In the article I discussed in my previous post, astronomer Seth Shostak cites estimates that there are between 10,000 and one million civilizations in our galaxy. But there is a strong reason for rejecting such estimates, and for thinking that the number of civilizations in our galaxy should be either very high (many millions) or very low (fewer than 100).
A spiral galaxy similar to our galaxy (credit: NASA)
The reason has to do with interstellar colonization. Let us imagine that there was a single civilization that was interested in colonizing the galaxy, which contains more than 100 billion stars. Such a civilization could spread throughout the galaxy in a time span of less than 100 million years.
You do not defeat such a conclusion by arguing that interstellar travel is impossible. It is quite possible that none of the fancy space warps or warp drives of science fiction will ever be created. It is also possible that there are practical engineering reasons why interstellar spaceships may never travel faster than an average of about five percent of the speed of light. But even if the fastest interstellar spaceship takes a century of more to travel from one star to another star, interstellar travel should be possible. A civilization would merely need to create relatively slow multi-generation ships to travel between the stars. Such ships might be manned by one generation as the ship left a solar system, and then when the ship finally reached another solar system, the ship might be manned by the descendants of the descendants of the descendants of the original crew members.
Two mathematicians have estimated how long it would take for a civilization to colonize the galaxy, using very conservative assumptions. They assumed a very slow rate of colonization, in which it takes 1000 years for one solar system to colonize another solar system about five light years apart. The mathematicians reached the conclusion that it would take about 50 million years for a civilization to spread across the galaxy.
That may seem like a vast amount of time, but it isn't very long compared to the age of the universe. The universe is believed to be some 13 billion years old. It seems that intelligent life might have appeared in our galaxy at any time in the past billion years. So there would have been plenty of time for such a galaxy-wide colonization effort to have occurred (a billion years is 20 times longer than 50 million years).
Some have tried to use this type of reasoning to argue that we must be the only civilization in our galaxy, on the grounds that if another civilization had arisen, it would have already colonized Earth. But such an argument seems weak, on two grounds. First, it is possible that there exist a very small number of extraterrestrial civilizations, none of which has any interest in colonizing the galaxy. Second, there is always the possibility that our planet has been preserved as a kind of zoo or nature reserve. Humans themselves have established quite a few nature reserves around the world, so extraterrestrials might have done that also; and our planet may be part of some “do not colonize” zone.
Let's suppose that one civilization had spread throughout most of the galaxy, something that seems likely if there originally arose a hundred or more extraterrestrial civilizations. Would it be correct to refer to all of the far-flung colonies arising from such a civilization as being parts of a single civilization? It would seem not. The speed of light would seem to make it very likely that these far-flung colonies (separated by many light years) would break up to become a very varied set of civilizations, rather than anything that could be called a single civilization.
Once a civilization had been established light-years away from its parent solar system, the parent solar system would have only a weak degree of control or influence over its colony. As each new colony was created in solar systems farther and farther away, this cultural attenuation effect would increase.
We can imagine a typical colonization progression:
- Home world colonizes a solar system 5 light-years away, colonizing Planet A. That colony is only 80% similar to the civilization that it was derived from.
- Planet A colonizes a solar system 5 light-years away, colonizing Planet B. That colony is only 80% similar to the civilization that it was derived from.
- Planet B colonizes a solar system 5 light-years away, colonizing Planet C. That colony is only 80% similar to the civilization that it was derived from.
- Planet C colonizes a solar system 5 light-years away, colonizing Planet D. That colony is only 80% similar to the civilization that it was derived from.
You can see the general trend here. As the colonies are established farther and farther out in galactic space, these colonies tend to differ more and more from some ancestral world that they are descended from. By the time planets are colonized hundreds or thousands of light-years away from the planet that started the wave of colonization, each of these planets is really becoming its own distinct civilization, with its own distinctive customs, philosophy, ethics, and so forth. So it's not really a case of a single civilization spreading across the galaxy. It's like some microorganism that mutates into a thousand different strains as it spreads across the world.
So if one extraterrestrial civilization were to spread to millions of different planets over the course of millions of years, by the time that process had finished we should really say that there would be millions of diverse civilizations in the galaxy, rather than saying these colonies are all a single civilization.
There seem to be only two plausible possibilities:
- Galactic civilizations are very rare. It might be that there are fewer than 100 civilizations existing on fewer than 100 planets in our galaxy. In such a case, it is quite possible that none of them (other than our civilization) was interested in colonizing the galaxy. For example, if there were only 50 other civilizations in the galaxy, it could be that ten of them have moral objections to galactic colonization, that 30 of them are inward-looking or absorbed with activities on their home planets, and that the other ten lack the confidence or drive to colonize the galaxy.
- Galactic civilizations are extremely common. It could be that the galaxy is teeming with civilizations, partly because of widespread galactic colonization. The total number in our galaxy could be in the hundreds of millions, or perhaps even more than a billion. It could be that our planet has been preserved as some kind of nature reserve or laboratory or zoo, which might explain the fact that our planet has not been colonized.
Due to the galactic colonization factor, it does not seem plausible that there are a medium number of civilizations in our galaxy – some number such as 10,000 or 100,000 or a million. If there were that many civilizations, some of those civilizations would have started colonizing the galaxy, leading to a situation where hundreds of millions of planets became occupied by civilizations. It strains credulity to imagine that fewer than 1 in 10,000 civilizations would be interested in colonizing the galaxy (conversely, it doesn't seem too implausible to guess that something like only 1 or 2 percent of civilizations are interested in colonizing the galaxy).
It's rather like this. Imagine you wake up from a 10-year coma, and are told only that there is smallpox in a distant city. We know that smallpox is extremely contagious. So your best guess (based on this limited information) is that either very few people in the city had smallpox, or that smallpox had spread all over the city. Given how contagious smallpox is, it would not make sense to assume that a medium number of people in the city had smallpox (such as one twentieth of the population). In a similar vein, on a time-scale of a billion years, civilizations are highly “contagious” (in the sense of being prone to spread from one solar system to another). So it seems that the best guess would be that civilized life has either spread though the galaxy, or that there are so few civilizations that such a “contagion” has not occurred.