Wow, I've been doing this for two months and am not done yet. Now I know how Proust must have felt. Assuming Proust was writing blog posts about the Fermi Paradox, of course. So far I've looked at all the Great Filters which lie in our past — barriers to the evolution of life, barriers to intelligence and technology, and potential civilization-ending disasters. They certainly do reduce the number of possible advanced civilizations in the Milky Way, but only to a point.
Right now we're stuck at an estimate of about fifty civilizations out there among the stars, of which perhaps two-thirds have the wealth and ability to construct radiotelescopes or interstellar probes. Call it thirty. That's an awkward number because there's a very strong chance that many of them would be thousands if not millions of years older and more advanced than we are, which should make their activities easy to spot even across interstellar distances. But as yet we see nothing.
Today I'd like to consider three possible Great Filters which might explain why those civilizations — assuming they exist — are not doing anything we can detect. So these are Space Filters.
The first is the simplest, and most likely. Space travel, particularly interstellar travel, is just really hard. We may be able to send microchip-sized probes to nearby stars, but no colonists, no flag-planters. And given that it's likely to be tens of lightyears to the nearest world with even simple microbial life, maybe the effort is too great.
We do know that dangers wait in interstellar space: in the past few years we've detected two comets originating outside the Solar System. Now since presumably they didn't just start showing up right after humans invented ways to detect them, this suggests that interstellar wanderers are fairly common, and that space between the stars may be studded with rocks and shoals.
So go slowly, then. One thing which Oumuamua did demonstrate is that it's entirely possible to travel between the stars, as long as you don't mind spending tens or hundreds of thousands of years doing it. But is it really possible for any civilization to create a self-contained ship which can keep running that long? Or for any civilization to want to send out an expedition which won't report back for a thousand centuries?
Those are strong objections. Even the notion of firing off a bunch of Von Neumann self-replicating probes gets a little unattractive if your civilization might not last long enough to get the data they send back.
Now, it's possible that individuals in a sufficiently rich civilization might be able to launch their own interstellar missions. Right now a few tech billionaires on Earth are funding interstellar research essentially out of personal whims. The inhabitants of a Dyson Sphere with the full energy output of their home star to play with could do it as the equivalent of a high school science club project. But it's very hard to imagine any sustained, large-scale, systematic interstellar project under those circumstances.
I am going to call this one another Great Filter. Let's assume that half of those thirty advanced civilizations out there find the prospect of interstellar exploration too difficult, and so never do it. That cuts the number down to fifteen. Still awkward.
The second Space Filter is what some have called the "Dark Forest" problem. The idea goes like this: if you make yourself known across interstellar distances, some other civilization might decide to conquer or annihilate you. So everyone just keeps silent. The analogy is of a group of armed men stumbling around in a dark forest at night. None of them dares turn on a flashlight or call out, because anyone who draws attention could become a target.
I've written another long-winded series of blog posts about why I don't buy the Dark Forest idea at all. In addition to the reasons cited, there's another factor which I've only become aware of since writing those original pieces. Quite simply, our existence disproves it.
An advanced civilization can construct immense space-based telescopes, both radio and optical. Our own observatories are already looking for "biosignatures" and "technosignatures" on exoplanets within several hundred light-years. A civilization with kilometer-wide telescopes could do the same across a significant fraction of the Galaxy. This would be an especially important project for a paranoid xenophobic civilization planning to shoot first — the equivalent of night-vision goggles for that dark forest.
Our planet has had biosignatures in its atmosphere for billions of years. It has had detectable technosignatures for a few centuries, depending on how clever any potential observers are. The point is, we can already be seen across hundreds of light-years. Maybe thousands. Yet here we are. No interstellar killer probes yet.
Also, the detection and industrial infrastructure needed to monitor the whole Galaxy and zap any emerging civilizations should itself be detectable. So I'm calling the Dark Forest filter a dud. I just don't think it's likely.
And now the final filter, the one that I actually do worry about. In my discussion of civilization filters a couple of weeks ago I mentioned the danger that some civilizations might reach some kind of local optimum and simply stop advancing. No Scientific/Industrial/Economic/etc. Revolutions, just endless generations of Iron Age peasants, ruled by an endless series of alternating warlords and bureaucrats, until the Sun swells in the sky and the oceans dry up. Not a bad life, really, but one which leaves no mark on the Galaxy.
We got through that filter . . . or did we? It's possible there are several such dead ends in history, including one or more in our future. We may be on the cusp of one right now.
It's alarmingly easy to imagine a future for humanity in which we never leave Earth at all. Unmanned probes explore the Solar System a few square kilometers at a time, but that's all. No space colonies, no terraforming Mars, no Dyson Sphere Kardashev II civilization for us. Just endless "sustainable" generations of Information Age consumers, ruled by an endless series of alternating demagogues and bureaucrats, until the Sun swells in the sky and the oceans dry up (because nothing is really sustainable over time).
And across the Galaxy there may be dozens of civilizations caught in just the same trap, stuck on a single planet and unable to bootstrap themselves up to becoming interplanetary. Maybe . . . all of them?
That's a Great Filter I can see coming, and one we may be able to avoid. It means taking action — investing, donating, and letter-writing. If the Universe isn't worth fighting for, what is?
Well, Jim, you’ve already convinced us we’ll probably get turned into space dust if we try interstellar FTL travel and that there’s no other way to really do it. ;-) But yet we should sent our $20 to the Interplanetary Society or Tesla or ... who? Our friendly local SF authors might be a better bet. Maybe we should send seeds out to the stars, instead. Maybe it’s already happened. Maybe we will have a Big Crunch and live it all over again in an endless loop.
Posted by: Monica Eiland | 10/02/2020 at 12:50 AM