From these two facts it follows that the evolutionary path to life-forms capable of space colonization leads through a "Great Filter," which can be thought of as a probability barrier. (I borrow this term from Robin Hanson, an economist at George Mason University.) The filter consists of one or more evolutionary transitions or steps that must be traversed at great odds in order for an Earth-like planet to produce a civilization capable of exploring distant solar systems. You start with billions and billions of potential germination points for life, and you end up with a sum total of zero extraterrestrial civilizations that we can observe. The Great Filter must therefore be sufficiently powerful--which is to say, passing the critical points must be sufficiently improbable--that even with many billions of rolls of the dice, one ends up with nothing: no aliens, no spacecraft, no signals. At least, none that we can detect in our neck of the woods.
The article prompted some thoughtful responses, first from Jamais Cascio who posits several scenarios in which intelligent life could be out there, but invisible to us; or in which they would not be motivated to colonize the entire universe in the way that humans were motivated to colonize the whole Earth.
Tim O'Reilly posed his idea about the Great Filter:
If indeed cheap oil is a prerequisite to the first flowering of technological civilization, might a Roman-Empire-style collapse due to some future disaster make it difficult to rebuild to spaceflight-capable levels due to lack of said resource the next time around? Many of the large scale energy technologies that we imagine replacing oil are energy intensive to build. They are, in a sense, themselves dependent on oil.
Good thinkers, both, and I agree with Tim that - for us, anyway - further development of alternative energy is dependent on oil. With regard to that point, it seems to me that a more basic question would be to wonder if our massive oil fields are unusual, and whether civilizations emerging on other planets would have to rely on other resources (wood, water, and wind) that simply aren't sufficient to propel a civilization into a Space Age?
As for Jamais' proposition that they may be out there, but we may not be able to hear them, I've wondered for a long time whether we'd even recognize an alien life-form. It seems like there are so many assumptions underlying our search for extra-terrestial life - which is not the same as a search for intelligence. What if we discovered a planet made of cognitive minerals? Would we even be able to figure that out? And even if we did, would they be able to recognize us?
