On 4/27/2022 6:09 PM, Bruce Kellett wrote:
On Thu, Apr 28, 2022 at 10:14 AM Brent Meeker <meekerbr...@gmail.com> wrote:

    On 4/27/2022 2:00 PM, smitra wrote:
    If you agree, and are prepared,
    with me, to throw out Everett, then we agree, and there is nothing
    more to be argued about (at least, until you present some different
    complete theory).

    I'm open to the idea that QM itself may only be an approximation
    to a more fundamental theory. The arguments in favor of no
    collapse are strong arguments but you then do get this issue with
    probability that you have discussed here. The disagreement with
    you about this is that I  don't see it as a fatal inconsistency
    that would prove the MWI to be wrong. Probabilities for the
    different branches do not have to be equal. But that doesn't mean
    that this looks to be a rather unnatural feature of the theory.
    This suggests that a more fundamental theory exists from which
    one could derive quantum mechanics with its formalism involving
    amplitudes and the Born rule as an approximation.

    If there are probabilities attached to the branches, then
    Gleason's theorem shows that the probabilities must satisfy the
    Born rule.  So I don't seen any inconsistency in simply saying
    they are probabilities of measurement results,  that's
    Copenhagen.  But if they are probabilities of results that implies
    that some things happen and others don't...other wise what does
    "probability" mean and what use is it as an empirical concept?

That is exactly right. If you try to claim that the probability of 'up' is 90% and the probability of 'down' is 10%, but that both results certainly happen (albeit on different branches), then you are talking nonsensical gibberish. That is why I say that Everett is incompatible with the Born rule.

I'm not such a purist as to hold that probability can't apply to many things; any measure that satisfies Kolomogorov's axioms qualifies as far as I'm concerned.  So maybe there's some such measure in Everett's theory, but it seems to have the same problems that were discussed in 1930's.  When is a measurement?  In Everett the answer seems to be "Never."  Or else it's purely subjective event, as in QBism.


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