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But consider the alternative, the prospect that at least some civilizations learn to live with high technology; that the contradictions posed by the vagaries of past brain evolution are consciously resolved and do not lead to self-destruction; or that, even if major disturbances do occur, they are reversed in the subsequent billions of years of biological evolution. Such societies might live to a prosperous old age, their lifetimes measured perhaps on geological or stellar evolutionary time scales. If 1 percent of civilizations can survive technological adolescence, take the proper fork at this critical historical branch point and achieve maturity, then fL ≈ 1/100, N ≈ 107, and the number of extant civilizations in the Galaxy is in the millions. Thus, for all our concern about the possible unreliability of our estimates of the early factors in the Drake equation, which involve astronomy, organic chemistry and evolutionary biology, the principal uncertainty comes down to economics and politics and what, on Earth, we call human nature. It seems fairly clear that if self-destruction is not the overwhelmingly preponderant fate of galactic civilizations, then the sky is softly humming with messages from the stars.

These estimates are stirring. They suggest that the receipt of a message from space is, even before we decode it, a profoundly hopeful sign. It means that someone has learned to live with high technology; that it is possible to survive technological adolescence. This alone, quite apart from the contents of the message, provides a powerful justification for the search for other civilizations.

If there are millions of civilizations distributed more or less randomly through the Galaxy, the distance to the nearest is about two hundred light-years. Even at the speed of light it would take two centuries for a radio message to get from there to here. If we had initiated the dialogue, it would be as if the question had been asked by Johannes Kepler and the answer received by us. Especially because we, new to radio astronomy, must be comparatively backward, and the transmitting civilization advanced, it makes more sense for us to listen than to send. For a more advanced civilization, the positions are, of course, reversed.

We are at the earliest stages of our radio search for other civilizations in space. In an optical photograph of a dense star field, there are hundreds of thousands of stars. By our more optimistic estimates, one of them is the site of an advanced civilization. But which one? Toward which stars should we point our radio telescopes? Of the millions of stars that may mark the location of advanced civilizations, we have so far examined by radio no more than thousands. We have made about one-tenth of one percent of the required effort. But a serious, rigorous, systematic search will come soon. The preparatory steps are now underway, both in the United States and in the Soviet Union. It is comparatively inexpensive: the cost of a single naval vessel of intermediate size—a modern destroyer, say—would pay for a decade-long program in the search for extraterrestrial intelligence.

Benevolent encounters have not been the rule in human history, where transcultural contacts have been direct and physical, quite different from the receipt of a radio signal, a contact as light as a kiss. Still, it is instructive to examine one or two cases from our past, if only to calibrate our expectations: Between the times of the American and the French Revolutions, Louis XVI of France outfitted an expedition to the Pacific Ocean, a voyage with scientific, geographic, economic and nationalistic objectives. The commander was the Count of La Pérouse, a noted explorer who had fought for the United States in its War of Independence. In July 1786, almost a year after setting sail, he reached the coast of Alaska, a place now called Lituya Bay. He was delighted with the harbor and wrote: “Not a port in the universe could afford more conveniences.” In this exemplary location, La Pérouse

perceived some savages, who made signs of friendship, by displaying and waving white mantles, and different skins. Several of the canoes of these Indians were fishing in the Bay.… [We were] continually surrounded by the anoes of the savages, who offered us fish, skins of otters and other animals, and different little articles of their dress in exchange for our iron. To our great surprise, they appeared well accustomed to traffic, and bargained with us with as much skill as any tradesman of Europe.

The Native Americans drove increasingly harder bargains. To La Pérouse’s annoyance, they also resorted to pilferage, largely of iron objects, but once of the uniforms of French naval officers hidden under their pillows as they were sleeping one night surrounded by armed guards—a feat worthy of Harry Houdini. La Pérouse followed his royal orders to behave peaceably but complained that the natives “believed our forbearance inexhaustible.” He was disdainful of their society. But no serious damage was done by either culture to the other. After reprovisioning his two ships La Pérouse sailed out of Lituya Bay, never to return. The expedition was lost in the South Pacific in 1788; La Pérouse and all but one of the members of his crew perished.*

Exactly a century later Cowee, a chief of the Tlingit, related to the Canadian anthropologist G. T. Emmons a story of the first meeting of his ancestors with the white man, a narrative handed down by word of mouth only. The Tlingit possessed no written records, nor had Cowee ever heard of La Pérouse. This is a paraphrase of Cowee’s story:

Late one spring a large part of Tlingit ventured North to Yakutat to trade for copper. Iron was even more precious, but it was unobtainable. In entering Lituya Bay four canoes were swallowed by the waves. As the survivors made camp and mourned for their lost companions two strange objects entered the Bay. No one knew what they were. They seemed to be great black birds with immense white wings. The Tlingit believed the world had been created by a great bird which often assumed the form of a raven, a bird which had freed the Sun, the Moon, and the stars from boxes in which they had been imprisoned. To look upon the Raven was to be turned to stone. In their fright, the Tlingit fled into the forest and hid. But after a while, finding that no harm had come to them, a few more enterprising souls crept out and rolled leaves of the skunk cabbage into crude telescopes, believing that this would prevent being turned to stone. Through the skunk cabbage, it seemed that the great birds were folding their wings and that flocks of small black messengers arose from their bodies and crawled upon their feathers.

Now one nearly blind old warrior gathered the people together and announced that his life was far behind him; for the common good he would determine whether the Raven would turn his children into stone. Putting on his robe of sea otter fur, he entered his canoe and was paddled seaward to the Raven. He climbed upon it and heard strange voices. With his impaired vision he could barely make out the many black forms moving before him. Perhaps they were crows. When he returned safely to his people they crowded about him, surprised to see him alive. They touched him and smelled him to see if it was really he. After much thought the old man convinced himself that it was not the god-raven that he had visited, but rather a giant canoe made by men. The black figures were not crows but people of a different sort. He convinced the Tlingit, who then visited the ships and exchanged their furs for many strange articles, chiefly iron.

The Tlingit had preserved in oral tradition an entirely recognizable and accurate account of their first, almost fully peaceable encounter with an alien culture.* If someday we make contact with a more advanced extraterrestrial civilization, will the encounter be largely peaceable, even if lacking a certain rapport, like that of the French among the Tlingit, or will it follow some more ghastly prototype, where the society that was a little more advanced utterly destroyed the society that was technically more backward? In the early sixteenth century a high civilization flourished in central Mexico. The Aztecs had monumental architecture, elaborate record-keeping, exquisite art and an astronomical calendar superior to that of any in Europe. Upon viewing the Aztec artifacts returned by the first Mexican treasure ships, the artist Albrecht Dürer wrote in August 1520: “I have never seen anything heretofore that has so rejoiced my heart. I have seen … a sun entirely of gold a whole fathom broad [in fact, the Aztec astronomical calendar]; likewise a moon entirely of s

ilver, equally large … also two chambers full of all sorts of weapons, armor, and other wonderous arms, all of which is fairer to see than marvels.” Intellectuals were stunned at the Aztec books, “which,” one of them said, “almost resemble those of the Egyptians.” Hernán Cortés described their capital Tenochtitlán as “one of the most beautiful cities in the world … The people’s activities and behavior are on almost as high a level as in Spain, and as well-organized and orderly. Considering that these people are barbarous, lacking knowledge of God and communication with other civilized nations, it is remarkable to see all that they have.” Two years after writing these words, Cortés utterly destroyed Tenochtitlán along with the rest of the Aztec civilization. Here is an Aztec account:

Moctezuma [the Aztec Emperor] was shocked, terrified by what he heard. He was much puzzled by their food, but what made him almost faint away was the telling of how the great Lombard gun, at the Spaniards’ command, expelled the shot which thundered as it went off. The noise weakened one, dizzied one. Something like a stone came out of it in a shower of fire and sparks. The smoke was foul; it had a sickening, fetid smell. And the shot, which struck a mountain, knocked it to bits—dissolved it. It reduced a tree to sawdust—the tree disappeared as if they had blown it away … When Moctezuma was told all this, he was terror-struck. He felt faint. His heart failed him.

Reports continued to arrive: “We are not as strong as they,” Moctezuma was told: “We are nothing compared to them.” The Spaniards began to be called “the Gods come from the Heavens.” Nevertheless, the Azecs had no illusions about the Spaniards, whom they described in these words:

They seized upon the gold as if they were monkeys, their faces gleaming. For clearly their thirst for gold was insatiable; they starved for it; they lusted for it; they wanted to stuff themselves with it as if they were pigs. So they went about fingering, taking up the streamers of gold, moving them back and forth, grabbing them to themselves, babbling, talking gibberish among themselves.

But their insight into the Spanish character did not help them defend themselves. In 1517 a great comet had been seen in Mexico. Moctezuma, captured by the legend of the return of the Aztec god Quetzalcoatl as a white-skinned man arriving across the Eastern sea, promptly executed his astrologers. They had not predicted the comet, and they had not explained it. Certain of forthcoming disaster, Moctezuma became distant and gloomy. Aided by the superstition of the Aztecs and their own superior technology, an armed party of 400 Europeans and their native allies in the year 1521 entirely vanquished and utterly destroyed a high civilization of a million people. The Aztecs had never seen a horse; there were none in the New World. They had not applied iron metallurgy to warfare. They had not invented firearms. Yet the technological gap between them and the Spaniards was not very great, perhaps a few centuries.

We must be the most backward technical society in the Galaxy. Any society still more backward would not have radio astronomy at all. If the doleful experience of cultural conflict on Earth were the galactic standard, it seems we would already have been destroyed, perhaps with some passing admiration expressed for Shakespeare, Bach and Vermeer. But this has not happened. Perhaps alien intentions are uncompromisingly benign, more like La Pérouse than Cortés. Or might it be, despite all the pretensions about UFOs and ancient astronauts, that our civilization has not yet been discovered?

On the one hand, we have argued that if even a small fraction of technical civilizations learn to live with themselves and with weapons of mass destruction, there should now be an enormous number of advanced civilizations in the Galaxy. We already have slow interstellar flight, and think fast interstellar flight a possible goal for the human species. On the other hand, we maintain that there is no credible evidence for the Earth being visited, now or ever. Is this not a contradiction? If the nearest civilization is, say, 200 light-years away, it takes only 200 years to get from there to here at close to the speed of light. Even at 1 percent or a tenth of a percent of the speed of light, beings from nearby civilizations could have come during the tenure of humanity on Earth. Why are they not here? There are many possible answers. Although it runs contrary to the heritage of Aristarchus and Copernicus, perhaps we are the first. Some technical civilization must be the first to emerge in the history of the Galaxy. Perhaps we are mistaken in our belief that at least occasional civilizations avoid self-destruction. Perhaps there is some unforeseen problem to interstellar spaceflight—although, at speeds much less than the velocity of light it is difficult to see what such an impediment might be. Or perhaps they are here, but in hiding because of some Lex Galactica, some ethic of noninterference with emerging civilizations. We can imagine them, curious and dispassionate, observing us, as we would watch a bacterial culture in a dish of agar, to determine whether, this year again, we manage to avoid self-destruction.

But there is another explanation that is consistent with everything we know. If a great many years ago an advanced interstellar spacefaring civilization emerged 200 light-years away, it would have no reason to think there was something special about the Earth unless it had been here already. No artifact of human technology, not even our radio transmissions, has had time, even traveling at the speed of light, to go 200 light-years. From their point of view, all nearby star systems are more or less equally attractive for exploration or colonization.*

An emerging technical civilization, after exploring its home planetary system and developing interstellar spaceflight, would slowly and tentatively begin exploring the nearby stars. Some stars would have no suitable planets—perhaps they would all be giant gas worlds, or tiny asteroids. Others would carry an entourage of suitable planets, but some would be already inhabited, or the atmosphere would be poisonous or the climate uncomfortable. In many cases the colonists might have to change—or as we would parochially say, terraform—a world to make it adequately clement. The re-engineering of a planet will take time. Occasionally, an already suitable world would be found and colonized. The utilization of planetary resources so that new interstellar spacecraft could be constructed locally would be a slow process. Eventually a second-generation mission of exploration and colonization would take off toward stars where no one had yet been. And in this way a civilization might slowly wend its way like a vine among the worlds.

It is possible that at some later time with third and higher orders of colonies developing new worlds, another independent expanding civilization would be discovered. Very likely mutual contact would already have been made by radio or other remote means. The new arrivals might be a different sort of colonial society. Conceivably two expanding civilizations with different planetary requirements would ignore each other, their filigree patterns of expansion intertwining, but not conflicting. They might cooperate in the exploration of a province of the Galaxy. Even nearby civilizations could spend millions of years in such separate or joint colonial ventures without ever stumbling upon our obscure solar system.

No civilization can possibly survive to an interstellar spacefaring phase unless it limits its numbers. Any society with a marked population explosion will be forced to devote all its energies and technological skills to feeding and caring for the population on its home planet. This is a very powerful conclusion and is in no way based on the idiosyncrasies of a particular civilization. On any planet, no matter what its biology or social system, an exponential increase in population will swallow every resource. Conversely, any civilization that engages in serious interstellar exploration and colonization must have exercised zero population growth or something very close to it for many generations. But a civilization with a low population growth rate will take a long time to colonize many worlds, even if the strictures on rapid population growth are eased after reaching some lush Eden.

My colleague William Newman and I have calculated that if a million years ago a spacefaring civilization with a low population growth rate emerged two hundred light-years away and spread outward, colonizing suitable worlds along the way, their survey sta

rships would be entering our solar system only about now. But a million years is a very long period of time. If the nearest civilization is younger than this, they would not have reached us yet. A sphere two hundred light-years in radius contains 200,000 suns and perhaps a comparable number of worlds suitable for colonization. It is only after 200,000 other worlds have been colonized that, in the usual course of things, our solar system would be accidentally discovered to harbor an indigenous civilization.

What does it mean for a civilization to be a million years old? We have had radio telescopes and spaceships for a few decades; our technical civilization is a few hundred years old, scientific ideas of a modern cast a few thousand, civilization in general a few tens of thousands of years; human beings evolved on this planet only a few million years ago. At anything like our present rate of technical progress, an advanced civilization millions of years old is as much beyond us as we are beyond a bush baby or a macaque. Would we even recognize its presence? Would a society a million years in advance of us be interested in colonization or interstellar spaceflight? People have a finite lifespan for a reason. Enormous progress in the biological and medical sciences might uncover that reason and lead to suitable remedies. Could it be that we are so interested in spaceflight because it is a way of perpetuating ourselves beyond our own lifetimes? Might a civilization composed of essentially immortal beings consider interstellar exploration fundamentally childish? It may be that we have not been visited because the stars are strewn abundantly in the expanse of space, so that before a nearby civilization arrives, it has altered its exploratory motivations or evolved into forms indetectable to us.

A standard motif in science fiction and UFO literature assumes extraterrestrials roughly as capable as we. Perhaps they have a different sort of spaceship or ray gun, but in battle—and science fiction loves to portray battles between civilizations—they and we are rather evenly matched. In fact, there is almost no chance that two galactic civilizations will interact at the same level. In any confrontation, one will always utterly dominate the other. A million years is a great many. If an advanced civilization were to arrive in our solar system, there would be nothing whatever we could do about it. Their science and technology would be far beyond ours. It is pointless to worry about the possible malevolent intentions of an advanced civilization with whom we might make contact. It is more likely that the mere fact they have survived so long means they have learned to live with themselves and others. Perhaps our fears about extraterrestrial contact are merely a projection of our own backwardness, an expression of our guilty conscience about our past history: the ravages that have been visited on civilizations only slightly more backward than we. We remember Columbus and the Arawaks, Cortés and the Aztecs, even the fate of the Tlingit in the generations after La Pérouse. We remember and we worry. But if an interstellar armada appears in our skies, I predict we will be very accommodating.