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It is kinda controversial. At least in relation to MWI; the validity and meaning of that assumption forms the basis of much of the criticism of MWI. If there were a rigorous derivation of the Born rule as a logical consequence of a global state undergoing unitary evolution, then I and many others would be much more convinced.

The other part of the problem is how/why observational outcomes occur in a probabilistic fashion in the first place. You keep saying that one or other branch "occurs" or "happens", but in MWI they're all happening. For some reason the observer only experiences one particular strand of their own superposition, and with a somewhat arbitrary probability to boot. It's not like that's the strand they're "actually in" but ignorant of. This is very different from subjective knowledge of a deterministic universe.




What's not controversial is that empirically the equal branches are equally likely. If that can be mathematically derived it probably requires some axioms of probability. But still, it's not a valid objection because the situation is much better than any other theory of probability, like frequentism!

The second problem is easy to see with the classical cloning analogy. Say, someone creates two clones of you and kills the original. Your experience will split, one version for each clone. I think it's clear how that would work classically and how it's analogous to the WMI with equal branch weights.


> If that can be mathematically derived it probably requires some axioms of probability.

I have no problem with axioms of probability being used. I just think you need to be explicit about what the fundamental postulates of the theory are and what is being derived. Clearly, in order to make predictions in line with experiment, a physical meaning must be assigned to the norm-squared of the wave-function. Most modern accounts don't make this a fundamental postulate, so it needs to be derived in a coherent manner.

> it's not a valid objection because the situation is much better than any other theory of probability

Beside the point. If our best theory of nature is flawed then we need to be honest about it.

> like frequentism!

OK- what about Quantum Bayesianism? That's a coherent and consistent account of quantum probabilities. It just lacks in what one can really say about the underlying reality.

> it's clear how that would work classically and how it's analogous to the WMI with equal branch weights.

I think its a false analogy. There aren't actually two copies of you in MWI, just a superposition of two different states. I need an explicit process by which classical probabilities emerge, not an intuitive allusion to how it's kind of like some classical process. A superposition is not classical; that's the whole issue!


We are not being dishonest here. Even if Born's rule was postulated, WMI would still have the most technical merit. Physics can never have a proof anyway.

And the Born rule is just assigning probability to the norm-squared of the wave function, so I'm not sure why you think it's assumed in a derivation of the Born rule itself. That would make the proof a tautology. The assumptions are laid out explicitly for the various proofs throughout the series of papers and critiques.

QBism is all about belief of agents and if you think that's a valid approach than the decision theoretic proof from Deutsch and Wallace shouldn't be hard to accept. Actually a derivation of the Born rule in QBism must take the same form.

A superposition of two different states is two copies after decoherence. They occupy different parts of the wave function and they share nothing, so can't interact. In configuration space (not classical space) they are separated "wave packets".




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