But not so fast. Maybe that's just a filter, not the filter. So we made it through the Cambrian Explosion and all -- good on us -- but the real trick is to develop a big brain and an opposable thumb. Or not to nuke or grey goo ourselves out of existence. Maybe the filter is time itself -- you just have to get to that colonizing level before an asteroid or cosmic ray blast or simply the lifecycle of your homeworld wipes you out. (Stephen blogged recently about some of these possibilities.)

And there, says Bostrom, lies the problem. Unlike developing flexible body plans or bigger brains, the nuke, grey goo, asteroid, and cosmic blast filters all (potentially) lie in our future. That means we haven't made it through the great filter yet and, seeing as nobody else has managed to do it, there's not much reason to expect that we will.

Say it with me:

Whoa, dude.

Bummer.

Sure, it's possible that others have made it through this filter and decided that they would rather stay home than colonize the galaxy, but what are the chances that everybody who reaches this stage makes that choice? Or maybe they're all out there hiding from us because they're guided by some kind of Prime Directive that protects us from exposure to their advanced awesomeness. But that one seems pretty unlikely. Plus, it's been argued that the Prime Directive is stupid and/or immoral.

So if life is out there, chances are it gets destroyed (or destroys itself) before ever making it to the galactic colonization stage.

Or maybe, just maybe, something else is going on. Let me offer up a few possibilities.

1. Colonizing the galaxy is a dumb idea.

Don't get me wrong. I personally think it would be the single coolest project ever, and hope to have literally billions of uploaded copies of my personality wending their way not just across the galaxy but the entire universe using those very Von Neumann probes that so far have not shown up here from anywhere else. But I have to admit that I come by this fondness for the idea from the standpoint of living in a civilization that is not quite ready to start the project. As we get closer to that goal, we might find that technology is opening up other possibilities for us that are in fact much more appealing. I don't know what those are. But then, let's face it -- I don't really know much. Maybe outlining a project that advanced civilizations must engage in -- if they exist at all -- would be a tad presumptuous on my part, seeing as I don't know what all the options are.

2. They're out there, but we can't see them.

Bostrom deals with this idea, but I want to take a slightly different angle. What if they aren't hiding or invisible, but just really, really small? Our own information technology has been heading in an exponentially smaller direction for some time now. And there are those who suggest that we are bound (sooner or later) to merge with that technology. Maybe by the time we get to the spacefaring stage, we won't be sending out Devil's-Tower-sized motherships, but very compact Von Neumann machines no bigger than a Snicker's bar. There could be millions of those orbiting our sun right now -- how would we ever know? The real population of the solar system might be in the trillions: a paltry six billion on earth and many, many more nano-colonists living lives we can't begin to imagine right here in our neighborhood.

3. Only would-be colonizers self-destruct.

There could be a correlation. Maybe civilizations that go in an information-technology-merger direction survive, but tend to stay more or less in one place, while those who would build the galactic empire inevitably destroy themselves. After all, what are the chances that, say, the Klingons ever would have made it out of hunter-gatherer stage, much less develop Warp Drive? Maybe colonizing the galaxy is a developmentally backward idea. (See possibility #1.) It would appeal to Klingons or to some humans at our current stage of development, but it's a dead end.

4. They stayed home.

Not because they have rejected technological development, but rather because the natural direction of development is in, not out. The miniaturization trend continues, and all the surviving advanced civilizations are now using femto-technology and basically working their way out of this universe altogether. As John Smart once explained it to me:

Fortunately, this perspective is quite falsifiable by future advances with SETI. If I'm right, in just a few more decades as the Moore's law-driven sensitivity of our sensor systems continues its exponential growth, we'll begin discovering "radio fossils" in the night sky, emissions of very weak electromagnetic signals (radio, TV, etc.) unintentionally emitted from the older intelligence-bearing planets whose past developmental record should already be detectable in our galaxy.

Once our antennas are powerful enough to detect unintentional EM emissions from the closest few million stars, something that Frank Drake tells me is almost possible now with the closest of our neighboring stars, we'll begin to discover these unmistakable signatures of nonrandom intelligence. We will also notice that every year, a small fraction (roughly 1/200th) of these radio fossils suddenly stop sending signals. Like us, these will be civilizations whose science invariably discovers that the developmental future of universal intelligence is not outer space, but inner space.

That's the destiny of species.

These possibilities are all variations on that same theme, I suppose. A couple things we ought to keep in mind about advanced aliens is that (if they exist) they are advanced and they are alien. Both of these facts put us at something of a disadvantage when it comes to making definitive statements about where they should be, things they might have done, and decisions they must have made.