We Don't See Sentient Extraterrestrials Because They Don't Exist*

I have wanted to write this essay for a very long time now. In the last few years, things kind of came together in a way which made writing it possible. The essay is quite long, and will be difficult to follow for those unfamiliar with the territory. I tried to keep things as simple and straightforward as I possibly could, but there's only so much I could do and still get the key points across. That said, I didn't write this for you, the reader, although I tried to consider you at every turn. I wrote this essay because I wanted to write it, and I enjoyed doing it. The last section can be read separately — Dave

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*... or they're very rare 

Generally speaking, there are two answers to the question Is There Intelligent Life In The Universe?, where the term "intelligent life" means technologically advanced sentient beings broadly similar to humans. In the first essay I discussed optimistic answers to this question. Optimists imagine a Universe teeming with more advanced versions of ourselves, an answer which coincides (not coincidentally) with their vision of a bright human future.

This week we look at the views of the pessimists, who constitute a small minority of those concerned with astrobiological questions. Pessimists believe that Homo sapiens is alone and unique in the observable Universe, or believe that species broadly similar to Homo sapiens are very rare.

I am a pessimist, a position which follows from prolonged contemplation of the Fermi Paradox, which Paul Davies called "the eerie silence" (see the first essay).  Let me begin with an illuminating quote from Lee Billings, whose book Five Billion Years of Solitude was recently published by the Penguin Group (October, 2013).