On 6/06/2017 10:21 am, David Nyman wrote:
On 6 June 2017 at 00:23, Bruce Kellett <bhkell...@optusnet.com.au
<mailto:bhkell...@optusnet.com.au>> wrote:
On 5/06/2017 8:42 pm, Bruno Marchal wrote:
I am not alone skeptical about inferring that the violation of
the Bell inequalities shows action at a distance. What is
wrong in Deutsch and Hayden? What is wrong in Rubin (Rubin,
M.A. Found Phys Lett (2001) 14: 301.
doi:10.1023/A:1012357515678), or in Maudlin's book?
They don't all necessarily make the same mistake as Price, but
they all make equally silly mistakes, and build in the
non-locality without realizing it. Last year I analysed the
argument by Tipler (arxiv:quant-ph/0003146v1) in detail and showed
where he made exactly this mistake of building the non-locality in
without realizing it.
Bruce, I'm reading The Emergent Multiverse by David Wallace at the
moment. He's well known as a prominent theorist of MWI. I don't know
whether he falls under your definition of competence in this area, but
as far as I've understood him, he fully accepts that MWI must be
consistent with QM in all respects, including of course nonlocality.
The distinction he makes is between nonlocality and the question of
whether this requires us to think in terms of instantaneous transfer
of information at greater-than-light speed, or "action at a distance".
I can't say I've been able to get my head around his full exposition
of this yet, but I'm pretty sure he doesn't go along with your
exposition of Price's seemingly faulty version of this.
It is interesting that Wallace has come to this view. He, with Deutsch,
was one of those who attempted to argue that MWI restored full locality.
They also tried to derive the Born Rule from within MWI, and failed in
that too.
I do not know the book you refer to, but if Wallace now accepts that QM
and Bell implies non-locality, then I fully agree. I have always argued,
on this list and elsewhere, that non-locality does not mean the
instantaneous transfer of physical information -- if you think about it,
that would, in a sense, be a local, albeit FTL, effect. The core of the
quantum singlet state is that it does not involve the physical positions
of the particles. It is expressed in configuration space, and the
difficulties appear to arise from interpreting configuration space as
though it were the same as ordinary 3-space. What has been said is that
the singlet state is always local in configuration space, which
translates to non-locality in 3-space. And this without some FTL
information transfer. If there were FTL information transfer, then that
could be manipulated to give FTL signalling, and there are all sorts of
theorems in QM that show that FTL signalling is not possible.
But it seems as though Wallace is coming to see these things as do the
majority of other physicists -- non-locality is intrinsic to quantum
entanglement.
As we know, MWI hypothesises multiple outcomes for each measurement
event. So on this basis, when Alice makes a measurement there is an
immediate split into branches consistent both with the measurement she
records and with its counterfactual partner. The same considerations
must apply equally to Bob. So we now have a spectrum of available
branches in which exist potential pairings of recorded measurements
that would be consistent with QM. The question then concerns which
pairings of Alice and Bob we (or they) should expect to observe in the
form of actual encounters for the purpose of comparing notes. QM tells
us that the results of any such observable pairings must be consistent
with violation of Bell's inequalities. Can we say, in terms of the
logic of MWI, why this might be so?
Yes. This is essentially the Tipler calculation that I have summarized
elsewhere. It is non-local, but it shows how the different branches
arising from each measurement must always match up to give the correct
correlations. Conceptually, what goes on is easier to understand if you
consider an EPR experiment at time-like separations. Then Bob can always
be in Alice's forward light cone, and there is no ambiguity as to what
splits occur, and when they occur.
Bruce
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