Cosmos - Carl Sagan [164]
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