"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." This is terribly silly. It reminds me of the Onion talks: "Step 1: Devise an idea to create a car that runs on compost. Step 2: Create the car. We’ve already compl…
> Specifically, the odds of abiogenesis are ~1/2^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's somewhere in there. If the odds of abiogenesis are 1/2^128, and if that is the source of life, then we're the only life in the Universe…
Re: There Have Probably Been Aliens
#81OP's argument here is slightly wrong I think, as it does not take into account the amount of time: Abiogenesis probably hasn't happened in a lab yet because the corresponding experiments have been running for a couple of years or decades at most, whereas nature's "experiments" have been continuously running at planetary scale for billions of years. If we count the number of attempts to invent life in the sense of putting together some anorganic and organic chemicals, supplying some energy and waiting for e.g. one year, then Nature has probably run 10^18 times more experiments than we have (10^9 years of experiments, each one running on a 1 m² surface [not taking into account depth]). And, fossil evidence points to life having started 4.1 billion years ago, or roughly 500 million years after the formation of the planet, which is a rather short amount of time (it's very likely that first lifeforms existed even earlier than that). So my naive guess would be that n < 100, though without further evidence (e.g. finding life on other planets) this is hardly more than a belief.