'We know that the universe contains many stars. There are some 100 billion of them in our galaxy alone, and the observable universe contains billions of galaxies. Thanks to recent astronomical discoveries, we now know that it's common for these stars to have planets. Many of these solar systems are much older than our own. You start with billions and billions of potential germination points for life, and you end up with a sum total of zero alien civilizations that developed technologically to the point where we earthly observers can detect them.
'So what's stopping them? Perhaps the most compelling theory is that there is some kind of barrier - what the economist and polymath Robin Hanson called a "Great Filter".' (Boston Globe article).
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