Check out Part 1 of the series here.
Now versions of the Great Filter argument had been around for decades (just like Fermi was not the first person to ask where everybody is), but the most comprehensive form of the argument comes from Robin Hanson in 1996, who is an economist. Now I know you’re thinking: an economist, really? But hey, who says astronomers get to have all the fun. Possible resolutions to Fermi’s paradox are less about physical theory and raw observations and more about statistics and probabilities, so hey economists, welcome to the party.
Here’s the simplest possible reduction of the great filter argument, the most distilled essence of the entire chain of reasoning to explain why we don’t see advanced space-faring civilizations roaming the galaxy: nobody makes it.
That’s it. nobody makes it to that stage. Question: where is everybody? Answer: nobody’s home. The logical chain that leads to Fermi’s paradox is broken in the assumption that intelligent space-faring civilizations are COMMON. According to the great filter answer, they’re not common at all, and so we shouldn’t be surprised when we don’t see anybody.
But wait, wait wait. Aren’t we on the cusp of achieving space colonization status? If the great filter is true, and nobody makes it to that level of sophistication, then…what does it mean for us? Is this…the end of the road?
Like I said, a little ominous.
Let’s breaks things down to see under what exact conditions things should start to feel a little spooky, and where we might be able to keep our blood pressure in a nice safe range.
The issue is that we don’t know exactly when or where the great filter actually happens. There are a lot of steps to go from “random planet with the right ingredients for life” to “vast interstellar empire”. For his part, ol robby hanson broke it down into 9 separate steps that life must go through to get from the little to the big leagues. These steps are: having the right star system to create the basic conditions for life, generating self-reproductive molecules (as in, abiogenesis), stepping up to basic cells with prokaryotic life, advancing to more complex and capable cells with eukaryotic life, achieving sexual reproduction, making the jump to multicellular life roaming around the world, achieving some vague sense of intelligence (like, say, mastering fire or using tools), then advancing to the status of a civilization with the POTENTIAL for space colonization, and finally, once all the pieces are in place, becoming a gigantic galaxy-spanning explosion of life.
We can debate the particulars of this classification scheme until the cows come home, but the point is that to get to make the galaxy your playground, you have to jump through a lot of hoops and pass a lot of tests. Some of those tests are early on, and some are a little later in the process. Sooooo the big question is: where’s the bottleneck? Where does the “Great filter” become so great? What stops nascent life from reaching the stage where we should be able to detect it? Is it at the beginning, with life-ready systems hard to come by? Somewhere in the middle, where life never gets a start or just spends billions of years swimming around in oceans, or is it towards the latter stages? And considering that WE OURSELVES ARE NEAR THE LAST STAGE, we’d really like to settle this question, because the location of the great filter means a lot for the future survival of our species.
Now I know there is an obvious bias in this argument: that life across the galaxy follows a similar path to us, which may or may not be true. But if we expand our definition of life, that makes fermi’s paradox even WORSE: there should be MORE evidence of life in that scenario, not less, so let’s stick with this restriction because it’s the most compatible with solutions to fermi’s paradox and move on.
So let’s talk about the early stages first, because…that makes sense. Maybe the great filter takes place at stage 1. Maybe the conditions for life are exceedingly rare. Well, to be perfectly honest, that doesn’t like that’s the case. Yes, I know that the Earth is the only known planet in the solar system, heck, the universe, where we know life can exist. But it seems based on our limited observations that life has had plenty of chances, even in our own solar system. We know for a fact that Mars once hosted liquid water oceans and a thick atmosphere, right around the same time that the early Earth did. Life happened here, and so it’s somewhat plausibly reasonable to assume that life got a start there. It just…kinda died. Same thing for Venus.
And we have to mention the icy moons of the outer planets, like Europa, Enceladus, and more. Under their icy shells those worlds host globe-spanning liquid water oceans, more liquid water than the Earth has! Those oceans just might be rich in minerals and nutrients..and might be homes for life.
I could go on. And I will. We see organic molecules, and even amino acids, in molecular clouds and on comets. The galaxy is swimming in the ingredients needed for our kind of life. So the basis, the stage 1, of just having the right conditions, seems to be very common indeed. No signs of a great filter here.
What about the next couple stages? You know, the bits about self-reproducing molecules and the evolution of single-celled organisms. Well, we don’t have much to go on here: we only have evidence for life on one single planet. But it’s still a data point that we can learn some lessons from. It’s not much – I’m definitely venturing into speculation territory here, so you’re welcome to make your own arguments – but it’s something. We know that life appeared basically ASAP once our planet cooled. Once the conditions for life were met – with the right ingredients, temperatures, and so on – life…happened. Life on Earth is almost as old as the Earth itself. So if we were forced to guess (and again it’s only a guess!) then a halfway decent reasonable guess is that the great filter is NOT in the earliest stages: if life has the right conditions, it probably shows up right away.
So where does that leave us?
