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.
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.
Bruno
Bruce
The simplest reason why QM-without-collapse is that the equation
are linear differential equation. Susskind argues on this by
showing that the density matrix of Bob remains unchanged when Alice
makes his measurement. I once verify this for the case of
teleportation.
And give the argument yourself -- do not take the lazy route of
referring to papers of dubious reliability.
Just read Everett himself. You are the one making the extraordinary
claim. I have not yet seen one proof that QM (without collapse)
entails non-locality. It entails only an apparent non-locality due
to our abstraction of the macro-superposition we are in.
To be sure, with computationalism this is an open problem, but if
there is non locality, it will need the first order modal logic of
the observable to be shown. Up to now, as far as we know, the comp
observable obeys the same quantum logic than QM, and it entails non-
locality only if we assume counterfactual definiteness, it seems to
me. I think that Price explanation is rather clear. If you think he
made some error, please show it to me.
Bruno
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