On 13 Jul 2017, at 13:12, Bruce Kellett wrote:
I have recently acquired Wallace's book and read his section on non-
locality, so I am re-opening this thread (under a different header)
to discuss Wallace's views in more detail. More below......
On 7/06/2017 9:24 am, David Nyman wrote:
On 6 June 2017 at 01:46, Bruce Kellett <[email protected]>
wrote:
On 6/06/2017 10:21 am, David Nyman wrote:
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.
Wallace uses the term non-separability. He makes an analogy,
to a certain extent, with the ontology of field theories such as
electromagnetism, about which he says "The structural complexity of
a given electromagnetic field is represented not in the properties
of very small spacetime regions (indeed in the limit as these
regions become point sized, the field's structure becomes almost
trivial) but in the way in which those properties vary across
spacetime. Furthermore, this general model is characteristic of
pretty much any classical field theory, except that vector fields
seem mathematically tame compared to the sorts of mathematical
objects used to represent the field values of many classical field
theories.". He gives a number of examples of these latter objects
including the affine connections of General Relativity. He then
goes on from this analogy to propose an ontology for quantum field
theory which he calls Spacetime State Realism. I can't really
attempt to elaborate on this here.
Moving on this basis to the question "Does Everettian quantum
mechanics display action at a distance?" he answers in the
negative. He justifies this by elaborating on the observation that
"In a quantum field theory, the quantum state of any region depends
on the quantum state of some cross section of the past light cone
of that region. Disturbances cannot propagate into that light
cone." To the question "Does Everettian quantum mechanics display
non-separability?" he answers in the positive. He justifies this by
elaborating on the observation that "Because of entanglement,
knowing the density operators of regions A and B does not suffice
to fix the density operator of (the union of) A and B. Some of the
properties of (the union of) A and B are genuinely non-local: they
have local physical manifestations only if we arrange appropriate
dynamics.".
That is a good extract from the heart of his exposition. In a way,
it is more a matter of words than of substance -- his description of
'nonseparability' is essentially what I have been calling 'non-
locality', and Wallace himself actually lapses into this usage from
time to time. His summary is:
"The overall story about locality in Everettian quantum physics,
then, is this: the dynamics of the theory are local: there is no
action at a distance, and no clash with relativistic covariance. But
quantum entanglement means that a great deal of the information
contained within the quantum state is non-local, associated with
large spatial regions but not with any given subregion of those
regions. As David Deutsch once put it, quantum theory is a theory of
local interactions and non-local states."
I cannot find anything in that summary to which I could object.
Wallace than goes on to discuss some examples. His first example is
of a single particle system, such as Schrödinger's cat. He describes
the initial local interaction, and the spreading of the branching
via decoherence. If the cat is system A, and the environment with
which it becomes entangled a set of systems B_i, (i = 1,2,...), then
the spread of the branching results in a situation in which the
individual states of the systems A and B_i become mixed, but the
combined state of (A B) remains pure. "The state A itself does not
change at all in the process; what changes are the non-local states
of successively larger regions including A." The spreading of
entanglement by decoherence is thus a process that introduces a
degree of non-locality. Wallace illustrates this in his Figure 8.1
on page 307.
Then we get to measurements on two independent particles, and,
finally, two entangled particles. Here Wallace really wimps out, and
does not give any systematic analysis. He does not consider the
entangled singlet state explicitly at all. All he says is that the
entanglement between the particle at A and the particle at B is a
non-local property of A B. "That property propagates outwards,
becoming a non-local property of the forward light cone of A and
that of B. Only in their interactions can it have locally
determinable effects--and it does, giving rise to the branch weights
which, in turn, give rise to the sorts of statistical result
recorded in Aspect's experiments and their successors: statistical
results which violate Bell's inequality."
That is just the standard quantum account, since it is always
accepted that the correlations only become apparent when the results
of measurements by A and B are combined at some later time, when
their light cones overlap.
Wallace seems to find this relatively uninteresting. Bell's result
entails non-separability (non-locality), but not action at a
distance -- but then, no one said it did involve action at a distance.
? The discussion rised because at least a participant maintained that
the violation of Bell's inequality involves action at a distances
(agreeing already that no transfert of information was possible in
that case).
I told you we were agreeing all along.
I think Wallace used the "standard" vocabulary of Bernard
d'Espagnat: inseparability (to keep "non-local" meaning "influence at
a distance").
He then claims that Bell's theorem does not apply to the Everett
interpretation anyway, because is assumes that experiments have
unique, definite outcomes. That is the usual MWI claim against Bell,
but Bell's results are not specifically quantum -- the inequalities
obtain for any theory in which the measurements at A and B are
independent, so this passing swipe at Bell is rather unnecessary.
?
The violation of Bell's inequality becomes non local if you have only
one universe. That is why Bohm's theory needs non local action of the
potential guiding the particle. It introduces p-zombies in universal
waves.
The upshot, it seems to me, is that Wallace acknowledges non-
locality as I have used the term, only he prefers to call it
nonseparability.
The point was just that Bell's violation does not entail physical
influence at a distance. If you agree with this convince that other
gentleman.
The change in terminology does not change the physics, so Wallace
accepts that the Everettian interpretation of QM does not eliminate
non-locality, despite the claims of many MWI supporters.
OK. It does not eliminate inseparability, but it eliminate non
locality in the (usual?) sense of action at a distance.
But that participant was agreeing with me, that "obviously", with one
and one one universe or real branch (like with the wave packet
reduction), the violation of Bell's inequality does entail the
existence of action at a distance (despite no-signaling). The point is
that with the MWI, we keep the inseparability (your non-locality) but
evacuate indeed all physical actions at a distance (and that was my
only point, to be clear).
(and so, like with the FPI, everyone agree except *one* gentleman)
Bruno
Bruce
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