The author John Scalzi asked for comments on The Fermi Paradox.
The Fermi Paradox was posed by Enrico Fermi, an Italian physicist and one of the fathers of the atomic bomb. Discussing the possibility of intelligent life in the galaxy, he asked “where are they?” Fermi was no dummy. He knew that, for 99% of the history of humanity, the aliens would have to find us by landing on Earth.
Implicit in his question, however, was the assumption that aliens would go forth, if not to look for us, just to explore. Some of the pessimists who argue that man is alone in the galaxy point out that, even at relatively low rates of population growth, eventually any alien species will fill the galaxy. I think both groups are missing something, namely demographics.
Consider humanity (since it’s the only species we can consider.) For 99% of human history, the demographic pressure has been to have as many children as possible. Worried that the kids will get eaten by a saber-toothed tiger? Have more. (It works for rabbits.) Worried that somebody needs to be around to run the farm when you’re old and senile? Have more children. In agriculture, you get a bonus – even very small children can do useful work.
All this changes when man becomes industrialized. Building steam engines is a non-trivial task. And you can’t have a rocket ship if you can’t build a steam engine. Education becomes more important, and guess what – children become a short-term liability.
Carried to extremes, as in Western Europe and Japan, birth rates fall below replacement levels. The only reason the US population is growing is due to immigration. Even third-world countries, like Mexico and Kenya, are seeing the rate of population growth fall.
What does this have to do with aliens? Plenty. Most mass migrations in human history were semi-voluntary at best. My great-great-grandfather came here from Lithuania because that was the only shot he had at getting enough cash to buy a farm in the old country. As soon as the pile under his mattress was big enough, back he went. Fortunately for me, a 14-year-old boy who became my great-grandfather stayed.
So, if populations stabilize or fall, the big pressure for immigration and colonization fall off as well. Granted, some small percentage will go no matter what. But then, you run into the problem of small numbers. Assume 1% of the alien race leaves. 1% of a billion is a million – roughly the population of Montana. Montana is a big state, but a damn small planet.
So, why haven’t we met aliens? They’re just not spreading out very fast. And I suspect neither will we.