From: *smitra* <[email protected] <mailto:[email protected]>>
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".
It has become clear that the real argument by advocates of MWI is that
many-worlds deflects Bell's theorem, so that its implications do not
apply in MWI. I have, as Saibal points out, been arguing against this,
and I still consider my proof that selecting one branch out of the MWI
superposition is sufficient to apply Bell's theorem in its full rigour.
Since the argument applies to any branch, it applies to the
superposition as a whole, and MWI does not avoid the implications of
Bell's theorem. The implication is that no local hidden variable theory
can account for the observed EPR-type correlations. In particular, any
common cause, or Bertlmann's socks type argument, fails in MWI for the
same reasons that it fails in a single world account.
I have no idea what Saibal means when he claims that Bell's theorem only
constrains extensions of "standard instrumental QM". Saibal has not
offered any convincing counter argument to my proof that Bell's theorem
applies in MWI.
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.
That does not follow. Just because you think you have shown that Bell's
theorem does not apply, it does not follow that MWI is thereby local, or
that a local account of the correlations is available in MWI. Similarly,
the claim that there are no hidden variables says nothing at all about
whether reality is local or non-local.
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,
Bell's theorem rules out Bertlmann's socks as an explanation. You seem
to accept that my argument establishes that Bell's theorem is valid in
any branch of the MWI superposition. It follows that Bertlmann's socks
is invalid in all branches, so the possibility of a common cause effect
is ruled out in MWI, just as it is ruled out in any single-world
interpretation.
while in the Copenhagen interpretation it cannot be explained in that way.
Why not? Many-worlds and single-worlds are entirely equivalent in this
respect -- they both have the same wave function, after all!
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.
That does not follow. If you want to advocate a common cause explanation
in MWI you have to actually develop the corresponding dynamical theory--
i.e., what is the common cause and how is it implemented? If you simply
appeal to the nature of the singlet wave function for entangled states,
then you have not given a local account since, as Maudlin correctly
points out, the wave function itself is intrinsically non-local. It
seems that you are talking out of both sides of your mouth here -- QM is
non-local, but the non-locality is entirely trivial in MWI?????
Bruce
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