There Have Probably Been Aliens
&quot;In other words, given what we now know about the number and orbital positions of the galaxy’s planets, the degree of pessimism required to doubt the existence, at some point in time, of an advanced extraterrestrial civilization borders on the irrational.&quot;<p>This is terribly silly. It reminds me of the Onion talks:<p>&quot;Step 1: Devise an idea to create a car that runs on compost. Step 2: Create the car. We’ve already completed Step 1. We’re half way there.&quot;<p>Increased certainty on three of the seven factors is irrelevant, because 99% of the uncertainty is concentrated in the other four. Specifically, the odds of abiogenesis are ~1&#x2F;2^n, where n is the complexity in bits of the smallest self-replicator. n is bounded below by ~100 or so (or it would have happened in a lab somewhere), and is bounded above by ~1 million (the smallest existing bacteria), so we know it&#x27;s somewhere in there. But where? We have only the vaguest guesses.<p>&quot;Specifically, unless the probability for evolving a civilization on a habitable-zone planet is less than one in 10 billion trillion, then we are not the first.&quot;<p>&quot;One in 10 billion trillion&quot; sounds like a very large number, and it&#x27;s put in italics for emphasis, but in information-theoretic terms it&#x27;s tiny. log2(10^22) is just 73 bits. If all that we know about a number is that it&#x27;s somewhere between a hundred and a million, saying that X would imply it&#x27;s higher than 73 (well technically 73 + log2(#incidences of molecular combination), so probably a few hundred) is not a disproof of X, nor really even much evidence against X.