Bostrom thinks that it will be bad news for us if we find life on Mars (or anywhere else in our neighborhood, for that matter). He starts off by saying that "the observable universe contains on the order of 100 billion galaxies, and there are on the order of 100 billion stars in our galaxy alone" and yet we haven't yet encountered intelligent extra-terrestrial life.
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.Bostrom's point is that if we find life on Mars, for example, it would suggest that the emergence of life is not that improbable and the Great Filter is most likely still to come. On the other hand:-
Now, just where might this Great Filter be located? There are two possibilities: It might be behind us, somewhere in our distant past. Or it might be ahead of us, somewhere in the decades, centuries, or millennia to come.
If the Great Filter is indeed behind us, meaning that the rise of intelligent life on any one planet is extremely improbable, then it follows that we are most likely the only technologically advanced civilization in our galaxy, or even in the entire observable universe.Maybe it's because I'm tired but I don't think things are as neat as Bostrom makes out. There are a whole lot of hidden assumptions in his thinking that make his article even more speculative than he imagines.
Two principle assumptions stand out for me: Bostrom assumes that intelligent life inexorably produces a technological society willing and able to colonize space and he assumes that interstellar travel is feasible. I don't have his blind faith in the latter and the former has scant evidence in its favor.
Having said that, it's a long and interesting article which I might return to once I've had some sleep and freshened up. In the meantime, if you liked Bostrom's piece, you might also be interested in Michio Kaku's The Physics of Extraterrestrial Civilizations.
