On 17/04/2016 7:35 am, John Clark wrote:
On Fri, Apr 15, 2016 Bruce Kellett <bhkell...@optusnet.com.au <mailto:bhkell...@optusnet.com.au>>wrote:

    ​ > ​
    Consider the usual case of a spin singlet that splits into two
    spin-half components that separate and are measured by A and B at
    spacelike separation. There are two possible measurement results
    for each observer, call them |+> and |->.
     The entangled state can then be written as:
    ​ ​
    |psi> = (|+>|-> - |->|+>).

​You can write it however you like but in many worlds it means there are 4 different outcomes and a observer will see each one. ​

    ​ > ​
    The first ket applies to observer A and the second to observer B.

​OK but keep in mind that in Many Worlds there must be some difference
between A and B for the universe to split; if there is no difference between them then it is meaningless to talk about two separate entities.

A and B are shorthand for the conventionally named experimenters, Alice and Bob. These two are different, regardless of the orientations they choose for their magnets.

    ​ > ​
    A and B perform their measurements at spacelike separation, but
    each chooses the measurement orientation outside the light cone of
    the other.

​In other words they choose it for reasons that don't exist inside their lightcone, in other words they ​choose it for no reason, in other words they choose it at random.

    There are four possible combinations of results, corresponding to
    four worlds in the MWI: |+>|+'>, |+>|-'>, |->|+'>, and |->|-'>.

​OK.​ All those results could happen and in many worlds everything that can happen does happen.

    Since each observer has a 50% chance of getting |+> and 50% of
    getting |->, and the two measurements are completely independent
    of each other, it would seem that each of these four worlds is
    equally likely.


    ​ > ​
    But this conclusion is contradicted by quantum mechanics:

​ It is?! Then
which of those four outcomes does quantum mechanics predict is more likely to happen?? ​

Read on......

    ​ > ​
    if the two observers, by chance, have their magnets aligned, then
    the |+>|+'> and |->|-'> combinations are impossible.

If A and B are identical BEFORE the measurement then there were not 2 experimenters there was only one, and in Many Worlds only when different results are observed does the universe split and only then would it make sense to talk about A/* and*/ B . If on the other hand A and B were different
BEFORE the measurement
​ and​ they observed identical results then A and B would still be different and thus be in different universes.

But A and B are different, regardless of this experiment. And there are copies of both A and B in every universe that might be generated by their respective experimental results.

    ​ > ​
    In general, the probabilities of the four possible joint outcomes
    depend explicitly on the relative orientation of the magnets of
    the A and B -- they are seldom all equal. How is this taken into
    account in the formalism?

In any formalism that corresponds to reality a spinning particle going through a Stern–Gerlach
magnet will be deflected either either up or down from the center line by the same amount, and the probability of observing either is exactly 50%. And from that experiment, depending on how the magnet is oriented, you can determine the part of the original particle's spin that was in the z axis or the part that was along the x y axis but NOT both.

If the world is realistic then the spin axis of the original particle had a exact orientation before the measurement you just didn't know what it was, before you a made a measurement for all you knew the north pole of the spin axis could have been pointing to any point on the inner surface of a sphere, after the measurement you've narrowed things down to any point on a circle but you still don't know what point on the circle.
This is counterfactual determinism. Unfortunately, quantum mechanics violates CFD in many situations, particularly EPR and the Kochen-Specker result.

If the world is not realistic then the spin axis did not point to anything before the measurement was taken because the spin axis did not even exist before the measurement.


    ​ > ​
    After the measurements are complete, A and B communicate their
    results to each other,

​ Then, assuming A and B were different to begin with and assuming the MWI is correct, the universe ​
​ splits into 4 strands with a observer in each one. ​

Sometimes: sometimes only two strands.

    ​ > ​
    So MWI does not give a local account of the EPR results on
    entanged states.

​I think that's right, some would disagree but they can only so by giving a somewhat tortuous meaning to the word "local", and when they do that they just increase the difficulty in making MWI realistic and deterministic. No quantum interpretation can get rid of quantum weirdness and make things realistic local and deterministic as common sense demands because common sense is just dead wrong.

It seems that we agree, then.

I like many worlds because, at least to me, it seems a little bit less ridiculous than any other quantum interpretation I know of. Maybe someday a better one will come along but I would bet money it will still be weird because we live in a weird place.

I don't like many worlds because it is baroque and unnecessary.


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