Sandberg writes:
It is based on a paper we are writing together that analyses how much harder the Fermi question (because it is not really a paradox, just a question with answers we tend to dislike/disagree on) becomes once you take modern ideas about self replication and exploratory engineering into account. The main finding is that intergalactic expansion is likely doable using local resources and a very high branching factor, and that makes the solar neighbourhood accessible to at least millions of times more potential alien civilizations. So either alien civilizations have to be even rarer than we think, they have to approach some non-visible behavioural attractor with very high fidelity, or they are here and hiding efficiently (in this case likely because the first expanding civilization used its probes to enforce some set of rules for everybody else).
The simplest explanation is that complex life is very rare (e.g. we are alone at least in our galaxy). The hydrogen hypothesis for the endosymbiosis of Eukaryote life appears to be a singularly rare event:
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Note the odds 3/4's of the way in the paper.
Thanks for the link, a really good talk!
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