On 16/04/2016 12:20 am, Bruno Marchal wrote:
On 14 Apr 2016, at 14:31, Bruce Kellett wrote:
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).
I do not understand what you are saying. Are you claiming that ordinary
QM with collapse is counterfactually definite because Einstein realism
does not apply? I.e., we cannot know with certainty what would have been
the outcome of an experiment that was not performed? (This is also the
consequence of the Kochen-Specker result that no set of hidden variables
can predict the results of all possible spin measurements on a spin 1/2
particle.) I would have thought that this was one possible definition of
counterfactual /indefiniteness/.
What additional fact about MWI changes this conclusion? Since in MWI all
possible experiments are performed in some word or other, I would have
thought that experimental outcomes are available for all possible
experiments -- nothing is /actually/ indefinite -- even though not all
outcomes occur in this one world that we happen to inhabit at the moment.
Bruce
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