The Milky Way - a relatively small corner of an unfathomably large universe - is teeming with billions and billions of solar systems. Inside those solar systems, different chemicals have been given eons to form into basic organisms which have had plenty of time to evolve into extra-terrestrial intelligences. Enter the Fermi paradox, coined by Enrico Fermi in 1950: if all that's true, where the hell is everybody? We haven't seen them, we haven't heard from them, and we certainly haven't talked to any of them. There are all kinds of other problems too. We should expect that somewhere in the infinite expanse there is at least one xenophobic advanced alien civilization. By now they should have sent out so-called beserker probes - simple self-replicating probes with a particularly nasty payload - to find and destroy rival civilizations. And yet here we are, blissfully not destroyed.

Three canonical solutions:

(1) Self-annihilation - there is a built-in threshold past which a civilization cannot evolve without something happening that causes it to destroy itself. In Frankfurt terms, it's an overdetermination problem - there are so many things that could go wrong (environmental catastrophe, apocalyptic war, etc) that one of them does. The Fermi paradox is so powerful that, rather than presenting a problem for the way we think about how evolution may work, it's taken as evidence for the very real probability of overdetermined self-annihilation. But this solution assumes that every civilization destroys itself - that in the infinity of the cosmos, there's not a single one that's avoided self-annihilation. Really? Not one?

(2) Digital revolution - Imagine what was happening if ETIs were looking for us on the radio spectrum. We've been broadcasting powerful radio signals for a century or so. In another century, we'll have certainly moved on to something else. Those 200 years are a blink of an eye over the course of human evolution. Maybe we've been looking in the wrong place. The problem with this solution is that it still doesn't explain why we haven't been contacted deliberately by an ETI - why no UFOs and why no beserker probes?

(3) Virtual worlds - Maybe alien civilizations don't contact us or bother us because they've got something more interesting to do than deal with reality. Given the relative paucity of physical experience compared to what's possible in virtual worlds - maybe there's a threshold that civilizations cross where they're able to create such fantastic worlds for themselves that they withdraw from physical reality. After the jump, a video that will creep you the hell out.