On 14/11/2017 10:01 am, Brent Meeker wrote:
On 11/13/2017 1:40 PM, Bruce Kellett wrote:
On 14/11/2017 2:07 am, Bruno Marchal wrote:
On 12 Nov 2017, at 23:05, Bruce Kellett wrote:
What really annoys me is the continued claim that many worlds
eliminates the need for non-locality. It does not, and neither
Bruno nor anyone else has ever produced a valid argument as to how
many worlds might restore locality.
But nobody has proved that there is non locality in the MWI.
EPR-BELL proves non-locality apparant in each branch, but the MWI
avoids the needs of action at a distance to explains them. Once
Alice and Bob are space-separated, their identity are independent.
It makes no sense to talk of each of them like if they were related,
(unless you correlate them with a third observer, etc) If they do
measurement, some God could see that they are indeed no more
related, but if they decide to come back to place where they can
compared locally their spin, they will always get contact to the
corresponding observer with the well correlated spin. The
independent Alice and Bob will never meet because they can't belong
to the same branch of the multiverse, by the MWI of the singlet
state. So Mitra is right. Although Bertlmann's socks are tyically
not working for Bell's violation in a MONO-universe, it works again
in the MWI, applied in this case to the whole singlet state.
Bell has proved non-locality in MWI, every bit as much as in each
branch separately. You appear not to have grasped the significance of
the scenario I have argued carefully. Alice and Bob are not
space-like separated in the scenario I outlined. Alice and Bob are
together in the same laboratory when the second measurement is made.
They are necessarily in the same world before, and branch in together
according to Bob's result. Your mumbo-jumbo about them only being
able to meet in appropriate matching branches does not work here,
because they are always in the same branch. And there is no reason to
suppose that their results in some of those branches do not violate
conservation of angular momentum.
It's that last point I don't understand. Why isn't conservation of
angular momentum a condition in every world. It's something separate
from QM.
Yes, sorry, I seem to have got my negatives confused! There is no reason
to suppose (in the time-like case proposed) that 'up-up' or 'down-down'
combinations are impossible, and such combinations of results would
violate conservation of angular momentum.
I'd say there is non-locality even when Bob's measurement is time-like
because there is correlation with no physical causal link. The
"common cause" of conserved angular momentum is not an explanation
because that doesn't work in the space-like case and there's no reason
to suppose QM is different in the two cases.
You are quite right here. The time-like and space-like separated
measurements must give the same results, and have the same explanation.
Common cause explanations don't work because space-like separations rule
them out. My reason for considering time-like separation is that in that
case one can precisely control the number of branches of the MWI
involved, so that Alice and Bob are in the same branch when the crucial
measurement is made. Thus there is no confusion over separate worlds
that have to be matched up according to AM conservation when they
finally meet -- they are always together, so there is no 'matching up'
issue.
Bruce
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