Copyright © 1983, by David Brin. All rights reserved. No duplication or resale without permission.

The following article was first published in the 1980s in Analog Magazine as a popular adaptation of the much deeper and more scholarly 'classic' review of the field -- The Great Silence -- which appeared in the Quarterly Journal of Royal Astronomical Society, fall 1983, v.24, pp 283-309, which is also available on this site.

What could early students of this new science say about extraterrestrial life-forms? No matter how daring, they were faced with one major limitation: a near total lack of data. The only known case of intelligent life is here on Earth. Until the Viking mission, some held on to Percival Lowell's dreams for Mars. Now the evidence seems to weigh against finding even microbes there. Putting aside, for the moment, speculations about dolphins, whales, and gorillas, it's pretty hard to extrapolate a graph from only one data point.

Still, three scientific discoveries and one useful philosophical tool gave researchers the courage to make crude estimates about the distribution of life among the stars.

The first discover came when it was found almost ridiculously easy to make amino acids, and other precursors to living matter, from abundant molecules such as methane, ammonia and cyanogen. Stanley Miller subjected a water solution of these substances to electrical discharge and ultraviolet radiation and got an organic "soup" in short order. Leslie Orgel of the Salk Institute accomplished the same thing by a freezing process. The high pressures of ice formation not only gave up amino acids, but the purine adenine as well. (Adenine is one of the four building blocks of DNA, and is the core of ATP, adenosine tri-phosphate, which controls the energy economy of the living cell.

So many mechanisms have been found that can change crude precursors into "biological" molecules that today organic activity seems almost an automatic consequence of the distribution of chemical elements in the universe.

The second major discovery supports this point of view. During the last two decades, radio astronomers -- listening to narrow emission lines from interstellar space -- have discovered great clouds of complex molecules: ethylene, formaldehyde, ethyl alcohol; some even claim evidence for -- you guessed it -- adenine.

(Astronomer and science fiction author Sir Fred Hoyle, looking at starlight scattered from interstellar dust, thinks that the dust itself may actually be something akin to bacteria... living cells about a micron in size, in diffuse colonies spanning light-years and outmassing suns. It's an extravagant speculation, but fun to think about.)

It's clear, then, from basic chemistry and radio astronomy, that the basic materials for life are out there. What about the right environments? We have to assume, until we have reason to think otherwise, that complex life must grow and evolve to intelligence on planets orbiting stable stars. Are there other "nursery worlds" like the Earth?

There are plenty of stable, long-lived G-type dwarf stars like the Sun out there... about 6 percent of the galaxy's several hundred billion stars. Are there planets circling many of them?

The data are still poor. It's hoped that the Space telescope will tell us more about the companions of nearby stars. Some scientists think there is good evidence that at least one of our neighbors, Barnard's star, has possibly two dark companions a bit more massive than Jupiter.

We do know that F-, K-, and G-type dwarf stars rotate much more slowly than larger, hotter stars. The Sun contains 99.9 percent of the matter in the solar system, yet it has only 0.5 percent of the angular momentum. The rest is distributed among the planets of the solar system, especially Jupiter. Most astronomers believe that those slowly rotating stars that aren't members of multiple-star systems have to possess dark companions that were used to "dump off" excess angular momentum early during star formation. Recent models of gas-cloud condensation tend to support this belief.

We've covered three discoveries, then, that help us believe that it's reasonable to talk about life outside the Earth. What is that "philosophical tool" we mentioned that caps the legitimacy of xenology? It is sometimes called the cosmological principle, or the assumption of mediocrity.

Since Copernicus, astronomy has been a series of lessons in humility, all leading to the conclusion that "there is nothing special about where and when we are." First the Earth was displaced from the center of the solar system, then the Sun became a nondescript traveler in orbit about the rim of the galaxy. The galaxy became merely one island universe among billions, and the universe seems to have no "middle" at all.

The cosmological principle tells us we should avoid the temptation to think that there's anything unique about the Earth in space, time, or situation. It is the major philosophical underpinning for the new study of xenology. It forces even the most cantankerously conservative astronomer to admit that someone, somewhere, might be peering up at HIS stars, among which insignificant motes is our own Sun.

If xenology has some justification, then, where did the first generation of scientific xenologists get their numbers? How did they estimate the population of our galaxy... or the probably distance to our nearest neighbors?

The Drake Equation is the most popular way to guess at the possible distribution of technological species. It was invented by Frank Drake when he was at the Arecibo National Radio Observatory. It remains the most widely accepted tool for xenological speculation.

Let N = the current number of technological civilizations in the galaxy. Then,

Here R is the average rate of production of suitable stars since the formation of the galaxy, approximately one per year. (The current rate is slower. R is an average that includes the burst of star creation early in the galaxy's history.) P is the fraction of stars that are accompanied by stably orbiting planets. Factor n(e) is the average number of planets per system that have the requisite conditions to support life.

The other factors include f(1), the fraction of these congenial planets on which life actually occurs; f(i), the fraction of these on which "intelligence" appears; f(c), the fraction of intelligent species that attain technological civilizations, and L, the average lifespan of each species.

For what then seemed fairly good reasons, Sagan and others chose to assign P and n(e) each values near 1. These guesses, within an order of magnitude, don't seem to conflict with what we now know about planets.
 
For purposes of discussion it was assumed that congenial planets normally develop life, [f(1) = 1], that about a tenth of the planets with life evolve intelligence, [f(i) = 0.1],and that about a tenth of the latter will see technological civilizations [f(c) = 0.1]. In other words, a likely planet will contribute roughly 0.01 technological races during its history.

A complete discussion of the Drake Equation can be found in books and in many recent technical articles. (Some references for the interested reader are given at the end of this article.) There are reasons to believe that the equation is, in fact, short about three factors. But suffice it here to say that the best guesses, with plenty of up-and-down leeway in every parameter, led Sagan and others a decade ago to a rough estimate.

This meant the average life span of technological races would determine the number present in the galaxy at any time. If self-destruction is the common fate of "civilized" species, then there might be no more than a handful of them in the Milky Way at a given moment, separated by vast tracts of silent starscape. If, on the other hand, a reasonable fraction of races live a long time, the galaxy might be teeming with life.

Cameron von Hoerner, Shklovskii, and Sagan all guessed at L, allowing for various ways in which a culture might end. Generally, their results suggested that the number of civilizations in the galaxy might be on the order of one million, most of them long-lived and patient species. The numbers giving rise to this estimate were a bit arbitrary, but not unreasonable.

If the planets of a million stars held sophont races, then about 0.001 percent of all eligible stars in the galaxy would be inhabited by thinking beings. The average distance separating these islands of technology would be on the order of several hundred light-years -- a gap that seemed unbridgeable corporeally, but easily crossed by radio waves.

This was the state of affairs in the early seventies. With interstellar travel virtually ruled out, the accepted model depicted isolated motes of intelligence separated by sterile tracts of space.

These speculations led to CYCLOPS, OZMA, SETI, and CETI. The search for extraterrestrial life was born. The radio astronomers who slapped together borrowed time and equipment to scan the sky were hopeful, and numerous articles about their endeavors came out on the pages of magazines.

We could take up several articles just talking about SETI. The early arguments over search strategy are fascinating reading. What kinds of antennae would be best suited for the job? Would extraterrestrial intelligent species (ETIS) transmit in the "water hole" frequencies? Should we transmit our own messages, or just wait and listen? If we wait, should we let our nearest neighbors get their first impressions of us from DEW-line radar and I Love Lucy?

Extraterrestrials might not use radio for long-range communication. If lasers carried their traffic, we might not be able to eavesdrop on interstellar conversations.

Even if we can't tape long-distance calls, though, we might still listen for leakage from a planet's commercial radio network... or search for a beacon... a signal meant to be picked up by new radio-using species like ourselves.

Many papers came out during the early seventies suggesting that advanced extrasolar sophonts would likely broadcast the interstellar equivalent of Sesame Street, to help younger species (like us) pass over their initial dilemma of survival or self-destruction. The reasoning went that it would be in the older species' interest to help its younger neighbors live long enough to get a decent conversation going.

The first formal search for extraterrestrial intelligence came when Frank Drake and his associates looked at the two nearest candidates, the two Sun-like stars that lie within twelve light years and are not members of multiple-star systems. Drake's team found nothing but star noise coming from the K2 dwarf, epsilon Eridani. Then they turned their telescope to the star Tau Ceti.

And lo! They hear something! For a brief instant they felt a thrill, as modulated signals came down the cable, obviously of intelligent origin! But then, as the telescope settled down, the "signal" faded away, never to return. They soon concluded that the signal was indeed coded noise from the nearest civilization -- some commercial traffic in nearby Milford, Massachusetts!

Undaunted, Drake and others expanded the search. The telescopes turned and scanned. Nothing was found. The Russians joined the search, enthusiastically. They reported only negative results.

No problem, astronomers suggested. Any advanced species wouldn't waste energy broadcasting over the entire bandwidth of, say, the hydrogen 21-centimeter line. To conserve power, and to attain a high signal-to-noise ration, they would modulate over a very narrow band. Just wait, they suggested, until we can develop fineband simultaneous multichannel analyzers!

Yet the second and third generation of eavesdropping devices have come up with nothing.

True, still better instruments are planned. The money and time spent in the search has been insignificant compared with the potential rewards, which might include clues to the very survival of the human race. (There is a battle under way as this is being written, to restore the piddling two million-dollar appropriation for SETI, which recently was Proxmired to death.

Still, just one decade ago some of the radio-xenologists were talking as if they expected to be cracking codes in short order.

Now a few even glumly propose that no one is "out there" after all at least, not in our vicinity.

How can this be? If we've been at the search for less than fifteen years, using spare time and borrowed equipment, how could anyone expect success so soon? Sure, it'd be nice to find neighbors twelve or twenty light-years away; you could hold a "conversation" within one person's life span, for example. But according to most calculations using the Drake formula, the average distance between technological civilizations might be a few hundred light-years. There are well over a million stars in a sphere a hundred parsecs across. It would take some time to search even the most likely of these, choosing only those radio hands we guess to be the best (not knowing whether our idea of "best" is universal).

Two hundred light-years makes "conversation" a little more difficult. But a Sesame Street beacon would be just as useful as ever at that range. Just knowing extraterrestrials existed might profoundly boost Homo sap's sagging morale.

It seems like we are presenting an argument to restore that appropriation from Congress, not laying out a case for doom and gloom. It only appears to be a matter of time and effort. Success, in the long run, seems assured to the persistent.

What has changed, then? What has caused this spreading anxiety?

It's not the sort of thing one would expect to be a cause for pessimism. At first hearing it sounds like very good news.

Starships are possible --

Next... is it time to launch starships?