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 silver, 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 Aztecs 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.*
* There may be many motivations to go to the stars. If our Sun or a nearby star were about to go supernova, a major program of interstellar spaceflight might suddenly become attractive. If we were very advanced, the discovery that the galactic core was imminently to explode might even generate serious interest in transgalactic or intergalactic spaceflight. Such cosmic violence occurs sufficiently often that nomadic spacefaring civilizations may not be uncommon. Even so, their arrival here remains unlikely.
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 reengineering 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.