# Non-locality in MWI

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DrChinese
Gold Member
Ah, got it.
My fault, reading my post it was not clear. I want a/b/c/d to be a constant that can be determined at the time there is a split.

Now the terms are dependent on something which is non-local, which is what we sought to avoid. Sorry I don't think I was clear on this point.
Looking at it another way: Bob's setting might not have even been determined at the time, so no way the values could be determined.
On that basis you could say the distance between Alice and Bob is non-local. There's nothing wrong with having non-local functions, the only problem with non-locality is when cause and effect are non-local.

PeterDonis
Mentor
2019 Award
On that basis you could say the distance between Alice and Bob is non-local.
Yes, it is.

Ah, got it.
My fault, reading my post it was not clear. I want a/b/c/d to be a constant that can be determined at the time there is a split.
As I recall I explained that worlds only split in the future decoherence cone of a measurement. And since then I've said that deciding exactly when the split happens is a bit arbitrary because it's just a picture of the end result after decoherence. I'll make your point even stronger for you. a/b/c/d can be known precisely as soon as you have the angles. Knowing the coefficients makes no difference whatsoever to the state, it just tells you how to calculate the outcomes.
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Yes, it is.
Thanks, I might never have been sure :)

Ah, got it.
Well come on, share it with the rest of us, then.

PeterDonis
Mentor
2019 Award
Well come on, share it with the rest of us.
I just meant I understood what @DrChinese was saying, and I don't have anything useful to add to it.

But according to MWI, there is no 3-dimensional world. According to MWI, the only object that exists is the wave function, which does not live in a 3-dimenional world. ... According to MWI, the 3-dimensional world is an illusion
On thinking about it a bit more, I don't think that's entirely true. Certainly it is often said that in MWI, the only reality is the wavefunction. Which of course can be expressed in multidimensional phase space. But it can also be expressed in three dimensions as a sum of products. So you can interpret it as a multidimensional object with a curious penchant for three dimensional observables or a three dimensional object which can exist in superposition. I don't think MWI is dogmatic about which. In fact there is no reason why the wavefunction itself need be ontic at all, it could be an abstraction that maps onto something else via an unknown transformation. MWI will work just as well with any of them.

One thing to add to this discussion, which think it quite important, is that I think a recent paper from Sean Carroll does a better job than Deutsch at giving an easy to understand and less rigorous explanation of why WMI can be considered local:

https://arxiv.org/pdf/1801.08132.pdf

Vaidman also has some to-the-point insights on the topic.

In fact there is no reason why the wavefunction itself need be ontic at all, it could be an abstraction that maps onto something else via an unknown transformation.
The reason the wavefunction must be ontic (which that I mean maps 1-to-1 onto reality) is given by the PBR and Colbeck and Renner theorems.

The reason the wavefunction must be ontic (which that I mean maps 1-to-1 onto reality) is given by the PBR and Colbeck and Renner theorems.
Indeed so but that's a weaker sense of ontic than I was using. I was just meaning that there is no reason why reality must actually be the wavefunction. A 1-1 map would be fine.