We should be able to estimate the number of detectable extraterrestrial civilizations in the Milky Way galaxy according to the Drake equation:
By plugging in realistic values we obtain the result that there should be other civilizations in our galaxy, and this is what fundamentally motivates the search for extraterrestrial intelligence effort.
Now, the fact is that we haven't detected any, leading to Fermi's paradox: where is everybody?
Carl Sagan proposed, in the 70's, that this could only explained by the fact that civilizations almost invariably self-destruct once they have thermonuclear arsenals.
I am skeptical of the following claim: That Fermi's paradox implies civilizations will self destruct inevitably in a short time. I would like to see scholarly articles supporting Sagan's claims or debunking it.
In Sagan's own words: http://www.youtube.com/watch?v=0Ztl8CG3Sys
