[alok-g]

>> Many Worlds - Quantum phenomena cause the world the branch into multiple worlds. The probabilities of quantum mechanics represent the 'share' of reality that branches in each direction.

I seem to have access to only one of these universes. How does nature decide which determinate state would my universe take? It still requires the magical collapse. (Or is it that I am branching off too and loosing access to myself in other branches?)

>> So long as you're willing to abandon locality, hidden variables can work.

(Asking, not questioning) Why is this so? I have read a bit or two about Bell's Inequalities, but do not understand why locality needs to be abandoned. With particles having random but entangled spins showing correlations even when separated out, how do we know there were no random hidden state associated with these particles when they separated. That is, how do we know that the decision on which particle has which spin did not happen during separation itself (getting stored as a hidden variable)?

[lmm]

> How does nature decide which determinate state would my universe take? It still requires the magical collapse. (Or is it that I am branching off too and loosing access to myself in other branches?)

You're branching too. It remains to be explained why your subjective probabilities of which branch you find yourself in follow the Born probabilities, but that's the only known way of assigning subjective probabilities that behave as probabilities should (in terms of things like "probability of A or B <= probability of A + probability of B").

> That is, how do we know that the decision on which particle has which spin did not happen during separation itself (getting stored as a hidden variable)?

Conway's "Free Will Theorem" offers a simplified proof that this can't happen: if you measure the particle along three orthogonal axes then you will always get two of one result and one of another, but there's no way to preassign values to all possible axes such that this remains true. So the result you get from measuring in a given direction cannot be fixed if you have a free choice about which set of directions you're measuring in.

[akvadrako]

> Or is it that I am branching off too and loosing access to myself in other branches?

Yes, you also split into branches. Of course the whole branches thing is a rough approximation - for example quantum computers work because they aren't branching.

> Why is this so? I have read a bit or two about Bell's Inequalities, but do not understand why locality needs to be abandoned.

I would recommend looking up the explanation from Dr. Chinese. Briefly it's because you measure each half of an entangled pair randomly on 1 of 3 angles, and you see that measurements of equal angles always disagree, yet measurements on different angles agree over 50% of the time. That's impossible to do with pre-determined values.

[ridewinter]

> quantum computers work because they aren't branching.

Or rather, the branch universes diverge and do their computing in parallel, and then finish by matching perfectly, with all universes having the same answer. Any interference from the environment outside of the qubits results in the universes not being perfectly similar at the end and you don’t get an answer back.

[ridewinter]

Question: where does the hidden variable/pilot wave interpretation say that the computation in a quantum computer takes place? Many-worlds says that it takes place in parallel universes.

[n4r9]

> how do we know that the decision on which particle has which spin did not happen during separation itself

It's difficult to explain concisely, but John Stewart Bell wrote a nice albeit lengthy essay on this exact question:

https://cds.cern.ch/record/142461/files/198009299.pdf