Tim Urban discusses the Fermi Paradox at Wait But Why:

Moving forward, we have no choice but to get completely speculative. Let’s imagine that after billions of years in existence, 1% of Earth-like planets develop life (if that’s true, every grain of sand would represent one planet with life on it). And imagine that on 1% of those planets, the life advances to an intelligent level like it did here on Earth. That would mean there were 10 quadrillion, or 10 million billion intelligent civilizations in the observable universe.
Moving back to just our galaxy, and doing the same math on the lowest estimate for stars in the Milky Way (100 billion), we’d estimate that there are 1 billion Earth-like planets and 100,000 intelligent civilizations in our galaxy.

With the staggeringly high probability of many other life forms out there, why haven’t we come across evidence of them?

I think I’m an “Explanation 1: We’re Rare” subscriber, but it’s tons of fun — and potentially terrifying — to hypothesize about the other options.

(Via Chase McCoy)