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.
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. 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. 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.
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.
Bruce
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