On 07-03-2022 23:01, Bruce Kellett wrote:
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.
your account has not eliminated the non-locality -- you have just
disguised it by calling it a "common cause" effect. That common
is only non-locally available to Alice and Bob.


Yes, but this non-locality is a trivial issue in the MWI, analogous
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
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
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
course, not Alice's exact state, only as so far the outcome of the
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
spin and in that sector the state of Alice is different that makes
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

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.
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
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.


The setting of the polarizers are also the result of only local interactions. Processes in your brain determine how you will end up choosing the setting of your polarizer. If the settings were not a priori agreed (a set of different setting can be agreed a priori for a Bell test), then that only makes the situation a bit more complex, you end up in a superposition of different polarizer settings and in each sector for a definite polarizer setting you are in a superposition of the different spin outcomes. This extra layer of complexity does not change anything.

As far as the explanation of the correlation is concerned, this follows the derivation of the correlations using quantum mechanics. For example, for two spins in the singlet state one can take Alice's results of up and down spins, represented by plus and minus 1's respectively, and Bob's up and down results are assigned the opposite signs. Measurements are done for an angle theta between the polarizers, the fraction of differences in the string of ones and minus ones is evaluated. This is repeated for an angle of 2*theta. Then for certain values of theta the fraction of differences become more than twice as large when the angle is doubled, this is inconsistent with local hidden variable theories due to the fact that the differences can be interpreted as mutations in one string and doubling the angle would then amount to adding the same fraction of mutations twice over. The maximum fraction of mutations would then be doubled if there is zero overlap, in case of an overlap would be less than double. It's impossible to increase the fraction of mutations by more than a factor of 2.

While this then excludes a local hidden variable theory underlying quantum mechanics, there is nothing nonlocal about quantum mechanics itself. Every step is a local process here, from the choice to set the polarizers, the creation of the entangled spin pairs, the measurement process etc. etc. That the correlations one observes cannot be explained by a local hidden variable theory, is a problem for such local hidden variable theories. There isn't (for all we know now) a hidden variable theory underlying quantum mechanics, nor is there anything in the dynamics (except for objective collapse) that's inherently nonlocal.

So, I don't see where the problem is.


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