Overcoming Bias covers two papers on SETI; importantly, the papers distinguish between the search for artifacts (like Dyson spheres) and the search for communication. There are problems with searching for communication, among them: do we know what medium they'd use, can we understand them, and should we expect the beacons to be on all the time, or just intercept them briefly, like the WOW signal? The search for artifacts can be divided into looking for massive engineering undertakings of far away civilizations that are solar system- or galaxy-wide, and looking for them right here in the solar system where you're reading this. The latter is not a frequently considered approach, but that's why I'm excited for Dawn to finally make it to Ceres; there are specific reasons to think low-gravity bodies with water and organics would be the places to look for evidence of extrasolar technology. (But until there's a probe that lands and gets good chemistry we won't have evidence.)
Yet, we've found no clear evidence as yet. Add to that the argument that if there is any chance different than zero for any species to develop interstellar travel, the galaxy is very likely to already be full - that is to say, if space-traveling life is anywhere, it should be everywhere, because it would be vanishingly unlikely for us to be the first. And we don't see such life everywhere. At this point we can't conclude that no one is out there, but we can be more certain that no one is everywhere out there. Maybe we're looking for the wrong things, but as we look further and include more types of phenomena, the more we find nothing, the more we should assume we're alone or nearly alone as a technology using intelligence.
Hanson's concern is about the great filter. As it seems the evolution of life seems more and more likely in many places, the great silence we observe means that something is stopping all these living things from leaving their homeworlds, and by some arguments that something is more likely to be in humanity's future than our past. One candidate is that intelligence is an evolutionary dead end which causes species to wipe themselves out, which was exactly Fermi's original fear - that intelligence creates a superpredator that not only exterminates its prey but itself. An interesting bit of trivia: we are currently living through a mass extinction at least as bad as the K/T event, and maybe the worst so far on Earth, and we're causing it.
The other question to ask is this: which of the following two propositions is more likely to be true?
1) That life evolves very frequently, and intelligence relatively frequently, but only very few (or no) species make it to the point of interstellar expansion, so that we don't see a galaxy chock full of waste heat from their engineering projects (i.e. that life is anywhere but NOT everywhere);
2) That they are everywhere out there, but we still don't know what we're looking for.
It may be instructive to work backwards. Start with the assumption that we are surrounded by massive (roughly galaxy-spanning) civilizations, as the papers envision them. We've been looking right at them since the first time a human paid attention to the night sky - how could we differentiate them from background? The uncontacted people in the Amazon are surrounded by nation states, and yet for a half century they've been growing up with the sound of planes in the sky, and they haven't inferred the rest of the world.
What are the things we already see that could be evidence? Dark matter is an intriguing candidate just because we understand it so poorly. The absence of obvious life could itself be a hint, i.e. still-extant species are hiding from or destroyed by others.
This is certainly a less depressing alternative than intelligence being an evolutionarily unstable strategy, which of course has nothing to do with its being true. I increasingly suspect that life in the universe is mostly space viroids that when seeded in a large, warm medium, incidentally produce replicators like life on Earth, that is then stuck there, because either it can't travel in space, or it gets smart enough to travel in space and therefore to kill itself.
The Sad World of Uncited Papers
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