Ethan Siegel has an excellent post up exploring the possibility of extraterrestrial civilizations.
With hundreds of billions of stars (visible, above, in infrared wavelengths) in our galaxy alone, and literally trillions of planets around them, we have many, many chances for life to have evolved similarly to how it did here on Earth. With at least 200 billion galaxies in the Universe, it seems unfathomable to us that we would be alone as the only self-aware, intelligent, sentient lifeforms in the Universe.
And yet, the titular question of this article — where is everybody — is one of the most famous puzzles in modern science: Fermi’s Paradox. If the Universe is so conducive to life, and if there are so many opportunities for it within our galaxy alone, why isn’t there any evidence of extraterrestrial life?
Ethan’s piece is excellent and I highly recommend reading it. But he only obliquely refers to a factor that I consider important in this question, which I’ve written about before: the degree to which interstellar travel is possible.
It seems to me that everything Siegel lays out is correct, if interstellar travel is effectively impossible. I say “effectively” because we know it’s possible in the sense of Voyager leaving the solar system and passing by other stars tens of thousands of years from now. But effective means getting there in some sort of usable time frame, and with technology that is able to function when it arrives.
For that to be conceivable, it seems like the probe needs to get there within a few centuries at most, which implies achieving speeds of at least 1% of the speed of light. Is that possible? I don’t think anyone can say for sure, but it seems very conceivable to me that it is using foreseeable technologies.
If it is possible, then it would only require self replicating probes around 100 million years to colonize the entire galaxy. The galaxy has been around for 13 billion years. We’ve haven’t been contacted by any of these probes, which implies that it is unlikely that there are any other advanced civilizations in our galaxy, putting the closest civilization possibly millions of light years away.
I’m admittedly ignoring the possibility of “prime directives” and such, mainly because it’s hard to imagine such a directive holding across multiple civilizations and billions of years.
If faster travel is possible, then the distance to the next civilization is further away. If some form of faster than light travel is possible, the next civilization may be outside of the visible universe. For a common science fiction trope to be reality, that of alien empires that haven’t noticed us yet, they’d most likely have to operate across distances of billions of light years.