On 7/06/2016 2:00 am, Bruno Marchal wrote:
On 06 Jun 2016, at 03:20, Bruce Kellett wrote:
On 5/06/2016 9:44 pm, Bruno Marchal wrote:
But it makes no sense to say that particles 1 and 2, when separated,
belongs to the same branches. Bell can say that because it assumes
only one branch (so to speak) in which case there is a mysterious
spooky action at a distance. But if they are space-like separated,
we get the non-locality appearances only for those Alice and Bob
wich will be able to meet at some points, and the math shows that
this linearly and locally implied such appearances, despite the wave
evolved locally at all time in the phase space. There should be no
problem as you seem to accept the definition of worlds by set of
events/objects close for interaction. If Alice and Bob are space
like separated, they just cannot belong to the same woirld: it makes
no sense.
That claim makes no sense. You are making an elementary logical
blunder -- Separate worlds do not interact, objects with spacelike
separation do not interact, therefore spacelike separation implies
separate worlds. That argument is equivalent to: all As are Bs,
therefore this B is an A.
Come on. It was not an argument in logic, but in quantum mechanics. It
is a consequence of the linearity of both the evolution and the tensor
product. Once you define a world by a set closed for interaction (or
possible interaction), space-like separations orthogonalize the
realities. It just makes no sense to singularize Alice and Bob in one
world/relative-branch when they are entangled with the singlet state.
Spacelike separations do not orthogonalize anything. A world is closed
for interaction, but that is not the best defining characteristic of a
world. In MWI, worlds are produced by decoherence following an
interaction (be it a measurement or some other interaction). Decoherence
into the environment inevitably results in the production of soft IR
photons that escape from the region. These photons are not recoverable,
so once decoherence has progressed to reasonable degree, the situation
is not reversible: the IR photons can never be retrieved and put back
into the interaction region, so once the possibilities have decohered,
the process is irreversible /in principle,/ not just FAPP. It is this
irreversibility that precludes further interference or interaction
between the worlds. So irreversibility is the defining characteristic of
separate worlds, not just lack of interaction.
Given this, Alice and Bob separate into different branches/worlds only
following an interaction -- only when they measure their part of the
singlet state. It makes no sense to claim that this happens before such
interaction with the state because before any measurement has been made,
the situation is completely reversible and there is only one world.
Separate branches arise only from decohered quantum interactions.
Not in the MWI. If you decide to fix some base, you can consider that
the branches are separated at the start. It is the differentiation
view of Deutsch, which works also for the universal machine's
"many-dreams" interpretation of arithmetic. The Y = ll rule. IN QM it
is just that
a(b + c) = ab + ac if a is an observer, he does not need to look at
the particle state b/c to be multiplied.
That is just playing with words, and Deutsch's approach reduces the
concept of "separate worlds" to meaninglessness -- the concept becomes
so fluid as to become useless. One is very much better advised to limit
the idea of separate worlds to the irreversibility following a decohered
interaction.
Of course, from the digital mechanist view, all this talk is
premature. It is just that I don't see any spooky action at a distance
in the MW.
Preparing a singlet state and sending the particles off in separate
directions does not create separate worlds -- particles 1 and 2 are
in the same world until the spin measurements are made. Then multiple
worlds are generated, which eventually pair up so that worlds in
which correlations can be defined appear. For the singlet state under
consideration, these correlations violate the Bell inequalities in
all branches. The wave function evolves locally and linearly in
configuration space -- that is seen as non-locality in physical space.
Somehow that would please a digital mechanist, as this would make the
physical even less real. But I am not convinced by your argument.
My logic is secure. You haven't refuted my basic arguments as yet.
There is no "outside view" of configurations space, so the
non-locality is intrinsic to the "bird" view of the wave function in
physical space, just as it is to the "frog" view from within a
particular branch. No local account of this physics exists.
I think we might disagree about what we mean by "physical world".
Space-like-separated world can interfere probabilistically without any
possible interactions in between. Quantum non separability can exist
between space-like separated worlds, but as we can hope, without any
need of physical interaction or causation between them.
That sounds like you actually do accept the standard concept of
non-locality in quantum mechanics! Spacelike separated particles can
interfere probabilistically without any possible interactions
(mechanistic force-field exchanges) between them: that is precisely what
is meant by non-locality in this context.
I think you have been too tied up with a mechanistic interpretation of
non-locality -- you appear to think that it necessarily involves FTL
exchange of some particle or other mechanistic influence. But this is
not necessarily the case -- we don't actually postulate non-local hidden
variables of this type because that would represent an attempt to give a
"local" account of "non-locality". All that is involved is that the
singlet state is a unity, even though the entangled particles might be
widely separated. This is reflected in the fact that the wave function
itself is intrinsically non-local -- it is local and deterministic only
in configuration space, not in 3-dimensional physical space.
Bruce
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