Thursday, September 18, 2008

Origin of life: The "billion billion" planets solution?

In Answering the New Atheism: Dismantling Dawkins’ Case Against God, Scott Hahn and Benjamin Wiker comment on Richard Dawkins’s approach to the question of whether life could have originated by chance. Accusing him of "selective confusion of the possible and the impossible when it suits his purposes" amounting to "intellectual slight-of-hand", they say ,

According to Dawkins, we can safely estimate that there are, somewhere in the vast universe, a "billion billion" planets that would be suitable for life. He then supposes what he takes to be long odds of one in a billion of life arising by chance (although he really doesn't mean life, but merely "the spontaneous arising of something equivalent to DNA.") Well then, concludes Dawkins, "even with such absurdly long odds, life will still have arisen on a billion planets - of which Earth, of course, is one." This is such a surprising conclusion, Dawkins remarks, "I'll say it again. If the odds of life originating spontaneously on a planet were a billion to one against, nevertheless that stupefyingly improbable event would still happen on a billion planets."
They reply,

It is such a surprising conclusion precisely because it doesn't follow. The entire argument is faulty. ... he assumes without argument that the spontaneous assembly of DNA is like getting a perfectdeal in bridge rather than being like tossing a perfect cardhouse in a hurricane. That is what he would have to prove rather than assume.

The real question, the prior question, is one of possibility and impossibility, not greater or lesser probability. If tossing a perfect cardhouse in a hurricane is impossible cause the cards would keep blowing away, then it wouldn't become possible by adding into the calculation a billion billion available planets, or even a trillion trillion.
In fact, the "one in a billion" odds Dawkins gives are ridiculously low and there is no particularly good reason to believe that a billion planets are well suited to life.

What are one in a billion odds? Well,

- according to the British Civil Aviation Authority, the post-2001 Concorde has a one in a billion chance of crashing.

- "A blonde hair found in the van of the man accused of killing schoolgirl Sarah Payne had a billion-to-one chance of not being hers, a jury has been told."

- There was said to be a one in a billion chance that brothers Thomas and Julius Jones "from a town of fewer than 6,000 residents both start in the N.F.L. at running back and play against each other on Thanksgiving." (This one was worked out by an expert, whose assistance was requested by Greg Bishop.)

But all this stuff happens in a universe where the vast sea of metabolisms that we call life already exists in great numbers. We can't assign anything like these relatively high probabilities to a purely accidental origin of life. The odds against that are comparatively astronomical, whatever Richard Dawkins may hope.

By the way, I highly recommend Hahn and Wiker's book.