dish of the National Radio Astronomy Observatory in Green Bank, West Virginia, Drake listened in on two nearby Sunlike stars: Epsilon Eridani and Tau Ceti. His Project Ozma (named for Queen Ozma in L. Frank Baum's Wizard of Oz books) slowly scanned frequencies close to the 21-cm wavelength for six hours a day from April to July 1960. The project was well designed, cheap, simple by today's standards, and unsuccessful.

Following the Ozma experiment, Drake organized a meeting with a select group of scientists to discuss the prospects and pitfalls of the search for extraterrestrial intelligence — nowadays abbreviated SETI. In November 1961, ten radio technicians, astronomers, and biologists convened for two days at Green Bank. Young Carl Sagan was there, as was Berkeley chemist Melvin Calvin, who received news during the meeting that he had won the Nobel Prize in chemistry.

It was in preparing for this meeting that Drake came up with what soon became known as the Drake Equation:

N = R x f_p x n_e x f_l x f_i x f_c x L

Nowadays this string of letters and symbols can be found on T-shirts, coffee mugs, and bumper stickers. It is simpler than it looks. It expresses the number N of "observable civilizations" that currently exist in our Milky Way galaxy as a simple multiplication of several, more approachable unknowns:

R is the rate at which stars have been born in the Milky Way per year, f_p is the fraction of these stars that have solar systems of planets, n_e is the average number of "Earthlike" planets (potentially suitable for life) in the typical solar system, f_l is the fraction of those planets on which life actually forms, f_i is the fraction of life-bearing planets where intelligence evolves, f_c is the fraction of intelligent species that produce interstellar radio communications, and L is the average lifetime of a communicating civilization in years.

The Drake equation is as straightforward as it is tantalizing. By breaking down a great unknown into a series of smaller, more addressable questions, the formula made SETI a tangible effort and gave the question of life elsewhere a basis for scientific analysis.

Astronomers and biologists alike have tried to "solve" the equation ever since. At first sight, coming up with a reasonable estimate for the answer might seem fairly straightforward. But the number of communicating intelligences can't be judged so easily. Several of the variables in the equation have been firmed up since 1961. But at least three remain very unknown.

The rate of star formation in our galaxy is approximately one per year, R = 1. The next factor, f_p, is fairly close to 1: most stars have planets, we finally know as of 2012. And the next factor, n_e, probably can't be a whole lot less than 1 either.

But from here on, things get much more tricky. Optimists would argue that life will form wherever it can (f_l = 1), that the Darwinian process of natural selection eventually favors the evolution of intelligence (f_i = 1), and that no intelligent civilization would exist for a very long time without discovering electricity and radio and feeling the urge to communicate (f_c = 1). In this most optimistic case, the Drake equation boils down to the simple observation that N = L (the average lifetime of technological civilizations, in years). If L is, say, 100,000 years, there would currently be about 100,000 chatty civilizations in our galaxy. And that's assuming that only one such civilization arises during a given planet's entire multi-billion-year lifetime.

That figure of 100,000 would mean there is one radio-emitting civilization right now per 4 million stars — reason enough to tune in on the heavens and start hunting for them. If they were scattered at random throughout the Milky Way, the nearest one would probably be about 500 light-years from us. That means a two-way conversation would require a time equal to a good fraction of recorded human history, but a one-way broadcast might be audible.

However, some 50 years of SETI have failed to find anything, even though radio telescopes, receiver techniques, and computational abilities have improved enormously since the early 1960s. Granted, the "parameter space" of possible radio signals (all the possible frequencies, locations on the sky, signal strengths, frequency drift rates, on-off duty cycles, etc.) is vastly larger than the tiny bit that has yet been searched. But we have discovered, at least, that our galaxy is not teeming with very powerful alien transmitters continuously broadcasting near the 21-centimeter hydrogen frequency. No one could say this in 1961.

Have we overestimated the values of one or more of the Drake parameters? Is the average lifetime of technological civilizations short? Or have astronomers overlooked some other, more subtle aspect?

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