On 20-04-2016 03:02, Bruce Kellett wrote:
On 20/04/2016 6:56 am, smitra wrote:
The mistake made is to invoke classical reasoning after the
measurements are made. If the choice for the orientation of the
polarizers were not made in advance, then Alice and Bob cannot have
said to have made any definite choices at all.
I think you need to learn something about decoherence , and the
emergence of the 'classical' from the 'quantum'. In the final
analysis, Alice and Bob meet to compare their results. By that stage,
their results, and their relative magnet orientations, are definite
and classical (FAPP if you wish). And that is the end result we have
to explain. All else is boondoggle.
Invoking FAPP is precisely where your argument goes wrong. While due to
decoherence the macroscopic world looks classical, in reality (assuming
MWI) it not classical. This means that when Bob meets with Alice that
the settings Alice chose are still not determined. It is only when Alice
communicates to Bob what her polarizer settings were that Bob becomes
localized in that particular sector of the multiverse where this is now
fixed.
If Bob were to be imagined being located in that particular branch were
Alice had made definite choices and had made definite observations, then
that implies the existence of an observable for Bob that only acts on
himself that will yield the exact details of what Alice has done. So,
Bob could in principle have psychic powers, the information of what
Alice did would already be present in his brain before Alice
communicates these to him!
Obviously, Bob's brain does not have any information about what Alice
did until the details are communicated to him. So, Bob's mind is
identical across the many branches where Alice and, due to decoherence,
the local environment is different. So, in the experiment the
effectively classical communication is not at all trivial, in the MWI it
is a crucial step localizing the observers in the multiverse as where
the measurements of the spins.
Saibal
Bruce
In some particular sector where Alice made some particular result and
found some particular result, she knows that Bob's spin state. But Bob
lives in larger sector of the multiverse which includes sectors where
Alic had made different choices.
Alice and Bob communicating later is not some trivial exchange of
information that existed a priori, it leads to a further de-facto
collapse of the wavefunction.
There isn't anything more to this that Alice measuring the spin of an
electron in a lab, and then letting Bob who doesn't know what
direction the spin was measured in, doing another measurement.
Saibal
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