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?
via Throwback Thursday: Where is Everybody? — Starts With A Bang! — Medium.
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.
SelfAwarePatterns 
Thank you for the link! It’s a really interesting look at interstellar travel.
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Thanks Michelle!
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You are right. And that almost certainly means that FTL travel and time travel are impossible even for the most advanced civilisations imaginable. Otherwise, they’d be here.
The positive to take away from this is that if we ever manage to begin colonising other star systems, then we will almost certainly never again find ourselves in a situation where we (or an alien race) could completely destroy everyone or be wiped out by some cataclysmic event.
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Good point. Although I suspect successful colonization will involve changing ourselves to survive in other biospheres. It makes me wonder how human we might consider some of those colonists if we could see them now.
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There’s a couple of issues with that. We don’t know how long it takes for intelligence to evolve on planets, other than that it took 4.6 billion years for it to evolve here on Earth, and the first wave of stars in the galaxy were not particularly hospitable.
We don’t know if it’s possible to build starships that even go that fast. You usually see someone drag out Orion, but Orion was on paper only. It’d be like saying that we could build a sea-floor tunnel train across the Atlantic Ocean from New York City to London – even if it might be possible on paper, the engineering challenges you come across in building it might make it effectively impossible.
Not sure about the self-replicating probe idea. It’d probably be more akin to sending a massive starship carry an entire set of industrial tools and equipment and some pretty impressive automation, both so it can do repairs on itself in the trip through interstellar space, and so it can build up and replace itself once it gets to another solar system. It’s not impossible if you’ve got the AI, the tools, and the engines, but that’s still quite a commitment in resources.
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I think you raise a key point. It might be ultimately possible to develop some mechanism to do it using nuclear pulse propulsion, beamed propulsion, or some other technique, but will we consider it worth the resources?
On the self replicating probe idea, I think the usual concept is that the seed probe is as small as possible, perhaps even microscopic since every gram of mass comes with appalling energy requirements, and uses raw materials at the destination to bootstrap an infrastructure.
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Or perhaps fast interstellar travel is possible, but astronomically expensive.
Personally, I think that life is probably quite likely, but bridging the gap between a society of greedy individuals and a society that works together toward cohesive goals quickly enough that you don’t kill yourself is damn near impossible.
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Carl Sagan thought much the same. It may simply be in the nature of intelligent life to destroy itself. It’s a dark possibility that I see no way to conclusively dismiss. It’s hard to imagine though that it would happen to every species, but maybe the Fermi paradox is a combination of somewhat rare intelligent life coupled with a high probability of self destruction.
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