On 14 Apr 2016, at 14:31, Bruce Kellett wrote:
On 14/04/2016 5:46 pm, Bruno Marchal wrote:
On 14 Apr 2016, at 06:20, Bruce Kellett wrote:
On 14/04/2016 2:27 am, Bruno Marchal wrote:
Hi Bruce,
Sorry I have been busy in March and lost track of some post(s).
On 06 Mar 2016, at 23:16, Bruce Kellett wrote:
On 7/03/2016 4:52 am, Bruno Marchal wrote:
On 04 Mar 2016, at 23:12, Brent Meeker wrote:
When Everett proposed QM without collapse many people were
attracted to it just because it was deterministic.
That is a motivation enough, but as I have explained, and is
not to badly explained in the book by Susskind and Friedman
(except that you have to read many pages before getting the
quasi-answer) it restores full locality.
This is a claim that is frequently made -- you yourself, Bruno,
have made it several times. But I think the claim is false. The
general consensus these days is that QM is irreducibly non-
local. If you have an argument that purports to show that
Everettian MWI restores locality, then produce it.
As I said, this is well done in the book by Susskind and
Friedman, but see also the explanation in the Everett FAQ of
Price. You can also read Deutsch and Hayden, or Tipler, who wrote
papers on this topic.
The general consensus that QM is not local applies to QM
+collapse, or QM+one-world. We know that this needs spooky action
at a distance since Einstein Podolski Rosen. Bell made this clear
and testable, but he assumes counterfactual definiteness, which
is not the case in the many world.
There seems to be some confusion as to what the term
"counterfactual definiteness" actually means. In the Wikipedia
article on the subject, "CFD is the ability to speak meaningfully
of the definiteness of the results of measurements that have not
been performed." I.e., the existence of Einstein's 'elements of
reality': "in each run of an experiment, there exist some elements
of reality, the system has particular properties <#a_i> which
unambiguously determine the measurement outcome <a_i>, given that
the corresponding measurement A is performed".
On this reading, counterfactual definiteness is equivalent to the
existence of hidden variables, or that every state has definite
properties, independent of experiment (non-contexuality), that
determine the outcome of any measurement. Ordinary quantum
mechanics, in any interpretation, rules out this form of
counterfactual definiteness: the Kochen-Specker theorem clearly
shows that no such set of hidden variables can exist.
The alternative meaning for the term, for example from Price's MWI
FAQ, is that "Bell and Eberhard had implicitly assumed that every
possible measurement - even if not performed - would have yielded
a single definite result. This assumption is called contra-factual
definiteness or CFD." So this is saying, not that the experiment
would have yielded a particular (predictable) result, but that it
could not have yielded any definite result. Frankly, I do not know
what this means! Deutsch and Hayden acknowledge that "Despite
there being, in general, no single 'actual outcome' of a
measurement, there is of course a well-defined set of actual
outcomes, and a probability for each member of that set." Again,
it is difficult to see this statement as being consistent with the
previous contention that there is no definite result.
So counterfactual definiteness seems problematic to me. Ordinary
QM is not counterfactually definite in that there are no pre-
existent 'elements of reality' that determine all measurement
results, but the formalism certainly predicts that all
experiments, even those that are not performed, will produce a
single result with a calculable probability. To deny this latter
contradicts the fundamental quantum association between
observables, operators, eigenfunctions and eigenvalues.
The "local" resolution of the violations of the Bell inequalities
that is proposed by MWI appears to amount to no more than the fact
that all actual measurements are local, and that correlations
between distant measurements can only be calculated after local
communication between the experimenters.
It means that if we look at the entire superposed picture there
have not been any action at a distance. It is only if we suppress
the superposition state in which we don't belong that things look
non local.
It is interesting that you have not answered my question about what
exactly you mean by 'counterfactual definiteness' so that we know
what you mean when you say that a theory is not counterfactually
definite.
It is hard to define, especially if we avoid being technical. But we
have a good example: QM-with-collapse (or QM with a single universe).
Like Einstein already explain at the Solvay Meeting: if QM (with a
single universe) is correct, we can't ascribe an element of reality
knowing a result that we would obtain with certainty if we would make
some measurement, but will not do. Then Kochen and Specker proved that
QM (+ a single universe) is precisely like that. The proof does not
apply to the many-world, although it might apply to some too much
naive rendering of the many world (notably if we interpret wrongly the
singlet state as I have explained in previous post).
If that appears to you to be a satisfactory resolution of the
violation of the Bell inequalities, then I can only say that you
have not really understood the problem.
Just show me one branch of the multiverse in which the violation of
Bell's inequality would entail a spooky action at a distance. That
would make the MWI incoherent with relativistic quantum filed
theory, if not just special relativity. It seems to me that Price
calculation show rather clearly what happens, and why we need to
believe in action at a distance *only* once we drop out the hidden
information, not of hidden variables, but about our "localization"
in the branch of the multiverse.
I do not find the account given by Price in the least satisfactory
because he assumes that the two separated observers agree in advance
about the directions in which they will measure spin components.
? I am not sure of this, nor if that plays some role here. You might
be correct as I remember needing still to verify that quantum
teleportation is purely local, despite it sounds non-local. Well,
after verification it is local, and that has been confirmed by other
treatment.
This is not a real test of the Bell inequalities since any local
hidden variable theory will give the correct results in that case.
The crucial test of non-locality is when measurement orientations
are chosen independently, and after the entangled particles are
separated.
Sure. But the separation is local, and keep the correlation intact in
all worlds. The Bell non locality is only apparentin most single
branch because we don't know in which partition of the multiverse we
belong too.
The quantum mechanical predictions are then not given in terms of
particular combinations of spin measurement results for single
trials, but in terms of correlations between results for arbitrary
measurement angles over many trials.
OK. But we can relinquish this by using GHZ instead of Bell. In all
case, I don't see why the violation of the inequality entails action
at a distance: it entails only an appearance of violation due to the
fact that Bob learns in which partition he belongs to, together with
Alice. That seems clear also with Deutsch & Hayden treatment in the
Heisenberg picture.
Although all possible combinations of measurement outcomes exist in
MWI, it is not clear what limits the results of the two observers to
agree with quantum mechanics when they meet up in just one of the
possible worlds.
Because they have separated locally, and Alice's measurement just
inform both of them (directly for Alice and indirectly for Bob once
some classical bit of information is communicated by Alice to Bob by
the usual means).
Bruno
Bruce
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