The Fermi Paradox Revisited

Via DeLong, Charlie Stross over at his blog:

The Fermi Paradox probably doesn't need much introduction; first proposed by Enrico Fermi, it's one of the big puzzlers in astrobiology. We exist, therefore intelligent life in this universe is possible. The universe is big; even if life is rare, it's very unlikely that we're alone out here. So where is everybody? Why can't we hear their radio transmissions or see gross physical evidence of all the galactic empires out there?

If you aren't familiar with the Fermi Paradox, click that Wikipedia link above. Truly, it's a fascinating philosophical conundrum — and an important one: because it raises questions such as "how common are technological civilizations" and "how long do they survive", and that latter one strikes too close to home for comfort. (Hint: we live in a technological civilization, so its life expectancy is a matter that should be of pressing personal interest to us.)

Anyway, here are a couple of interesting papers on the subject, to whet your appetite for the 21st century rationalist version of those old-time mediaeval arguments about angels, pin-heads, and the fire limit for the dance hall built thereon:

First off the block is Nick Bostrom, with a paper in MIT Technology Review titled Where are they? in which he expounds Robin Henson's idea of the Great Filter: 

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.