On 16-07-2018 23:04, Brent Meeker wrote:
On 7/16/2018 8:18 AM, Bruno Marchal wrote:
I would like to think that this were the case, but you keep coming up
with irrelevancies that contradict the straightforward account of
these phenomena. If you forget about the metaphysics and just
concentrate on Alice and Bob making real measurements and recording
them in their lab books, then all these superfluities vanish. There
are no counterfactuals, no worries with other unobserved worlds, and
Bell's theorem goes through exactly as he intended. Many-worlds does
not invalidate Bell's argument. In fact, deflecting Bell's theorem
would do no more than allow for the possibility of a local hidden
variable account. That alone does not prove that many-worlds is local
-- that would still have to be established by developing such a local
hidden variable theory. No one has to date developed such a theory.
But since Bell's theorem has not been deflected, we do not have to
worry about such contingencies.
So we really agree. You have been probably misguided when trying to
defend John Clark who claimed that there are still FTL influence in
Everett, when the Bell’s inequality relations implies FTL only when we
assume unique outcomes of the experiences (i.e. some collapse, or
Bohm’s type of hidden variable).
No need of patronizing remark either, especially when rephrasing what
I was just saying. If you agree that there is no FTL in the
many-worlds, we do agree, that was the point I was making to J. Clark.
Not sure why you defended it, especially that you have shown
implicitly that you have no problem with the step 3 of the Universal
Dovetailer Paradox. You might eventually understand that with
mechanism, Everett’s task is still incomplete, as we need to justify
the wave from all computations, as seen from some self-referential
modes (fortunately and constantly implied by incompleteness).
Not to reignite the argument, but it originated because Bruno claimed
that MWI does away with non-locality in QM.
Brent
It reduces the non-locality to trivial common cause effects. Bruce has
been trying to prove that it doesn't by invoking the argument that you
can pick a single branch where Alice and Bob wrote their measurement
results in their lab books, and that one should therefore be allowed to
apply Bell's theorem by pretending that the other branches do not exist
and reach the same conclusion as in collapse theories. However, one has
to ask here what the violation of Bell's inequalities implies. It only
constrains extensions of "standard instrumental QM".
If we assume that, in general (and not just in case of Bell-type
experiments) measurement results are deterministic, that they are
specified by hidden variables, then the violation of Bell's inequality
implies constraints on such theories. Such theories must necessarily be
non-local. But then there is no evidence for a hidden variable theory,
so there is no need to invoke non-locality on these grounds.
Now, what is true is that if Alice and Bob perform measurements on
entangled spins such that their results are perfectly correlated and
they are space-like separated, that the non-existence of local hidden
variables has a non-local aspect to it because Bob has the information
about what Alice will find and the non-existence of local hidden
variables rules out that this piece of information is not somehow
present locally at Alice's location.
But this non-local effect is entirely due to a correlation mediated by
the entangled spins, in the MWI this is a common cause effect, while in
the Copenhagen interpretation it cannot be explained in that way.
Bruce's elaborate argument about verifying the violation of Bell's
inequality in single branches doesn't change that conclusion. Yes, you
can verify that Bell's inequality is violated in single branches, but as
pointed out above, that violation is part of the argument why in the MWI
the non-local aspects of entangled states are completely trivial.
Saibal
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