In discussions of filters and Fermi paradoxes, questions are often asked with the word "civilization". Why, we wonder, do we not see evidence of non-human civilizations?
The concern for the absence of evidence of civilizations has been that it is probably unreasonable to assume that humans are special (the self-indication assumption), and that, since the evolution of life elsewhere in the galaxy seems more and more likely (more planets discovered, more ways of making heredity chemicals), it's worrisome that we don't see evidence of other civilizations. Why worrisome? Because it may mean that the "filter" that stops civilizations from filling the sky seems less and less likely to be between the primordial chemical soup of young planets and the evolution of living things, and therefore more and more likely to be after the evolution of life and their surviving long enough to colonize the galaxy. That is to say, whatever it is which seems to have consistently stopped the others' expansion is probably still ahead of us in time. By this argument, any observation which makes it more likely on average for planets to get at least as far as humans are on the way to intelligence and interstellar diaspora is bad news, because it means the filter must still be in front of us.
There are two assumptions here which, if falsified, break the logic of these arguments. One is that the sky really is empty. We've only just started looking and it's not at all clear we know what to look for, or where (related posts here.) Second is that at this point it is totally
unwarranted to insist that matter-based replicators which move between stars must necessarily have, or be the product of, a "civilization". The provinciality of such an assumption cannot be over-stated. Certainly with most Earth organisms, there is no conceivable way to move between star systems without a specialized representational tissue that allows behavior-changing information to be cooperatively shared by large numbers of entities. This is what we call "civilization" in the one species that we know has developed it. But if it is indeed possible for non-intelligent replicators to spread between stars (even if slowly; see calculations here) there's still a good chance we'll find it. If it's possible for non-intelligent life to spread, and we don't find it, there's a good chance that the filter is in fact the evolution of life in the first place (despite all mounting findings apparently to the contrary), not the stability or longevity of "civilizations" that would otherwise be thought necessary to help intelligences escape the quarantines of their solar systems. In that case, we're out of the woods, and we're on our own.
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