On 13/11/2017 7:19 am, smitra wrote:
On 12-11-2017 11:21, Bruce Kellett wrote:
On 12/11/2017 9:14 pm, smitra wrote:
On 12-11-2017 07:57, Bruce Kellett wrote:
On 12/11/2017 5:39 pm, Brent Meeker wrote:
In Bruno's model the "influence at a distance" is determing which
world you're in.
If that means anything at all, it is still non-local because Bruno
has to rule out the worlds in which angular momentum is not conserved;
he has not shown how he can do this. If it is simply that you cannot
find yourself in a world in which AM is not conserved, then that is
just an unabashed appeal to magic, since such worlds have not been
shown not to exist.
Bruce
There are two correlated copies of Alice and Bob induced by the
correlated spins, there is nothing nonlocal about that in the MWI.
There is only a non- locality problem here if you assume a collapse
interpretation of QM. In the MWI the correlation arises via an
originally local common cause.
There is no collapse assumption in anything that I have written about
this scenario. What is the local common cause in MWI? Is that a local
hidden variable? Such would work in the time-like case, but not in
general -- that is Bell's result.
But you have still not answered the fundamental question as to what
causes Bob to necessarily measure spin down when Alice joins him with
a spin-up result? What turns Bob's particle from an unpolarized to a
polarized state so that the probabilities change from 50/50 to 100%
for spin down?
Bruce
Bob's particle never changed due to anything Alice did in the MWI.
Right.
All that happened was that Alice got entangled with her particle (and
not just Alice, her entire environment gets entangled with the state
of her particle), which in turn was entangled with Bob's particle. So,
Bob has the same probabilities for finding spin up or down,
Right. In fact, when Alice_up meets Bob, and Alice_down meets Bob in the
parallel universe, they can both tell Bob their result, and the
direction in which they measured the spin. This makes no difference,
since Bob is now entangled with the Alice's that have definite results.
None of this makes any difference to the particle Bob measures, because,
by the definition of locality, nothing has interacted with Bob's
particle, so it must be in the same spin state as when it was produced.
except that he can now measure the state of his particle by performing
a measurement on Alice, by asking her what she found for her spin.
That is not a measurement, that is making a prediction based on the
conservation of angular momentum.
It's not true that before Bob knows what Alice has found that only one
of the two version's of Alice has arrived and that the information of
her spin state is then already present in Bob's sector. This is not
true in the MWI, decoherence simple means that you can't demonstrate
the existence of the two versions of Alice via an interference
experiment. But the inability to do so, doesn't by itself imply that
only one version really exists.
I don't think you have fully understood the scenario I have outlined.
There is no collapse, many worlds is assumed throughout. Alice splits
according to her measurement result. Both copies of Alice go to meet
Bob, carrying the other particle of the original pair. Since they both
have now met Bob, the split that Alice occasioned has now spread to
entangle Bob as well as the rest of her environment. So there are now
two worlds, each of which has a copy of Bob, and an Alice, who has a
particular result. Locality says that Bob's particle is unchanged from
production, so when he measure its spin, he splits into two copies,
according to spin up or spin down. Since Alice is standing beside him,
she also becomes entangled with his result. But Alice already has a
definite result in each branch, so we now have four branches: with
results 'up-up', 'up-down', 'down-up', and 'down-down'. However, only
the 'up-down' and 'down-up' branches conserve angular momentum. How do
you rule out the other branches?
Bruce
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