I'm puzzled by the controversy over this issue - although given that I'm 
not a physicist and my understanding comes from popular renditions of MWI 
by Deutsch and others, it may be me who's missing the point. But in my 
understanding of Deutsch's version of  MWI, the reason for Born 
probabilities lies in the fact that there is no such thing as a "single 
branch". Every branch of the multiverse contains an infinity of identical, 
fungible universes. When a quantum event occurs, that set of infinite 
universes divides proportionally according to Schroedinger's equation. The 
appearance of probability arises, as in Bruno's comp, from multiplication 
of the observer in those infinite branches. Why is this problematic?

On Saturday, October 5, 2013 2:27:18 AM UTC+10, yanniru wrote:
> Foad Dizadji-Bahmani, 2013. The probability problem in Everettian quantum 
> mechanics persists. British Jour. Philosophy of Science   IN PRESS.
> ABSTRACT. Everettian quantum mechanics (EQM) results in ‘multiple, 
> emergent, branching quasi-classical realities’ (Wallace [2012]). The 
> possible outcomes of measurement as per ‘orthodox’ quantum mechanics are, 
> in EQM, all instantiated. Given this metaphysics, Everettians face the 
> ‘probability problem’—how to make sense of probabilities, and recover the 
> Born Rule. To solve the probability problem, Wallace, following Deutsch 
> ([1999]), has derived a quantum representation theorem. I argue that 
> Wallace’s solution to the probability problem is unsuccessful, as follows. 
> First, I examine one of the axioms of rationality used to derive the 
> theorem, Branching Indifference (BI). I argue that Wallace is not 
> successful in showing that BI is rational. While I think it is correct to 
> put the burden of proof on Wallace to motivate BI as an axiom of 
> rationality, it does not follow from his failing to do so that BI is not 
> rational. Thus, second, I show that there is an alternative strategy for 
> setting one’s credences in the face of branching which is rational, and 
> which violates BI. This is Branch Counting (BC). Wallace is aware of BC, 
> and has proffered various arguments against it. However, third, I argue 
> that Wallace’s arguments against BC are unpersuasive. I conclude that the 
> probability problem in EQM persists.
> http://www.foaddb.com/FDBCV.pdf
> Publications (a Ph.D. in Philosophy, London School of Economics, May 2012)
>   ‘The Probability Problem in Everettian Quantum Mechanics Persists’, 
> British Journal for Philosophy of Science, forthcoming
>   ‘The Aharanov Approach to Equilibrium’, Philosophy of Science, 2011 
> 78(5): 976-988
>   ‘Who is Afraid of Nagelian Reduction?’, Erkenntnis, 2010 73: 393-412, 
> (with R. Frigg and S. Hartmann)
>   ‘Confirmation and Reduction: A Bayesian Account’, Synthese, 2011 179(2): 
> 321-338, (with R. Frigg and S. Hartmann)
> His paper may be an interesting read once it comes out. Also available in:
>   ‘Why I am not an Everettian’, in D. Dieks and V. Karakostas (eds): 
> Recent Progress in Philosophy of Science: Perspectives and Foundational 
> Problems, 2013, (The Third European Philosophy of Science Association 
> Proceedings), Dordrecht: Springer
> I think this list needs another discussion of the possible MWI probability 
> problem although it has been covered here and elsewhere by members of this 
> list. Previous discussions have not been personally convincing.
> Richard

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