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. 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.
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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