Originally Posted by
przyk
What "traditional definition" are you working with? MWI is deterministic in the sense that if you know the initial quantum state of the entire universe (or an isolated subsystem), and you know everything there is to know about the interactions and evolution taking place, then the Schrödinger equation predicts a unique future state at any future time. But individual observers generally won't have access to complete information about that state for various reasons.
Huh? The branching that would occur according to an MWI account of the experiment you cited in the OP is probably quite a bit more complicated than you imagine and it doesn't happen all at once. Assuming "free will" (for simplicity, not by necessity), then the global quantum state generally splits into a number of branches corresponding to the number of measurement outcomes for every measurement performed. So if we just look at one iteration of the experiment, then by the time Alice and Bob have performed their measurements the global state has split into four branches (because Alice and Bob each have two outcomes), and when Victor performs his measurement (which ideally has four outcomes), the global quantum state further splits into a total of sixteen branches. Each time, exactly which branching happens depends on the measurements that Alice, Bob, and Victor choose to perform.
If you drop the "free will" assumption and you imagine modeling Alice, Bob, and Victor as physical systems, then there's additional branching depending on the number of decisions they could make. For example, as you say, in the experiment Victor was a quantum random number generator with two possible outcomes, and in a more complete MWI description, you'd consider that the measurements Victor performs are correlated with this outcome, which doubles the number of branches. In the experiment, Alice and Bob each chose between 3 different measurements they could perform, so if you also think of Alice and Bob as quantum random number generators, that's a total of 16 x 2 x 3 x 3 = 288 branches. That's 288 branches per iteration (generation of four photons) and assuming Alice, Bob, and Victor are no more complicated than two- or three-outcome quantum random number generators.
With all that said, if these responses don't make too much sense to you, I wouldn't worry about it too much. I've never really considered determinism a selling point of MWI anyway.
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