Extraterrestrial Civilizations - Isaac Asimov [100]
Counting in all the successive civilizations existing on a planet, we might suppose that the average total duration of civilization upon a planet, during the course of its star’s stay on the main sequence, is perhaps 10 million years.
This is a conservative enough estimate. It means that civilization would be present on a planet like Earth for only 1/740 of the time the planet will endure as a home for life after the first civilization has arisen. That means that only one star out of 570,000 shines down on civilization that is now existing.
Remembering our calculation that 390 million civilizations have come into being, we now have a thirteenth figure:
13—The number of planets in our Galaxy on which a technological civilization is now in being = 530,000.
EXPLORATION
Even a consideration of the mortality of civilizations leaves us with over half a million of them now existing in our own Galaxy. We must, therefore, still ask the question: Where is everybody?
And yet, just because these half-million advanced civilizations are in our own Galaxy, let us not overestimate their closeness to us. They are not our next-door neighbors by any means.
Here on the outskirts of the Galaxy (where we have decided the civilizations must exist) the distance between two neighboring stars that are not connected gravitationally in the form of multiple-star systems is about 7.6 light-years.
If only one star out of 570,000 shines down upon an advanced civilization now existing, the average separation of two civilizations is 7.6 light-years multiplied by the cube root of 570,000. This comes to about 630 light-years.
This is a long distance, and it may be that of all the reasons I have so far advanced as possibly explaining the lack of visits from other civilizations the impracticality of negotiating such distances is the most nearly compelling.* It may well be that every civilization, no matter how advanced, is isolated in its own planetary system and that visits among them are out of the question.
Nevertheless, it is possible to take the view that interstellar travel seems difficult to us only at our present level of technology. A hundred years ago it might have seemed to us that reaching the Moon was a matter of insuperable difficulty; that jet planes and television were mad fantasies. Yet such things are now so common we give them no thought.
Give us another hundred years—or another thousand of the prospective million-year-existence of our civilization—and might not interstellar travel become commonplace and easy?
We’ll discuss the pros and cons later, but for now let us assume that interstellar travel is a reality for the half-million civilizations of the Galaxy and that traveling from planetary system to planetary system offers no difficulties. If that is so, why haven’t they visited us?
Can it be that as one civilization after another ventures out into space, there is intersection and conflict? Even granted that each civilization, in order to get out into space, develops a planetwide political unit, might there not nevertheless be wars among the worlds?
If we want to wax dramatic, we can imagine civilizations killing each other off with devices that explode whole planets or induce stars to leave the main sequence.
Yet that seems wrong to me. Civilizations that had managed to suppress undue violence on their home worlds would have learned the value of peace. Surely they would not forget it lightly, once off their planet.
Besides, it is not likely that the struggle would be so even that, like the fabled Kilkenny cats, the various civilizations would destroy each other until none was left. Those that were more advanced might win out and establish sway over broader and broader sections of the Galaxy. Indeed, the oldest civilizations, intent on imperial growth, might take over scores,