The idea arises from the Fermi Paradox which notes the absence of any contact or visible evidence of extra-terrestrial life despite predictions made by the Drake Equation that there should be at least ten thousand detectable civilizations in our Milky Way galaxy.
Our Sun is a young star; there are stars in our galaxy, with habitable planets, that have existed for billions of years longer. If "intelligent life" were probable you might expect some of these systems to support civilizations that are millions of years more advanced than we are, and it is certainly expected that they would have, by now, colonized the entire universe. Yet here we are in the Great Silence.
Enter the Great Filter: before a colonization explosion is possible life must evolve through several steps. It seems that one of these steps is so improbable that life rarely, if ever, makes it through the whole series.
- The right star system (including organics and habitable planets)
- Reproductive molecules (e.g. RNA)
- Simple (prokaryotic) single-cell life
- Complex (archaeatic & eukaryotic) single-cell life
- Sexual reproduction
- Multi-cell life
- Tool-using animals with big brains
- Where we are now
- Colonization explosion
The filter may applied at any of the earlier steps, or it may be somewhere between steps 8 and 9. Earth seems to have life that is capable of developing the technology to one day colonize our own galaxy and even beyond, but in the thousands or hundreds of thousands of years it would take to spread throughout the universe there may be a point beyond which we can go no further; maybe some physical restriction, or maybe we would just destroy ourselves. The optimistic view - depending on your politics - would be that we have already gotten through the improbable step.
Watch this video to see some guy with a goatee talk about the great filter:
Ah yes, the Fermi Paradox. Stephen Baxter has had some interesting takes on this in his 'Manifold' novels about twelve years ago. Long story short, there are any number of filters that might come into play. One is indeed the possibility that civilisations like ours self-destruct (or, perhaps less dramatically, peak and decline, the cheap energy that fueled their rise gone for good). It isn't inconceivable that our galaxy (which is after all, NOT the universe) is full of somewhat intelligent species that lack the means and/or the will to move offworld (and if they're operating at a pre-radio (or post-radio) level, we won't be hearing them.
ReplyDeleteAnother 'Filter' of course could be something like a... forgive me if my jargon is rubbish... gamma ray burst, some kind of supernova that sterilises vast regions of any kind of life.
Manifold, eh? Sounds interesting.
ReplyDeleteI guess it might be more than optimistic to think that a civilization could seed the entire universe, but given a million years and the absence of any filter event, I think it makes sense to expect that one would colonize the entire galaxy, more or less. Of course, our solar system is out in the sticks.
It makes good sense, it does. Assuming a civilisation bent on expansion, and assuming it gets through its version of our stuck-on-earth/sustainability/energy bottleneck, then a Filter of some kind is pretty plausible as to why we don't see them.
ReplyDeleteOf course there have been so many workarounds to this stuff. Maybe we don't see them because we don't know what we're looking for. Maybe they stopped already and moved on because we are of no interest to them.