From: *Bruno Marchal* <marc...@ulb.ac.be <mailto:marc...@ulb.ac.be>>
On 22 Apr 2018, at 06:39, Bruce Kellett <bhkell...@optusnet.com.au
<mailto:bhkell...@optusnet.com.au>> wrote:
From: *smitra* <smi...@zonnet.nl <mailto:smi...@zonnet.nl>>
On 22-04-2018 04:51, Bruce Kellett wrote:
From: SMITRA <smi...@zonnet.nl <mailto:smi...@zonnet.nl>>
I think the confusion arises from a failure to distinguish between
'local interactions' and 'non-local quantum states'. In the
entangled
singlet case we have a non-local state since it involves two
particles
that are correlated by angular momentum conservation no matter
how far
apart they are, or whether measurements on the separate
particles are
made at time-like of space-like separations. No one has ever denied
that the interactions involved in the separate measurements on
thetwo
particles are all local, or that decoherence effects that
entanglethe
particles with environmental degrees of freedom are all local,
unitary
interactions. Decoherence leads to the effective diagonalization of
the density matrix, and the effective separation of copies of the
experimenters that obtained different results, but this effective
collapse of the wave-function is brought about by purely local
interactions.
The usual many-worlds argument for the absence of non-local
effects
points to the fact that all the interactions involved in
measurement
and decoherence are purely local to argue that there is no
non-locality. But this entirely misses the fact that the original
singlet state:
|psi> = (|+>|-> - |->|+>)/sqrt(2)
is intrinsically non-local. It refers to correlations due to
angular
momentum conservation that persist over arbitrary separations, and
these correlations are neither enhanced nor destroyed by any
number of
purely local interactions.
So many-worlds or many-minds interpretations of quantum theory
do not
obviate the need for non-locality: they cannot, because the basic
state that is talked about in all interpretations is non-local. The
point to be made is that in no theory, either a collapse or a
non-collapse theory, are there any non-local interactions: all
interactions in measurement and decoherence are local. But that
does
not mean that what one does to one particle of the singlet does not
affect the other particle -- directly and instantaneously. It
is just
that this effect is not instantiated by a local (or non-local)
hidden
variable. There are no faster-than-light physical transfers of
information. That would involve a local hidden variable, and
there are
none such.
The point is that quantum mechanics is weirder that you think
in that
it is intrinsically non-local, even though all physical
interactions
are necessarily local. Thinking of the 6 spatial dimensions of the
separated singlet particles as forming a single point in
configuration
space may help one to visualize this. Alternatively, one can
note that
the tensor product Hilbert space of the two spin states is
independent
of spatial separation.
Bruce
Quantum mechanics is a lot weirder w.r.t. to its non-locality
aspects
in single world theories. It is there that Alice, after she
makes her
measurement, has to wonder how the implied information about Bob's
measurement result popped up at his place. This is not an issue
in the
MWI.
Saibal
There is no difference between collapse and no-collapse theories in
this regard. MWI does not eliminate the non-locality in the
wave-function for the singlet state. This can easily be seen by
following the unitary development of my state |psi> above
through its
interactions with the measuring device, observer, and the
environment.
The extra worlds in MWI just come along for the ride -- they do not
add anything of substance to the argument. All the discussion about
whether Bell's theorem is invalid for MWI because he assumed
collapse,
or he assumed counterfactual definiteness, or he assumed that
measurements had only one outcome, etc, is totally irrelevant
to the
issue of non-locality. It is in the original quantum state, so it is
not eliminated by simply retaining all possible measurement results.
Bruce
In the MWI the non-locality becomes a common cause effect that can
be traced back to the creation of the entangled spins. As pointed
out by Vaidman here:
https://youtu.be/jKGuGptafvo?t=1876
<https://youtu.be/jKGuGptafvo?t=1876>
it's in the ordinary collapse models where there is real problem.
Saibal
Vaidman seems to be trying to demolish Bohm in this video -- nothing
intelligent about any "common cause" effect for Bell-type
correlations. It seems that Vaidman is really playing with idea of
retro-causality. And such things are orthogonal to many worlds.
Indeed, the whole thing seems very confused. The only thing that was
clear was that Vaidman adamantly rejects non-locality -- which is not
a very scientific stance.
Action at a distance does not make sense with special relativity. It
implies making sense of simultaneity.
Maudlin spends most of his book on Non-Locality and Relativity trying to
make sense of the notion that non-locality violates relativity. But this
is largely misguided because, as you say, there is no FTL physical
exchange of information -- the non-locality is inherent, it is not a
direct physical effect.
Bell defended this, and the possibility that special relativity is
false, just to avoid the MW. But Aspect showed him wrong.
I would say, rather, that Aspect proved Bell to be right in that there
are no local hidden variable that "explain" the correlations at
space-like separations.
Aspect experience is a strong evidence for the MW
No, it is nothing of the sort. Non-separability and non-locality are
built into the wave-function (quantum state) so it is exactly the same
in both collapse and non-collapse models. MWI adds nothing to our
understanding of this effect.
(which by the way, is a quasi trivial consequences of Mechanism in
cognitive science, so “apparent non-locality without action at a
distance” was to be expected, even without quantum mechanics.
Well, that just shows that your "mechanism:" is entirely unnecessary --
physics does perfectly well without it.
Bruce
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