On 14-11-2017 09:23, Bruce Kellett wrote:
On 14/11/2017 5:51 pm, smitra wrote:
On 13-11-2017 03:52, Bruce Kellett wrote:
On 13/11/2017 12:15 pm, smitra wrote:
On 12-11-2017 22:54, Bruce Kellett wrote:
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
The splitting as an apparent nonlocal aspect to it, which is due to
a common cause effect, the spins were entangled, and that
entanglement happened when the spins were created due to a local
interaction in the past.
Yes, the entangled spin zero state was created in the past.
If you then let Alice and Bob measure the space-like separated
spins, they'll split up, and that happens in a correlated way,
because the spins are correlated. It is just the MWI variant of
Bertlmann's socks
That is not correct. The correlation you seems to be relying on is
non-local once the particles have separated. As Bell pointed out,
Bertlmann's socks are the wrong way to look at it! Besides, my
variation was to look at time-like separated measurements so that one
could keep explicit track of the splitting into separate worlds --
Bob
and Alice, in their various versions, are always in the same world as
each other. This eliminates the confusion that seems to being used to
escape the fact of non-locality.
Bruce
Whether Bob and Alice are really in the same worlds has to be
considered very carefully within a precise model. If Bob is modeled as
a robot with it's conscious thoughts defined by some bitstring stored
in his electronic brain and similarly in case of Alice, then
decoherence does not lead to a splitting in the bitstring sector.
While the atoms out of which the logical units are made out of do
decohere, the logical states are macroscopic states that can be "0" or
"1" , and Bob as defined by its bistring specifying it's subjective
state can be simply factored out of the complete quantum state.
You're floundering. Whether Bob and Alice are robots, bitstrings, or
flesh and blood human beings, they get split by being entangled with
the environment and the experimental result. What they know or don't
know is entirely secondary. Once they are split by being entangled
with a particular result, that is that -- nothing can change that, and
their knowledge is secondary.
Within this model, Bob does not decohere until that time he is told
what Alice has found.
That is simply not true. Decoherence is not subject to a particular
person's knowledge. When Alice and Bob are next to each other, they
are jointly entangled with a particular result.
I think that this needs to be discussed in more detail. Decoherence
does not cause a superposition to get reduced to either one of the two
possible outcomes. Given what Bob knows, he cannot locate himself in
either sector. If this were not true then given everything Bob is aware
of, he could get to better odds than 50-50 for guessing the spin. But
that implies information transfer to something his brain can access.
So, it boils down to decoherence acting on a microscopic degrees of
freedom, while brains and computers must be robust systems that would
not function well if they would be affected by such effects. Thermal
noise would make computing impossible.
If you have robust bitstrings that are only going to be affected by
information present in the environment at a sufficiently coarse grained
level, then that bitstring is never going to pick up any information
about Alice's result other than via direct communication or any leakage
of information e.g. if Bob is in to cold reading, or if there are other
correlations at the macroscopic level that Bob can exploit.
So, we have two decohered parallel worlds, but Bob is mentally identical
in the two sectors despite decoherence having split his body. As long as
he cannot access that information, he is the same person in both
sectors.
Saibal
If Alice and Bob are computers and whatever they are aware of is
contained in a bitstring, then it follows that anything one isn't aware
of, but is different in the two sectors cannot be present in the
bitstring. Otherwise Bob could know Alice's result before asking her or
measuring his own spin. At least, you cannot rule this out.
All that decoherence does is that the Alice
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