you meant what's the effect on entangled particles at different locations?
No, I just meant that your Hilbert space is associated with a preferred foliation. The states in the Hilbert space are superpositions of configurations on the slices of that foliation. If you follow Copenhagen, observables are real, wavefunctions are not, and this foliation-dependence of the wavefunctions doesn't matter. It's like fixing a gauge, doing your calculation, and then getting gauge-invariant results back for the observables. These results - expectation values, correlation functions... - don't require any preferred foliation for their definition. The wavefunctions do, but they are just regarded as constructs.
So Copenhagen gets to be consistent with special relativity at the price of being incomplete. Now according to Many Worlds, we can obtain a complete description of physical reality by saying that wavefunctions are real. What I am pointing out is that wavefunctions are defined with respect to a reference frame. Time is not an operator and you need surfaces of simultaneity for Schrodinger evolution. The surface of simultaneity that it lives on is one of the necessary ingredients for defining a wavefunction. If the wavefunction is real, then so is the surface of simultaneity, but the whole point of special relativity is that there is no absolute simultaneity. So how do you, a wavefunction realist, get around this?
If you want an explanation of how you get a probabilistic state from an entangled state ("how the theory predicts what we observe"), check out partial traces.
So, let's return to your example. The wavefunction of the universe is "|U> x ( 3/5 |1> + 4/5 |0> )". Well, this isn't a great example because the wavefunction factorizes. But anyway, let's suppose that the reduced density matrix of your two-state system is c_00 |0><1|. You still need to explain how the Born rule makes sense in terms of a multiverse.
Perhaps an analogy will make this clearer. Suppose I'm a car dealer, and you place an order with me for 9 BMWs and 16 Rolls-Royces. Then you come to collect your order, and what you find is one BMW with a "3" painted on it, and one Rolls-Royce with a "4" painted on it. You complain that I haven't filled the order, and I say, just square the number painted on each car, and you'll get what you want. So far as I can see, that's how MWI works. You work with the same wavefunctions that Copenhagen uses, but you want to do without the Born rule. So instead, you pull out a reduced density matrix, point at the coefficients, and say "you can get your probabilities from those".
That's not good enough. If quantum mechanics is to be explained by Many Worlds, I need to get the Born rule frequencies of events from the frequencies with which those events occur in the multiverse. Otherwise I'm just painting a number on a state vector and saying "square it". If you don't have some way to decompose that density matrix into parts, so that I actually have 9 instances of |1> and 16 instances of |0>, or some other way to obtain Born frequencies by counting branches, then how can you say that Many Worlds makes the right predictions?
Once you get into field theory you have x, y, z and t all treated as coordinates, not operators. The universe realio trulio starts to look like a 4-dimensional object, and reference frames are just slices of this 4-dimensional object. And I guess you're right, if you don't use relativstic quantum mechanics, you won't have all the nice relativstic properties.
If you want your probabilities to be frequencies, I suppose you could work out the results if you wanted. The run-of-identical-experiment frequencies should actually be pretty easy to calculate, and ...
These are extracts from some Facebook comments I made recently. I don't think they're actually understandable as is—they're definitely not formal and there isn't an actual underlying formalism I'm referring to, just commonly held intuitions. Or at least intuitions commonly held by me. Ahem. But anyway I figure it's worth a shot.
A proposal to
rationalizederive magick and miracles from updateless-like decision theoretic assumptions:(On Google+ I list my occupation as "Theoretical Thaumaturgist". ;P )