This is a question about the Fermi paradox, a topic that can attract a lot of opinions. However, the scope of this question is strictly limited to a specific factual topic, and I've edited it quite substantially to try and make this clearer.
Enrico Fermi originally introduced his paradox in the following terms:
- We expect life / advanced civilisations to be common in the Universe
- If technologically advanced intelligent life existed, we would expect it to visit Earth
- Therefore its absence requires an explanation. (There is a huge body of literature that attempts to do this.)
However, it seems common in modern popularisations to replace (2) above with:
2$*$. If technologically advanced intelligent life existed, we would be able to detect it remotely.
My question is about this point (2*). The claim here is that we should expect extraterrestrial civilisations to have observational signatures that we would have already detected if they were present. I am somewhat skeptical about this claim, and so I would like to know more details about the assumptions and measurements behind it.
The reason I'm skeptical is that, as far as I'm aware, SETI has concentrated for most of its existence on detecting deliberate signals, i.e. radio signals that have been directionally beamed at Earth with the objective of making contact with us. However, one can think of all kinds of reasons why signals of that kind might not currently exist, even if advanced civilisations are common in the galaxy.
If we rule out this possibility, what else is left? Listening to popular accounts of the Fermi paradox, I'm left with the impression that the presence of extraterrestrial civilisations would just be obvious if they were there. But until the 1990s we could not detect exoplanets at all, and even now we can obtain very little information about their composition or surface details. It does not seem at all trivial to make measurements that would reveal the existence or absence of an extraterrestrial civilisation.
Therefore my question is, which specific observations - by which instruments - have been cited to support the idea that extraterrestrial civilisations do not exist, or are not common, in our galaxy? Note that I'm asking for observations that we have made already, rather than ones we potentially could make in the future.
Of particular interest to me are the observational signatures of megastructures such as Dyson spheres, or of waste heat from civilisations sufficiently large for that to be detectable. It seems to me that, given some assumptions about the size of the civilisation involved, these might have observational signatures that would allow them to be ruled out to some extent, and I am interested in whether this is possible and/or has actually been done. But more broadly, I am interested in published, referenced versions of claim (2*), so that I can understand the assumptions and the astronomical measurements behind them.