Today's post, If Many-Worlds Had Come First was originally published on 10 May 2008. A summary (taken from the LW wiki):
If early physicists had never made the mistake, and thought immediately to apply the quantum laws at all levels to produce macroscopic decoherence, then "collapse postulates" would today seem like a completely crackpot theory. In addition to their other problems, like FTL, the collapse postulate would be the only physical law that was informally specified - often in dualistic (mentalistic) terms - because it was the only fundamental law adopted without precise evidence to nail it down. Here, we get a glimpse at that alternate Earth.
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Let's contrast the three options of: phenomenological QM (Copenhagen interpretation); an objective collapse theory; and many worlds.
Phenomenological QM assumes a Born rule, and makes no sense as a final objective theory (unless you're a solipsist). An objective collapse theory has an objective world-picture including a probabilistic axiom (the physical law of collapse) and you can logically prove that this implies the validity of a phenomenological Born rule for observers. Many worlds has an objective world-picture not including a probabilistic axiom, and cannot logically prove that this world-picture implies a phenomenological Born rule for observers.
What most many-worlds believers are doing, is assuming the truth of the many-worlds picture of reality, and also assuming the truth of the Born rule just as if they were doing ordinary subjectivist QM. But their picture of reality all by itself ought to determine what observers typically see; having opted for a specific objective theory - and one which supposedly has no probabilistic axioms - there shouldn't be room, logically speaking, to "assume" anything further about the truth of the Born rule. Either the phenomenological Born rule is a true implication of their theory, or their theory implies that observers see something different to reality.
This seems to be a subtle point. One analogy would be if there was a quantum "interpretation" which consisted of saying "I believe that reality is described by this enormous unsolvable differential equation, and I also believe that if you could solve it, it would look like reality as described by the Born rule." That's not much of a theory, because having decided that reality is described by the equation, the equation should imply how reality looks, not your further assumption.
Many worlds is a little like this, except that their equation is already part of QM; it's the part of QM they find acceptable, the Schrodinger equation. But the same critique still applies. If "wavefunction evolving according to the Schrodinger equation" is everything, then the Born rule must be an implication of this equation. And they cannot exhibit any such implication. All attempts to do so either involve a long chain of reasoning in which the Born rule is implicitly reintroduced at some point (this is what Deutsch does) or involve focusing on specific substructures in the wavefunction and saying "those are the worlds" (this is what Hanson does). The latter step is surely one that has to be performed anyway, just to make sense of the wavefunction - from the perspective of Hilbert space, even just by talking about configurations, you're preferring a particular basis - but a lot of many-worlds believers don't want to take this step. As a rule, how it works is that non-physicist MWI believers just assume that configurations are the preferred basis, because spatial configurations are the everyday reality they're already familiar with, and physicist MWI believers import whatever they need from Copenhagen QM in order to avoid having an ontologically preferred basis, even though a self-sufficient objective theory should have no logical room for such "imported" extra components.
Sorry for the complexity of this answer, but the facts about how people are thinking, and the logical relations, dissimilarities, etc between the various approaches, and the normative evaluation of the relative merits of the different approaches, are just not simple. Strictly phenomenological QM has a certain simplicity. A final objective theory which really does the job of explaining QM will have some form of simplicity. But the path from the first to the second - which is taking decades - passes through a great deal of confusion, not just because there are wildly dissimilar theoretical options to be considered, but because people lose sight of what "an objective theory that fully explains QM" actually requires, because there are preachers saying "turn back! Copenhagen positivism was already the final answer, this other way leads only into the desert of philosophy", and for many other meta reasons.
How, do you think, does EY think about it? I could not find it stated clearly anywhere in the sequence, beyond "MWI is decoherence, decoherence is MWI", which is not overly helpful.