- At a lower bound, it's estimated on average there is one 17,000 LY away. The number that is being reported is that this means at least 36 civilizations in the galaxy.
- They mention the problem of relying on M-class stars as abodes for life - because they're quite unstable (flares). I have not read the paper in detail, but it seems hard to understand, if there are only 36 star systems, why those couldn't all be G-class stars.
- They also estimate a lower bound of communicating for only a century (since we've been communicating for that long so we know it's possible.) If it's only a 100 year period, if we're hearing them now, they were active before agriculture.
- There's also the problem of being able to discern signal from noise at that distance - and not knowing what type of signal we're looking for. A useful thought experiment is the C-index, which is the distance at which we could detect a twin Earth with identical EM emissions. By most estimates, even if there were a twin Earth orbiting Alpha Centauri, we still today could not hear them. This leads the authors to conclude that interstellar communication is for all intents and purposes impossible.
- Therefore, any persisting civilization is plausibly more likely to be detected by self-replicating artifacts. This all reinforces the greater relative importance of looking for artifacts in our own solar system, which is something we can conceivably do with known technology in the near future, with less of a signal-to-noise problem.
Westby T. and Conselice CJ. The Astrobiological Copernican Weak and Strong Limits for Intelligent Life. The Astrophysical Journal. 2020 June 15.
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