On 1/2/2014 11:54 AM, LizR wrote:
On 3 January 2014 07:07, Jason Resch <jasonre...@gmail.com <mailto:jasonre...@gmail.com>> wrote:



        You can find out more and find out exactly where is is but to do that 
you're
        going to need to get your hands dirty and perform a experiment, then 
the squared
        wave function collapses from everywhere to one specific dot on a 
photographic
        plate. This is the measurement problem and the problem that the MWI 
elegantly
        solves that most other quantum interpretations do not; it's the only 
reason I
        think MWI is better than the competition.


    There are other reasons to prefer it besides it's answer to the measurement 
problem
    without magical observers, including:

    - Fewer assumptions
    - Explains more (appearance of collapse, and arguably also the Born rule 
(with
    Gleason's theorem))
    - Explains how quantum computers work
    - Fully mathematical theory (no fuzziness, or loose definitions)
    - No faster-than-light influences
    - Explains universe at times before there was conscious life to observe it
    - Preserves CPT symmetry, time reversibility, linearity
    - Is realist on things other than our observations (here is "something 
else" out
    there, besides what is in our minds)

    I would say the evidence for MWI isn't just strong, but overwhelming, given 
the
    evidence for QM is overwhelming and MWI is the only theory of QM consistent 
with
    other (overwhelmingly established theories such as special relativity).


I await Brent's response with interest.

Then I'll start by saying I don't reject MWI, I just have reservations about it, not so much that it's wrong, but that it doesn't really solve the problems it claims to - which implies criticism of the position that MWI has solved all the problems of interpreting QM. A lot of the above claimed advantages knocking down straw men built on naive interpretations of Bohr. Some are just assumptions, e.g that physics must be time reversible and linear.

The basic problem of the Copenhagen interpretation was the Heisenberg cut. Bohr essentially said it was our choice. Somewhere there had to be a classical, irreversible result if the theory was actually to predict anything, BUT we could chose where to put it. Where ever we put it, on the classical side probabilities were predicted with the Born rule.

MWI says we there are different orthogonal worlds corresponding to the different experimental outcomes. This is just the Heisenberg cut in another form. MWI helps itself to the CI view that the experimenter/instrument choice determines what variables will be measured. Now decoherence theory has come along and tried to make this objective - not dependent on what the experimenter had in mind. It is proposed that an instrument, by it's interaction with the environment defines a "pointer basis" or "einselects" a basis in which the system+instrument reduced density matrix will evolve to be approximately diagonal. Notice that from a mathematical standpoint "reduced" means "doing an average over a randomized environment" - so this isn't so deterministic as advertised - it's statistical-mechanics deterministic. But the problem remains that finding the pointer basis or even proving that there is one is an open problem which is the same (fuzzy) problem as the CI problem of defining the Heisenberg cut. I think there's a solution, but that's not the same as MWI has solved it.

The question of whether MWI derives the Born rule or not also seems unresolved. Gleason's theorem and Everett's own arguments prove (I think) that if QM predicts probabilities they must be proportional to the norms of projections of the Hilbert space state. But this implies inherently continuous probabilities. It's not clear how this relates to the existence of multiple worlds. Deutsch has given frequentist interpretation, i.e. the number of worlds with a given outcome is proportional to the probability of that outcome. But this implies and infinite number of worlds to realize an irrational probability value. But if you don't take Deutsch's frequentist model, then "probability" is an extra variable you tag onto branch worlds; which seems pretty much like collapsing the wave-function.

Whether MWI has FTL influence seems like a muddled question to me. MWI happens in Hilbert space, not spacetime. So it's not clear what is meant by entanglement traveling out along lightcones. Is this a dynamic evolution that is derived from the SE? from QFT? The examples seem to imply that there is no entanglement until there is a measurement, but experiments like the Bucky Ball EPR show that decoherence doesn't require a measurement in the usual sense.

Brent

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