I recall reading in Technology Review an interesting article about the possibility of a galactic "filter" for intelligent life.
The gist of it is that since most of the universe is older than us, one would expect that if intelligent space faring life existed elsewhere it would have come into contact with us. Life on this plant typically expands very rapidly into every available niche. The supposition is that life advanced enough to colonize other worlds wouldn't stop with just their own solar system, and would rapidly fill the universe.
The alternatives are that it has chosen not to, is unrecognizable to us, or that there is some sort of "filter" that at a certain stage wipes out all technologically advanced societies.
I.e. they discover something they try to use that causes the destruction of the entire species.
The thrust of the article was that we should really really be hoping that life has never existed elsewhere. Because that would mean we are the most advanced life in the universe, and the filter if it exists is most likely behind us.
If advanced life does exist, and it wiped itself out, it would mean that the filter is probably ahead of us. And since we are now a space faring species it is probably rapidly approaching ("rapidly" in geologic terms anyway).
I'm not doing the article justice here, but it was a very interesting piece of food for thought.
EDIT - here is the link. Registration is free.
Where Are They? Why I hope the search for extraterrestrial life finds nothing- Nick Bostrom
Quote:
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.
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. Let us ponder these possibilities in turn.
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