On Mon, Mar 7, 2022 at 7:41 PM smitra <smi...@zonnet.nl> wrote:

> On 04-03-2022 00:17, Bruce Kellett wrote:
> >
> > But the effect of that initial entanglement is only non-locally
> > available at the point of Alice's and Bob's separate measurements. So
> > your account has not eliminated the non-locality -- you have just
> > disguised it by calling it a "common cause" effect. That common cause
> > is only non-locally available to Alice and Bob.
> >
> > Bruce
> Yes, but this non-locality is a trivial issue in the MWI, analogous to
> common cause effects in classical physics,

If by "common cause" you mean what happens at the formation of the
entangled pair, then your claim is manifestly false. Classical common cause
correlations must always satisfy the Bell inequalities while we know
experimentally that these inequalities are violated.

while in case of a real
> collapse the nonlocal effect would be present in the dynamics of Nature,
> die to collapse itself being a fundamental part of the dynamics.

I don't know why you are obsessed with collapse theories. My question does
not concern such theories. I am asking for an account of the claimed
locality of MWI -- completely independent of any collapse.

For example, in the MWI picture from Bob's point of view, when he
> measures his spin, he knows that the spin state of Alice in his sector
> is the opposite.

That is the question. How does he know this? He is at a spacelike
separation and cannot, locally, know either Alice's polarizer setting, or
her result.. He cannot know that his spin state is opposite, because he
cannot know 'opposite to what?'

So, if he knows what Alice's polarizer setting is,

He cannot know this, so the rest of your statement is otiose.

> knows the superposition in which Alice and her spin will end up in (of
> course, not Alice's exact state, only as so far the outcome of the spin
> measurement is concerned in the formal form of a |up, Alice finds up> +
> b |down, Alice finds down>).
> It's the fact that Bob knows that he has a copy who found the opposite
> spin and in that sector the state of Alice is different that makes this
> not a dynamical non-local effect.

That knowledge is surely relevant, since Bob does not know either Alice's
setting, or her result. Such a logical possibility has no bearing on
locality or non-locality.

But if collapse is real and the other
> Bob does not exist, then there is a real fundamental problem with this
> non-locality.

Again, your irrelevant obsession with collapse theories.

Note that we don't need to get into the Bell-type
> correlations here, these are only relevant to prove that the random
> results after a measurement cannot be due local hidden variables. Given
> that this is an established experimental fact we can just assume this it
> be true.

In other words, are you simply giving up on the possibility of an
explanation of these correlations?

So, Bob collapsing not just his own spin but also Alice's spin
> is a problem if the collapse is real. But in the MWI there is no
> collapse, all the other sectors objectively exist, it's just that his
> and Alice's sector are correlated with all non-local effects having
> arisen via local dynamics.

You have not demonstrated this -- you have merely assumed it. Zero out of
ten for your explanatory effort. I am still waiting for someone who
actually understands the problem to explain how MWI can give a local
account of these correlations.


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