# paulfchristiano comments on The Born Probabilities - Less Wrong

14 01 May 2008 05:50AM

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Comment author: 05 April 2011 12:51:41PM *  1 point [-]

Suppose that the probability of an observer-moment is determined by its complexity, instead of the probability of a universe being determined by its complexity and the probability of an observation within that universe being described by some different anthropic selection.

You can specify a particular human's brain by describing the universal wave function and then pointing to a brain within that wave function. Now the mere "physical existence" of the brain is not relevant to experience; it is necessary to describe precisely how to extract a description of their thoughts from the universal wave function. The significance of the observer moment depends on the complexity of this specification.

How might you specify a brain within the universal wavefunction? The details are slightly technical, but intuitively: describe the universe, specify a random seed to an algorithm which samples classical configurations with probability proportional to the amplitude squared, and then point to the brain within the resulting configuration.

Of course, you could also write down the algorithm which samples classical configurations with probability proportional to the amplitude, or the amplitude cubed, etc. and I would have to predict that all of the observer-moments generated in this way also exist. In the same sense, I would have to predict that all of the observer-moments generated by other laws of physics also exist, with probability decaying exponentially with the complexity of those laws (and notice that observer moments generated according to QM with non-Born probabilities are just as foreign as observer moments generated with wildly different physical theories).

Why do we expect the Born rules to hold when we perform an experiment today? The same reason we expect the same laws of physics that created our universe to continue to apply in our labs. More precisely:

In order to find the blob of amplitude which corresponds to Earth as we know it, you have to use the Born probabilities to sample. If you use some significantly different distribution then physics looks completely different. There are probably no stars behaving like we expect stars to behave, atoms don't behave reasonably, etc. So in order to pick out our Earth you need to use the Born probabilities.

You could describe a brain by saying "Use the Born probabilities to find human society, and then use this other sampling method to find a brain" or maybe "Use the Born probabilities everywhere except for this experimental outcome." But this is only true in the same sense that you could specify a configuration for the universe by saying "Use these laws of physics for a while, and then switch to these other laws." We don't expect it because non-uniformity significantly increases complexity.

As far as I can tell, the remaining mystery is the same as "why these laws of physics?" An observation like "If you use the probabilities cubed, you get one messed up universe." would be helpful to this question, as would an observation like "it turns out that there is a simple way to sample configurations with probability proportional to amplitude squared, but not amplitude," but neither observation is any more useful or necessary than "If you used classical probabilities instead of quantum probabilities, you wouldn't have life" or "it turns out that there is a very simple way to describe quantum mechanics, but not classical probabilities."

This question no longer seems mysterious to me; someone would have to give a convincing argument for me to keep thinking about it.

Comment author: 26 July 2011 02:32:35PM 0 points [-]

Does your argument work as a post hoc explanation of any regular system of physics and sampling laws, provided you're an observer that finds itself within it?