There’s a lot of debate here as to whether life, perhaps even the intelligent kind, exists somewhere else in the cosmos.
The question then becomes when will we have the tech to divine the presence of life on distant worlds? I am aware of postulated probes such as the Terrestrial Planet Finder, and a new terrestrial telescope which might be able to block light from the parent star-in both cases they would be able to discern the spectral signature from a given planet, deducing the presence of various atmospheric gases, such as nitrogen or oxygen.
Is there even more audacious future tech being envisioned? At what point could we say with a great deal of confidence that we’ve detected (or ruled out) life on any planets within X light years?
We have the tech now. It’s just the non-trivial matter of getting it there. I think that very soon - within the next couple of decades - we will discover that there was life on Mars and there is life on Europa.
If ‘basic’ forms of life such as microbial, we may get a lucky break if we can find something that drifted in from what we determine to be from another star. We may find life in other planets of the solar system (though that life may share a single origin with earth, so doesn’t prove intersteller life)
But if you are looking to find interstellar advanced civilizations we would have to figure out faster then light (FTL) travel and communication, because what we currently use (i.e. electromagnetic ‘radio’ waves) will not be used by them, it is just too slow for interstellar use. If they are a intersteller civilization they have figured out how to go FTL.
Once we figure out how to communicate FTL, that will give us a new way to search for them, and if (I’d say when) we figure out FTL travel we can go find them - or perhaps they will visit us in some ‘first contact’ gesture.
Now if it is impossible to go FTL then intersteller travel becomes something like generational/stasis/‘artificial womb create people when you get there’ ships. These ships could very well use radio wave communication but for the most part these ships one might ‘conject’ would operate as a isolated unit and not have much to say. So this society, even though may be able to travel between the stars and colonize other planets around them would not really be a ‘interstellar civilization’ as in the above sense just a star seeding one. It would most likely be a one way journey and take quite a bit of time for the spore colony to evolve to the point where it could do likewise.
There is always more audacious future tech being envisioned. At any given time in any given area of technology, there will always be a few papers about projects three or four generations beyond what we currently have. These are usually incremental changes: The next step after Terrestrial Planet Finder, for instance, would be yet another space-based optical interferometer telescope, just with a larger baseline, higher-resolution sensors, more precise optics, etc. Sometimes, the next step is as simple as “take the best thing we currently have, and make a whole bunch more copies of it in different places”, which can, depending on the project, have significantly greater than linear advantages.