From: *Bruno Marchal* <marc...@ulb.ac.be <mailto:marc...@ulb.ac.be>>
On 5 Jul 2018, at 17:20, Lawrence Crowell
<goldenfieldquaterni...@gmail.com
<mailto:goldenfieldquaterni...@gmail.com>> wrote:
John Bell proved that any objective theory giving experimental
predictions identical to those of quantum theory is necessarily
nonlocal.
Assuming a unique reality. I prefer the term “inseparable”, because
“non-locality” is often interpreted the existence of FTL influence
(even if they cannot be used to transmit information), but such FTL
influence seems to me suspicious. Some might disagree, but I have not
yet seen a proof that any FTL subsists when we abandon the collapse
postulate. Bell assumes that experiments gives univocal results.
You might not have seen a proof that non-locality remains when we
abandon the collapse postulate, but that does not mean that no such
proof can be given.
Consider the following scenario. Alice and Bob are given a large number
of entangled pairs, which they measure when they are at large spacelike
separation. Each measurement is made at some angle, and gives a '1' for
'up' or 'passed', and '0' for the opposite result. Both record the
sequence of such results that they obtain in their individual lab books,
together with the corresponding polarizer orientations. Their lab books
then contain a random sequence of say N, '1's and '0's. There are 2^N
possible such sequences in the many-worlds case, but since each observer
keeps the same lab book for the whole sequence, each series of
measurements is necessarily made in the same one world. Basically, this
is because the worlds are disjoint, and the observers and/or lab books
cannot move between worlds.
When Alice and Bob meet up at the end of the run of N trials, they take
their lab books with them. When they meet they are clearly in the same
Everettian branch. And since their lab books cannot have jumped between
branches, the sequence of results that they each bring must also have
all been recorded in this same one branch. So when they come to use
their data to calculate the correlations between the measurements on
their individual particles of the entangled pairs, they are in exactly
the same situation as they would be if they had assumed a collapse model
from the outset. The correlations they observe are necessarily
single-world correlations. So the conditions of Bell's theorem are
exactly satisfied, and since the correlations violate the Bell
inequalities, their experiment has demonstrated the impossibility of a
local hidden variable account. They have demonstrated that the quantum
correlations require non-locality, even with Everett's many-worlds, just
as Bell proved.
And all this happens whether they assume many-worlds or a collapse model.
Bruce
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