A little while back, I read a great piece on the internet about the Fermi Paradox and the possibility of other life in our galaxy (blogged about it here). To quickly summarize, there are tons of earth-like planets out there in our galaxy, and so a pretty fair number of them likely have the potential to harbor life. If we are just one amongst the multitudes, then some civilizations must have formed hundreds of millions or billions of years ago. Now, there’s a credible argument to be made that a civilization that is a few hundred million years more advanced than we are should actually have developed into a “Type III” civilization that has colonized the entire galaxy (gets into the somewhat spooky concept of the von Neumann probe). The question then is why haven’t we actually met any aliens in a galaxy that seemingly should be teeming with life.
There are two general answers. One is that life is out there, but we just haven’t detected it yet, and that online piece does a good job of going through all the possible reasons why we might not yet have detected any life out there. But the other possibility, and the one that I think is frankly a bit more plausible, is that there aren’t any Type III civilizations out there. Yet. Will we be the first? That’s what this piece by Nick Bostrom is all about. The idea is that somewhere in the history of a Type III civilization is an event known as the great filter. This is some event during the course of civilization development that is exceptionally rare, thus providing a great filter between the large number of potential life-producing worlds out there and the complete and utter radio silence of the galaxy as we know it. What are candidates for the great filter? Well, the development of life itself is one. Maybe the transition from prokaryotic life to eukaryotic life. Or maybe all civilizations are doomed to destroy themselves. So in many ways, the existential question facing humanity is whether this great filter is behind us (yay!) or ahead of us (uh-oh!). One fun point that Nick Bostrom makes is that it’s a good thing we haven’t yet found life on Mars. If we did find life on Mars, then that means that the formation of life is not particularly rare, meaning that cannot be a great filter event. The more complex the life that we found on Mars, the worse and worse it would be, because that would eliminate ever greater number of potential great filter candidates behind us, meaning that it is likely that the great filter is ahead of us. Ouch! So don’t be rooting for life on Mars. But while the presence of life on Mars would likely indicate that the great filter is ahead of us, the absence of such life doesn’t say anything, and certainly doesn’t prove that the great filter is behind us. Hmm.
So for a while, I thought this was a classic optimist/pessimist divide: if you’re an optimist, then you believe the filter is behind us, pessimist, ahead of us. But I think there’s actually a rational argument to be made that it’s behind us. Why? Well, I think there are two possible categories of great filter events ahead of us. One is destruction of all life by outside forces. These could be asteroid impacts, gamma ray bursts, etc. Bostrom makes a good argument against these being great filters because a great filter has to be something that is almost vanishingly rare to get past. So even if only 1/1000 civilizations made it past these asteroids and bursts and whatever, then it’s still not a great filter, given the enormous number of potentially life-sustaining planets out there. The other category of filter events (which is in some ways more depressing) are those that basically say that intelligent life is inherently self-destructive, along the lines of “we cause global warming and kill ourselves”, or global thermonuclear war, etc. This is the pessimists line of argument.
Here’s a statistical counterargument in favor of the filter being behind us, or at least against the self-destruct scenario. Suppose that the civilizations are inherently self-destructive and that the filter event ahead of us. Then I would argue that we should see the remnants of previous civilizations on our planet. The idea is that as long as a civilization’s self-destruction doesn’t cause the complete and total annihilation of our planet (which I think unlikely, more in a bit), then conditions would be favorable for life to again evolve until it hits the filter again. And again. And again. Statistically speaking, it would be very unlikely for us, right now, to be the very first in this series of civilizations. Possible, but unlikely.
Now, this argument relies on the notion that whatever these potential future filter events are, they don’t prevent the re-evolution of intelligent life. I think this is likely to be the case. Virtually every such candidate we can think of would probably destroy us, maybe even most life, but it’s hard to imagine them killing off all life on earth, permanently. Global warming? It’s been hot in the earth’s past, with much higher levels of CO2, and life thrived, probably would again. Nuclear war or extreme pollution? Might take a billion or two more years, but eventually, intelligent cockroaches would be wandering the earth in our place. Remember, it doesn’t have to happen overnight. I think there are very few self-destruct scenarios that would lead to COMPLETE destruction–all I can think of are events that literally destroy the planet, like making a black hole that eats up the solar system or something like that. I feel like those are unlikely.
So where does that leave us? Well, I can think of two possibilities. One is that we are not destined for self-destruction, but that the “filter” event is one that just prevents us from colonizing the galaxy. Given our current technological trajectory, I don’t think this is the case. Thank god! Stasis would just feel so… ordinary. The other much more fun possibility is that we are the first ones past the great filter, and we’re going to colonize the galaxy! Awesome! Incidentally, I’m an optimist and an (unofficial) singularitarian. So keep that in mind as well.
So what was the great filter, if it really is behind us? I personally feel like the strongest candidate is the development of eukaryotic life (basically, the development of cells with nuclei). You can get some sense for how rare something is by seeing how long it took to happen, given that conditions aren’t changing. This is hard, because conditions are always changing, but still. Take the development of life itself. Maybe a couple hundred million years? That’s a long time, but not that long, and moreover, conditions on early Earth were changing a lot, so it could be that it didn’t take very long at all once the conditions were right. But eukaryotic life? Something like 1.5-2 billion years! Now that’s a long time, no matter how cosmic your timescale. And the “favorable conditions” issue doesn’t really apply: presumably the conditions favorable to eukaryotic life aren’t really any different than for prokaryotic life, since it's just different rearrangements of the same basic stuff. So prokaryotic life just sat around for billions of years until the right set of cells bumped into each other to make eukaryotic life. Seems like a good candidate for a great filter to me.
Incidentally, one of the things I like about thinking about this stuff is how it puts life on earth in perspective. Given all the conflicts in the news these days, I can’t help but wonder that if we all thought more about our place in the universe, maybe we’d stop fighting with each other so much. We should all be working to better humanity and become a Type III civilization! The wisdom of a fool, I suppose...