On 17-03-2022 05:21, Bruce Kellett wrote:
On Thu, Mar 17, 2022 at 10:44 AM smitra <smi...@zonnet.nl> wrote:
On 17-03-2022 00:09, Bruce Kellett wrote:
You don't need to specify irrelevant details. What is relevant or
irrelevant depends on the particular case, but there is generally
no
confusion about this.
If it is irrelevant then we should be able to take the angles to be
known a priori to both Alice and Bob without that affecting the
conclusion.
I thought it was understood that we were discussing an Aspect-type
experiment where the detector angles are set while the entangled
particles are in flight. So that the relative angle is fixed at
spacelike separations. I am sorry if I did not make this clearer.
The earlier Freedman-Clauser experiments were criticized because they
used fixed polarizers, and switched to different relative orientations
between runs. It was suggested that this did not rule out the
possibility that the photons were able to anticipate the measurements
by some as yet unknown physics, and thus the observed correlations
could be due to some (unknown) local hidden variables. This
possibility was ruled out by Aspect when he was able to switch his
polarizer angles very rapidly while the photons were in flight.
While that's a way to rule out photons being able to anticipate the
polarizer settings, since that's not what happens the MWI, it's also
irrelevant to the discussion about how the MWI explains the correlations
in a local way.
Except that when the measurements are local, the relative angle
is
unknown to both participants. So neither knows locally that the
polarizers are aligned. That knowledge can be available only
non-locally. And that is how it happens.
It's then not clear that the polarizers will actually be aligned.
The
information about the angles cannot appear out of thin air, the
laws of
physics conserve information. Not specifying the process,
assuming that
it's not known but then later assuming the case of the relative
angle
being zero is an invalid way of reasoning.
You cannot reasonably ask for a local explanation of a non-local
effect. I have made it abundantly clear that this knowledge of
relative polarizer angles is known at the measurement source
non-locally. To ask for a local explanation is silly -- none is
available since it is non-local.
This could, perhaps, have been explained more clearly. If, for
convenience, we take Alice's measurement to be first. Then that
measurement fixes the spin angle of the particle that Bob will
measure. When this reaches Bob's polarizer, the angle of that
polarizer is available, and since the particle effectively carries
Alice's angle, the relative angle is immediately available to the
measured particle, and it responds according to the appropriate
probabilities. The non-locality of this process lies in the relative
angle via the fact that the spin direction of the particle Bob
measures is set by Alice's result for the spin singlet state. If she
gets up, then Bob's particle is automatically set to down (in Alice's
orientation), and if Alice gets down, Bob's particle is set to up (in
Alice's orientation). This is non-local since the particles are at a
spacelike separation when Alice makes the measurement that determines
what happens to Bob's particle. In MWI, the different results obtained
by Alice are realized in separate branches of the wave function.
Yes, in each separate sector for Alice where for some choice of her
angle she finds a certain result, there is a nonlocal effect. Since in
the MWI both of Alice's sectors exist for Bob (the state of Bob's local
environment factors out of the entire superposition describing Alice's
state), there is no nonlocal effect for Bob on his spin measurement
results due to Alice choice of the angle. The correlations between
Alice's and Bob's result only exist at the level of the measurement
results, and from the point of view of one observer, the results of the
other observer do not exist until they meet.
Not for the relative angle but rater for the angles at each
polarizer site.
The angles at each measurement site are recorded along with the
up/down results in both Alice's and Bob's separate lab books. The
correlations are determined by gathering data for each relative angle
separately. One you know the separate angles (and there are generally
only a few, such as 0, 60 and 120 degrees) Both Alice and Bob select
randomly (at spacelike separations) between these possibilities. Then
the relative angle can be readily determined for each entangled pair
when the data are later analysed. But this relative angle is not known
to the experimenters at the time they make their measurements.
Yes, if you bring in a nonlocality due to choosing the angles at
spacelike separations then you'll end up with a nonlocal feature in the
correlations. What matters is that unlike in collapse interpretations,
in the MWI there is no nonlocal effect in addition to what you put in by
hand.
The non-locality is in the knowledge of relative polarizer
orientations at the time of measurement.The possibility that this
is a
common cause effect is ruled out by Bell's theorem. See his paper
on
Bertlmann's socks.
Not in the MWI.
That is your assertion. It has not yet been proved. Nor has there been
any account of how the violations of the Bell inequalities arise
locally.
If you start with a locally constructed state and you don't put in
nonlocal effects in the experiment by hand (so, you use predetermined
angles, there is no need to eliminate the possibility of particles
interacting with polarizer settings in some unknown way as that's not a
feature of the MWI), then the MWI clearly does yield a local explanation
of the correlations while it's also clear that in collapse
interpretations you have nonlocal features. So, those nonlocal features
that are present in collapse interpretations doe get eliminated by the
MWI.
Also nonlocality of the relative angle plays no role in
Bell's theorem. It's only done in experiments to rule out the class
of
all local realistic theories.
Bell's theorem is a general result. We are discussing the
demonstration of non-locality in Aspect-type experiments. In addition,
restricting the discussion to the case of aligned polarizers is
probably not sufficient because a local hidden variable account can be
given in this limited case. To see the violation of the inequalities
you really have to look at several different relative orientations.
Such as in the CHSH inequality.
Yes, but the MWI is not such a local hidden variable theory, and it's
also not necessary to choose the angles non-locally when discussing the
MWI.
Saibal
Bruce
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