Have you ever seen an alien? Neither have I, or anybody else, as far as we know. That is actually quite strange, statistically speaking. This apparent paradox is known as the Fermi Paradox. It’s a very important paradox for science fiction, so I’m going to ramble about it a bit.
Life in the universe
To understand the Fermi paradox, you first have to understand the likelihood of intelligent life. Let’s do some math – did I really just say that?
As you can see in the picture above (CC-licensed from the European Space Agency) there are a lot of stars out there. In fact, there are a lot of stars; estimates for the milky way – our home galaxy – run from 200 billion to 400 billion. If you think that’s much, there are actually some 170 billion to 200 billion galaxies in the universe that we can see, each containing their own hundreds of billions of stars.
Not every star has a habitable planet, but a lot of them do, and as we start finding exoplanets, those estimates are only going up. Estimations for how many stars have habitable planets exactly varies, but it could be as high as one in five. Note that ‘habitable’ means that a planet is in an orbit around its star that allows the possibility of liquid water, the so-called goldilocks zone. That would lead to 40 billion in our Milky Way alone. Being pessimistic and dividing that by 10 or even 100 still leaves a staggering number of habitable planets in our galaxy alone. In the universe as a whole, that number is increased 200-billion-fold. That’s trillions of habitable planets.
The next step is to take into account the age of the universe. The universe is 13,7 billion years old. Stars only formed after a few hundred million years, but that still leaves over 10 billion years. To put that in perspective, that is over 5.000 times the period that homo sapiens has existed (1,8 million years) .
So we have trillions of planets and thousands of times the lifespan of our entire race. How much life would you expect in the universe?
The Fermi paradox
The physicist Enrico Fermi wondered, given this number of habitable planets and the enormous stretch of time that they have existed, even with a very small chance of intelligent life evolving, why are there no signs? In the time since the dawn of the universe, intelligent life could have evolved thousands of times.
If only a single intelligent alien race had evolved 1 billion years ago in our milky way, they could have easily colonized the entire milky way by now, even without faster-than-light travel. Even if alien races never wanted to leave their home system, they could still have started sending out probes.
Our race is less than 2 million years old, and we’re already creating very advanced probes. Self-replicating AI probes that could traverse the galaxy should be within our reach in a couple of decades. So, where are they? Where are the signs of aliens or their probes? Where are the radio signals? The ruins of an advanced colony on earth?
This lack of evidence of alien life, given the statistical analysis that they should be there, is the Fermi paradox.
What’s going on then?
The Fermi paradox is an ongoing mystery. There are many theories about it, but no proof as of yet. This mystery is the base for many a science fiction novel.
The fact that we exist means that there is a non-zero chance of intelligent life evolving. However, meaningful communication might not be possible in a universe where faster-than-light travel and communication appears to be impossible.
It could also be more grim, and mean that the likelihood of a space-colonizing race is prohibitively low. This last is known as the Great Filter hypothesis. It could very well be that any intelligent race is doomed to burn itself out before they can colonize the universe. Say, for instance, by making their planet uninhabitable through overpopulation and pollution.
Any science fiction work that includes alien life should include some kind of explanation for the Fermi paradox. It could be as simple as communications all using something else than radio waves so we never picked up the aliens. It could be that they are keeping us in the dark on purpose until we’re ready to enter the intergalactic arena. Or something is killing all life when it is found, meaning any day we could be next, once we’re detected.
The possibilities are as endless as our imagination.
Will we ever solve it?
There are three ways the Fermi paradox could be solved: either we find aliens, we find a reason why intelligent life is extremely rare, or we go extinct.
Finding aliens could happen any day now, or not in the next thousand years, or even never. Things like the SETI projects are not guaranteed to have results at any set moment. They just go on, trying to find a needle in a haystack.
Finding a reason why intelligent life is extremely rare is a more dubious solution to the problem. Since we exist, the chance of intelligent life must always be non-zero, and it’s very hard to prove that that chance is so close to zero that it can ‘prove’ that no other aliens will evolve anywhere near us.
The third option is that we go extinct. If we manage to kill our race in the next few centuries, we may have proven the Great Filter solution to the Fermi paradox, namely that intelligent races kill themselves before being able to leave a mark.
The Fermi paradox is an intriguing mystery, at least to science fiction writers. If you want to know more, just start reading some of those novels.