Earlier quoted context omitted.
> But that’s the thing, if our galaxy had lots of 20-21st century Earth level civilizations, we would be able to detect them with our existing SETI programmes. But we couldn't. In general, SETI couldn't even detect such a civilization even if it was in the star system closest to earth: > If an extraterrestrial civilization has a SETI project similar to our own, could they detect signals from Earth? > In general, no.…
> But we couldn't. In general, SETI couldn't even detect such a civilization even if it was in the star system closest to earth. Your quote relies on us detecting re-runs of "I Love Lzcwwdrrr", but our telescopes can reliably do spectrographic analysis of faraway stars and planets these days. We'd likely be able to detect things like unnaturally high levels of hydrocarbons in otherwise oxygen-full atmospheres, and ot…
We don't know what "unnatural" levels of CO2 on other planets are.
> If Alpha Centauri had a planet with an industrial civilization, we might be able to spot city lights.
We do not even remotely have that capability.