We haven't had an off-topic Friday night discussion for a while, but I've got one I've been meaning to post for a while. Might as well do it now.
This question has to do with Fermi's paradox, which contrasts a couple of basic realities. We know from observation that the universe is vast and filled with billions of stars. Probabilities tell us that if even a small fraction of those stars have planet capable of producing life, the universe ought to be teeming with civilizations. Yet we haven't made contact with or observed concrete evidence of life beyond our own planet.
Which raises the question, "Where are they?"
Really, we should be able to pick up a signal with SETI or something, at least, right?
If you've seen Christopher Nolan's excellent movie Interstellar, you may have some added perspective on this question. The film underscores the extreme difficulty of Earth-based organisms with relatively short lifespans making any attempt to explore the vastness of space—and, given the way relativity works, time. The exploration of deep space will involve extreme sacrifice for the individuals involved, and who knows what they might encounter in terms of a changed human culture once they return to Earth.
Considering the vastness of space and time together, and their interconnectedness, perhaps Fermi's question ought to be: "Where and when are they?"
I'm fascinated by the question. What are the odds that we'll discover evidence of extra-terrestrial life, especially intelligent life, in our lifetimes?
Or have we already?