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Jonathan Colvin's avatar

Are we talking about the "this universe" vs. "some universe" objection to anthropic fine tuning? For example  http://arxiv.org/ftp/arxiv/...  

I've always found the "this universe" objection highly unconvincing, because it appears no different than the clearly mistaken "this planet" objection (the "this planet" objection says that an ensemble of planets can not explain why *this* planet is anthropic). For example: http://www.apologeticsinthe...

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Overcoming Bias Commenter's avatar

 Thanks Silent Cal, that actually changed my mind (I can be taught!).   I now think that the confusion revolves around the fact that Q (which I had been thinking of as an assertion about a universe) is, given Katja's problem statement, really an assertion about a universe-as-experienced-by-an-observer.

So when computing P(Q) via Bayes' formula we need to compute ratios of various kinds of experienced-universes, not ratios of various kinds of universes.  In other words, when counting universes we must weight them based on their respective number of observers.

If we assume that the number of intelligent observers in a universe is proportional to the number of planets w/ life, conditioning on "this planet has life" (or "planet 3 has life", or, what I believe to be equivalent, "a randomly-chosen planet has life") effectively weights each universe in just this way.

One can dream up variants of Katja's scenario for which conditioning on "there exists a planet with life" (which effectively weights each universe equally independent of its number of observers) is the right approach, but given Katja's scenario, together w/ the above assumption, I agree it makes sense to condition on a given planet having life.

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