From: *Bruno Marchal* <[email protected] <mailto:[email protected]>>
On 17 Apr 2018, at 00:58, Bruce Kellett <[email protected]
<mailto:[email protected]>> wrote:
From: *Brent Meeker* <[email protected] <mailto:[email protected]>>
On 4/15/2018 8:33 PM, Bruce Kellett wrote:
We have discussed this, and I have never agree with this. The
singlet state (in classical non GR QM) describes at all times an
infinity of combinations of experimental result.
This is false. Even in Everettian QM there are only two possible
outcomes for each spin measurement: this leads to two distinct
worlds for each particle of the pair. Hence only 4 possible
parallel universes. Where do you get the idea that there are
infinitely many parallel universes? This is not part of Everettian
QM, or any other model of QM. But even if you can manufacture an
infinity of universes, you still have not shown how this removes
the non-locality inherent in the quantum formalism.
Bruno's ontology is all possible computations, so he's already
assumed (countably) infinite worlds. When there are only four or
two outcomes of an experiment it just means his worlds are divided
into four or two equivalent subsets.
That might very well be the case. But then that has absolutely
nothing to do with Everett or quantum mechanics. Bruno's long-held
claim is that Everett's many worlds obviate the need for
non-locality. But he has never been able to produce a coherent
argument to this effect. It is always this bullshit about an infinite
number of worlds -- as if that made any difference at all.
You are the one making the extra-ordinary claims. I don’t say much
more than maudlin on this issue in his book on Nonon-Locality: it
makes no sense in the many-world.
You seriously misrepresent Maudlin. To make this as clear as possible, I
have taken the third edition (2011) of Maudlin's book "Quantum
Non-Locality and Relativity" and typed out all the sections under the
heading of "many-worlds theory" from the index.
"The many-worlds theory is incoherent for reasons which have been often
pointed out: since there are no frequencies in the theory there is
nothing for the numerical predictions of quantum theory to mean." (Page
4, Note 1.)
"So we must either abandon locality or abandon the predictions of
quantum theory for events at space-like separation. I have sketched how
some versions of the many-worlds interpretation of quantum theory appear
to do the latter, and considered in some detail how locality might be
abandoned in a technically precise way." (Page 224, Chapter 10)
"Other, more popular approaches, though, are taken quite seriously even
though they offer no clear account of local beables at all. Most
obviously, many-worlds theorists typically do not postulate any local
ontology in the foundations of the theory: all there is is the
wave-function. A lot of attention is paid to "observables" and
"decoherence", but it is not at all clear how to generate a local
ontology if all one has to work with is the wave-function. ... But since
the wave-function is not itself a local beable, nothing about its
dynamics can yield a local ontology." (Page 250.)
Then the most extensive discussion of many-worlds appears in Chapter 10,
which was new for the third edition of his book.
"Standard quantum theory asserts that measurements always have outcomes,
and furthermore have unique (albeit unpredictable) outcomes. It is
exactly because such experiments always have outcomes that we can ask
after the predictions of the theory for the correlations between the
outcomes: if I measure the polarization of a photon in some direction on
one wing of an experiment and the polarization of an entangled photon on
the other wing, how like is it that the polarization outcomes will be
the same (both passed or both absorbed) or different (one passed and the
other absorbed)?
"If a many-worlds interpretation insists that there are no local
beables, then this is the situation. It cannot possibly reproduce the
predictions of standard quantum theory about the outcome of experiments,
and so is not relevant to our discussion of theories that agree with
these predictions. But the many-worlds interpretation is never presented
in this way. It is rather presented as if instead of no local beables,
there is a (largely invisible) profusion of them. That is, instead of
nothing happening on either wing of the experiment, the standard story
is that everything happens on both wings: on both wings, there is "a
world" in which the photon passes its polarizer and "a world" in which
it is absorbed, no matter how the polarizers were oriented.
"... If the wave-function never collapses, then the matter density
evolves into a rather indistinct blob, consisting in all the "possible"
outcomes of the experiment (passed and absorbed, for example, with all
these results being recorded in macroscopic ways) literally superposed
on one another in the same space-time region. One then tries to argue
that different components of the blob are causally disconnected from one
another, and so would be mutually transparent: many outcomes co-existing
but unaware of each other. One will typically appeal to decoherence of
the wave-function and a functional analysis of how to separate the blob
into distinct worlds to make out this conclusion.
"But two facts must be kept in mind. First, as we have seen, the matter
density ontology is not implied by the existence of the wave-function
/per se/. ... If a many-worldser wants there to be a local matter
density in space-time, then that has to be postulated in addition to the
wave-function. Second, if we produce an account like this, then there
still has to be discussion of what it means to say that the outcomes on
the two wings of the experiment are correlated to some degree. If
whenever a polarization experiment is done, with any orientation of the
polarizers, both outcomes are always produced, then it is not obvious
what it might mean to say that these outcomes are correlated. If no
sense can be made, then again the theory does not reproduce the
predictions of standard quantum theory, which predicts definite
correlations for outcomes at space-like separation. And if some sense
can be made of the existence of correlations, we have to understand how.
In particular, if appeal is made to the wave-function to explicate the
sense in which, say, the "passed" outcome on the right is paired with
the "absorbed" outcome on the left to form a single "world", then we
have to recognize that this is not a *local* account of the correlations
since the wave-function is not a local object." (Pages 250-252.)
So Maudlin does not support your notion that many-worlds removes the
need for non-locality. In fact, Maudlin is clearly claiming the exact
opposite. He clearly does not like the many-worlds approach, calling it
incoherent. But even if sense can be made of such a theory, there is
still no possibility of a *local* account of the correlations in
polarization measurements of the entangled singlet state.
Bruce
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