Well, I'd recommend to read the whole series. It's not so bad as it sounds. There are so many steps from where you are to appreciating the utter weirdness of Bell's experimental result. Not the weirdness of any theory (or an interpretation, which Many Worlds actually is) but of the basic experimental result.

If you are properly amazed by it, rejecting MWI or any crazy-ish borderline-conspiracy theory seems suddenly a lot harder.

I feel the whole Yudkowsky's QM series in fact served to deliver that one post.

Why isn't the MWI another form of hidden variables (a supremely non-parsimonious one at that), where the hidden variable is which of the many worlds you happen to inhabit?

I think you can make an argument for viewing it that way, depending on exactly what you mean by "you".

But IIUC, one of the remarkable things about MWI is that it would be a local hidden variable theory!

This is a very important property to have because the principle of locality is deeply ingrained in the way the Universe behaves. Note that (almost?) no other quantum interpretation is both realist and local at the same time.

Maybe you wonder, how is it possible that MWI can be considered a local hidden variable theory if Bell's theorem precisely shows that local hidden variable theories are not possible?

I think that it was Bell himself who said that the theorem is only valid if you assume that there is only one outcome every time you run the experiment, which is not the case in MWI.

This means that MWI is one of the few (the only?) interpretation we have that can explain how we observe Bell's theorem while still being a local, deterministic, realist, hidden variable theory.

For it to be local (causality does not propagate faster than light), it must be superdeterministic (all the many worlds that ever will be, already are). For it not to be superdeterministic (many worlds decohere at the moment of experimentation), it is also not local (the decoherence happens faster than the speed of light, across the universe).

I'm sorry but I don't follow.

If you take the Bell test experiment where Alice and Bob perform their measurements at approximately the same time but very far apart, I think you and I both agree that when Alice does a measurement and observes an outcome, she will have locally decohered from the world where she observes the other outcome.

But I don't see why the decoherence necessarily has to happen faster than the speed of the light.

It makes sense that even if Alice decoheres from the world where she observes the other outcome, the outcomes of Bob's measurement are still in a superposition with respect to each Alice (and vice-versa).

And that only when Alices' and Bobs' light cones intersect each other will the Alices decohere from the Bobs in such a way that the resulting worlds will observe the expected correlations (due to how they were entangled or maybe even due to the worlds interfering with each other when their light cones intersect, like what happens in general with the wave function).

I admit I'm not an expert in this area, but is this not possible?

An awesome question. That is exactly what I have been wondering without being able to put it into words, and this is core of why the MW seems completely uselsess to me as a scientific theory. (As a philosophical one-maybe? But science?)

To be clear, I don't reject Many Worlds at all and in fact consider it a promising candidate due to it sort of "falling out" of the Schrodinger's equations taken literally unless you add complexity.

But the fact remains that it is impossible to prove and it is conveniently well equipped to handle this situation. I'd prefer an argument that presupposes the Copenhagen interpretation as that is when my intuition fails.

>But the fact remains that it is impossible to prove and it is conveniently well equipped to handle this situation. I'd prefer an argument that presupposes the Copenhagen interpretation as that is when my intuition fails.

Is that not like trying to get a better intuition for planetary movement by using an epicycle-based model? The fact that the interpretation is conveniently shaped in a way that a paradox isn't an issue is not a coincidental thing that should be overlooked in the spirit of fairness to alternative interpretations. Regardless, I think my post below is useful for answering your want.

>So now we're back to the essence of the original question - if these things seem random why do we know that they're in fact deterministic without any hidden variables?

The world is only deterministic under Many-Worlds, and it's deterministic in the sense of "each outcome happens (mostly) separately". It doesn't make any sense to try to make sense of the "deterministic" part separately from MWI. MWI is the only deterministic QM theory (unless you're going to consider "superdeterminism", but there's nothing concrete to that interpretation besides "what if there existed a way that we had QM+determinism but not MWI". There's no basis to it, besides a yearning from people that like the abstract idea of determinism and don't like the abstract idea of MWI).

EPR doesn't tell us that the world is deterministic. It tells us that local hidden variable interpretations (where experiments have a single outcome) of QM can't work, because it shows that a measurement on a particle can appear to you to affect the measurement made by someone else on a distant particle. The Copenhagen interpretation response to this is that the wave function collapse must be faster than light. Therefore, the Copenhagen interpretation is not a "local" theory. (The Copenhagen interpretation doesn't give us any answer for who we should expect to trigger this wave function collapse first when two measurements are taken simultaneously at a distance though.)

If experimenters disprove Many Worlds, they've also disproved Copenhagen. These are exactly the same equations after all.

Theoreticians choose very different mindsets about the same equations, which (they say) somehow create them grounds to form various new hypotheses. As far as I know neither approach was very fruitful so far in terms of new science, so people try multitude of others.

What I've meant to say above, I have much trouble using Copenhagen to understand Bell's experiment. MWI fits the bill here for me.