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.

The Third Xenological Revolution began sometime in the mid-seventies, when several prominent scientists challenged the conventional wisdom that intelligent life arises upon isolated islands, forever separated by the wide gulfs of interstellar space. Sanger, Bracewell, Forward, Bussard, and others demonstrated that it's possible to build to cross the emptiness between the stars. No "magic" is needed. It isn't necessary to repudiate Einstein. Whether by lightsail or by antimatter rocket, humanity may be launching starships within a few centuries.

These "Starships" would be nothing like the good old Enterprise. Limited to possibly a tenth of the speed of light, they could not travel terribly fast by interstellar standards. But clearly they could carry people, possibly living several generations in transit. The "slow-boat" generation ship of science fiction fame has been mathematically vindicated.

This is bad news?

Of course not. But the possibility of starships places a new and awesome burden on xenology. It presents us with a paradox that is very difficult to overcome.

What would we do if we had starships? If both history and literature tell us anything, we would look around for nice real estate and start colonizing. In fact, we wouldn't even need to find nice planets; stable stars with asteroid belts would do. Our own "belters" might by then prefer such virgin territory to "dirty planets" anyway.

Once the new colonies reached a high level of industry, say in a few hundred years or so, what would they do? Why, they'd send out more colony ships, of course. It seems obvious to almost anyone holding a book like this one.

Imagine a sphere of human settlement slowly expanding through space. How long would it take for colonies to be planted three hundred light-years from Earth? Even limiting ship speed to a tenth of the speed of light, and allowing each colony plenty of time to industrialize? Ten thousand years? Thirty thousand years?

Mankind has hardly changed at all, physically, in the last thirty thousand years. If we make a few social advances and avoid self-destruction, we should be able to fulfil the above scenario.

And why shouldn't anyone else? If this sort of expansion can occur once, why not for each of the million sophont races we calculated earlier? In well under 100,000 years the 200 light-year "average spacing" between races would be filled up!

Recent calculations by Eric Jones of Los Alamos Laboratories indicate that the scenario we have just described, of a slowly expanding sphere of settled solar systems, could fill the entire galaxy within sixty million years. It's not unreasonable to imagine at least one out of a million civilized races living that long. So why do we see no signs that the Earth has been colonized in the last sixty million years?

Why have we picked up no radio signals, when the stars should be humming with information and commerce?

Where are they?

This question marks the first traumatic awakening of the new science of xenology. It marks the end of a very short period of innocence. Starting around 1975 and building toward the present, the Third Xenological Revolution commenced. The dust has not yet settled, but one thing is clear. Some of our assumptions are wrong. The universe might turn out to be considerably more complicated than the scientist optimists of the late sixties had at first thought.

Of course, science fiction writers and readers could have told them that all along.

The Third Revolution in Xenology came with the realization that space should be filled with intelligent life. There appears to be no excuse any longer for the failure of SETI.

Indeed, why hasn't the Earth itself been colonized! The question, Where are they? might better be put, Why aren't they here? The quandary can be called the Mystery of the Great Silence.

We see no evidence for ancient alien cities in the Earth's crust. Venus and Mars apparently never were terraformed, though many now think we could tackle the job in a few centuries. The asteroids of the solar system appear to be untouched.

Most significantly, the Earth, until less than a billion years ago, was populated for two billion years by only primitive prokaryotic organisms. A visiting starship need not have landed colonists. All they'd have had to do was be careless with their garbage or latrine and the history of the Earth would be totally different.

It certainly looks as though we've been alone a very long time.

There have been several imaginative suggestions to explain the Great Silence. At the end of this article we'll compile a partial list.

Dr. Eric Jones, Dr. Frank Tipler, and Dr. Michael Hart all think it means that the early calculations of the probabilities of intelligent life were greatly overoptimistic. They suggest that the apparent absence of ETIs simply means that this part of the galaxy is uninhabited... that not race has got out there ahead of us to make an impact by colonization. Their Uniqueness Hypothesis implies that some or all of the factors ÿ(l,i,c) in the Drake Equation are really very small. For instance, some contend that intelligence such as ours is an evolutionary fluke.

Dr. Thomas Kuiper of JPL has presented strong arguments in refutation, showing that convergent evolution has happened frequently on Earth and might well occur elsewhere.

Dr. John Ball has dredged up the science fictional idea that the Earth is a "zoo" or wildlife preserve, and that extraterrestrials are already here, observing us. There are many variants to this concept, including "quarantine" (ETIs awaiting humanity's social maturity), a noninterference "Prime Directive," and many others. All imply we should add to the Drake Equation a factor to account for ETIs purposely avoiding contact.

Contact optimists, such as William Newman of Princeton and Carl Sagan of Cornell, have tried to make excuses for extraterrestrials. In a recent paper Newman and Sagan suggested that truly advanced cultures would practice zero population growth and thus feel less pressure to expand into virgin territory. The rate of "galaxy-filling" calculated under their extremely conservative assumptions is slow enough to make it barely possible that the nearest expanding space-faring race simply has not reached us yet.

Sagan and Newman further propose that techniques of life extension -- immortality -- would make individuals of a race very conservative. If a passion for risk-avoidance took hold, a species' rate of expansion, V, could drop to nil.

Might a race naturally graduate to other interests after a certain amount of time? Science fiction is filled with possibilities, from extra dimensions to realms of the mind far more attractive than drifting through space and clearing land on some new world. Such "maturity stages" would affect L in the Drake Equation, as well as the velocity of expansion.

Our assumptions for factors ÿ(l) might be too high. Although the precursors of life -- sugar, amino acids, nucleic acids -- seem likely to be about as common as stardust, it's possible that the next steps to life might be much, much harder to reach, requiring some rare catalyst to set the process off.

From physics and SF comes the dreadful idea of "deadly probes." Saberhagen's "Berserkers" might make life rare if some technological civilization accidentally let loose something so monstrous. Gregory Benford's variant on the idea is hardly more optimistic. A particularly paranoid advanced species might not want any potential competition to rise up elsewhere. Self replicating autonomous probes might be sent out to reproduce and fill the galaxy. Whenever new radio traffic indicates that new sentients are loose, these preprogrammed probes would home in on the signals with powerful bombs and stop the infection before it spreads.

It's already too late to call back the spherical wave of I Love Lucy, etc., that's already spreading through nearby space.

All of the hypotheses given above have their problems. Some seem to contradict the best knowledge we have in the field. Others, like the "zoo" theory, are almost innately untestable.

What we hope to do is to compile a list of these possibilities. I will start things off by talking about a few hypotheses that the xenologist speculators have mostly passed up. Some are a bit frightening.

In the Drake Equation the combined factor ÿ(i,c) -- the fraction of life-planets on which intelligence and technology eventually evolve -- is generally assigned a value of about 1 in 100. The xenologists who put forward the one percent argument support it by citing the apparent fact that it took four billion years for the Earth to give rise to merely one technological race. This is almost half the viable life span of the planet. Intelligent life would seem to be a rare and wonderful thing.

But is this assumption tenable? It appears to be the weakest link in the chain of logic.

Let's consider the life cycle of a "Nursery World," a planet with a stable biosphere in which the slow evolution to intelligence can take place.

Evolution appears to have proceeded gradually at first and then at an accelerating pace for over three billion years. Except for (maybe) the introduction of sex, and later of flowering planets, there is no evidence in the fossil record to support the idea that the Earth was ever suddenly invaded by extraterrestrials who, "with kith and kine," introduced advanced flora and fauna. The Great Silence seems, at first glance, to have stretched through the entire Paleozoic.

If we assume the Earth lay untampered with until at least the time of the Jurassic, we can guess that it takes about three billion years for life on a Nursery World to evolve to a level of complexity that makes intelligence feasible.

What if humanity suddenly vanished? Would it take another three billion years for intelligence once again to arise on Earth? If so, it's reasonable to accept the guess that the number of technological species to erupt per habitable planet is of order less than one.

But Homo sapiens is not the only species to have benefited from three billion years of evolution. Today's German cockroach may look a lot like his distant ancestors, but he has accumulated many little tricks his cousins in the Triassic never heard of. The size of genome of the raccoon and wolf is hardly smaller than that of man.

Consider what's happened since the Cretaceous-Tertiary Catastrophe approximately sixty five million years ago -- the disaster that wiped out, over a period of a few hundred thousand years, almost every species of land animal whose adults massed more than forty kilos.

The creatures whose descendants went on to dominate the planet were small mammals: the early equivalents of mice, lemurs, and tree shrews. These humble animals expanded and diversified to all of ecological niches left vacant by the demise of the large reptiles. We are among their descendants.

In spite of the present arms race, man still lacks the ability to exterminate mice, although he will probably soon be able to do an efficient job on himself. The sudden demise of this star system's current technological race would not finish off the Earth as a nursery. If "mice" did it once, they could probably do it again.

We are led to suggest that suitable worlds must pass through long initial "fallow" periods before attaining a level of biological sophistication for intelligence. Afterward such planets should be able to produce sophont species at fairly short intervals, depending upon the time needed to recover from the damage done by the previous sentient race.

The interval between the Cretaceous Catastrophe and the present is a reasonable estimate for the time it takes to build a civilized race, once small and sturdy creatures have reached a high level of sophistication.

Next... so why would space travelers travel?