Given Galaxy's age (~13 Gy) and likely prevalence of habitable worlds, why is there no observable evidence of extraterrestrial civilisations? Candidate resolutions: great filter (upstream or downstream of us), rare Earth, zoo hypothesis,…
Given Galaxy's age (~13 Gy) and likely prevalence of habitable worlds, why is there no observable evidence of extraterrestrial civilisations? Candidate resolutions: great filter (upstream or downstream of us), rare Earth, zoo hypothesis,…