On 3/06/2017 9:16 am, smitra wrote:
On 02-06-2017 10:05, Bruce Kellett wrote:
On 2/06/2017 5:31 pm, smitra wrote:
On 02-06-2017 04:54, Bruce Kellett wrote:
On 2/06/2017 12:00 pm, smitra wrote:
On 02-06-2017 02:07, Bruce Kellett wrote:
On 2/06/2017 7:28 am, smitra wrote:
On 01-06-2017 02:26, Bruce Kellett wrote:
On 1/06/2017 4:43 am, Bruno Marchal wrote:
Non-locality is not removed in MWI as you appear to believe.
For me the abandon of the collapse is the solution of the EPR
"paradox", and Aspect experience is somehow the confirmation
of our belonging to macrosuperposition.
The non-local (paradoxical) nature of EPR remains even without
collapse. As on the previous occasion we discussed this, you were
unable to demonstrate where the notion of 'collapse' is used in
Bell's
theorem - all Bell requires is that measurements give results, and
that is what the whole of physics is based on: in MWI as well
as in
any other interpretation.
Bruce
In the MWI there are only trivial common cause like effects.
There is no non-locality in some mysterious sense like there is
in the CI where somehow there is a non-local effect but you
don;t have information transfer. The mistake most people make
when arguing that the MWI doesn't resolve the problem is that
they can't get their head around the fact that even when Alice
and Bob meet and Alice has not yet communicated everything
that's necessary to Bob, that the Alice that Bob sees has yet to
collapse in the branch corresponding to whatever she is going to
say (Bob's consciousness is thus located in many different
branches of Alice, even if the atoms in his body will be in
different states due to decoherence).
I think that either you or someone else said something like this
when
this was last discussed. I have a couple of points to make:
1. This is not quantum mechanics, or the many worlds
interpretation of
QM. It is your own idiosyncratic theory that has no bearing on the
question of non-locality in QM.
2. Even in its own terms, this theory is nothing more than an
undisguised appeal to magic. You want consciousness to be
unrelated to
the decohered body. That conflicts with the overwhelming
experimental
evidence in favour of the supervenience on consciousness on the
physical brain -- they move in lockstep, so if your body has
decohered
having obtained a particular measurement result, all copies (if
there
be such) of this consciousness are conscious of the same measurement
result. By the identity of indiscernibles, there is then only one
body
and one consciousness. That is what QM and MWI tell you, any
deviations are simple fantasy.
Bruce
Suppose Alice and Bob are robots with classical processors, the
states of each register can only be 0 and 1. Whatever Bob is
conscious of must then be contained in the bitstring that
specifies the state his processor is in. Decoherence has no
effect on that bitstring. Until that time that Bob asks Alice
about her setting of her polarizer, Bob's consciousness is exactly
the same across the different branches in which Alice gives a
different answer,
False.
It's trivially true.
Bob's consciousness reflects the results he got. There are only four
branches: '++', '+-', '-+', and '--'. Bob's conscious state is
different for the two Alice '+' branches, and also different for the
two Alice '-' branches. So it is not invariant across Alice's
branches.
What matters for the MWI discussion is simply that after Bob has made
his measurement and he gets located in a branch corresponding to
whatever he has measured, that whatever Alice does is not going to
further localize Bob in a narrower set of branches due to decoherence
caused by Alice's measurements.
Yes, in the case under discussion, there are only two branches for Bob
(and two for Alice).
Here you then do need to extract Bob's consciousness from Bob's exact
physical state.
We are doing quantum mechanics here, not a theory of consciousness. So
the discussion must be restricted to the formalism of QM. Introducing
Bob's conscious state is an obfuscation, because that does not appear in
the equations -- only his physical state is relevant to QM.
It's only when Alice communicates the details that Bob's consciousness
get's located in Alice's branches.
How? By magic?
despite rapid decoherence.
Real human beings can be expected to fit well within this model.
It is known that there typically is a lot of information present
in the unconscious mind that we're not aware of. So, your
consciousness could be identical across many branches even if your
brain had split and the unconscious mind is already diverging.
Take e.g. experiments where you are making a random choice and on
the basis on functional MRI scans the experimenters are able to
predict your choice before you have even made up your mind.
So, this is same good old QM in the MWI where an observer's
consciousness is modeled as a finite state machine described by a
finite bitstring. We can then work in the basis where the
observables for the bits of the bitstrings are all diagonal, this
then corresponds to the observer having a definite conscious
experience.
The definite conscious experience of relevance here is of observing
and recording the result of the spin measurement.
In practice, what this means is that you can be in a macroscopic
superposition long after decoherence has for all practical
purposes made the superposition inaccessible to be probed using
interference or other experiments.
For example, if a spin is polarized in the positive x-direction
and the z-component is measured, and I'm not aware of the outcome
of the result then my consciousness, as specified by the bitstring
that contains all the information that I'm aware of, cannot
possibly contain the result of the outcome of the measurement
before I'm told what the result is.
In a sector where my consciousness is described by bitstring X
after the spin has been measured, the state will have to be of the
form:
1/sqrt(2) |X> [|up, Universe(up)> + |down, Universe(down> ]
where Universe(up) and Universe(down) are different states of the
rest of the universe, but my consciousness is described by X and
this is not affected by the decoherence caused by measuring the
spin. In general there will be a summation of such terms where X
takes different values and Universe(up) and Universe(down) will
then depend on X, however, I can only ever find myself in a branch
were X takes some definite value, and any such branch will look
like the above state where the norms of both terms are equal.
That makes no sense, and is not the case in question. Alica and Bob
make measurements, observe the results and write them in their lab
books. Their subconscious mental states are of no relevance -- we need
look only at their lab books. That is when the non-locality becomes
apparent.
Bruce
There is no problem here because as soon as the spin is measured by
Bob, he becomes entangled with the entangled spin pair; that in
either of his sectors the probabilities for Alice's results are
immediately affected is a rather trivial effect.
It might be trivial, but it is also non-local, which was my point. The
only way this can be avoided is for you to claim that Bell's theorem
is not valid. If that is your claim, then you are no longer talking
about quantum mechanics, and your theory is not valid.
Bruce
There is only an apparent non-locality in the MWI due to common
cause effects. The many words aspect leads to correlated branches and
everything stays strictly local. Bell's inequality is violated but
that doesn't imply that the MWI must have non-local features. The
violation of Bell's inequality only precludes local hidden variable
models, but the MWI is not a local hidden variable model.
No, it is a non-local model. You seem to be under the misapprehension
that MWI is somehow a different theory from standard QM. Many worlds in
only an interpretation of QM; it is entirely the same theory, and gives
exactly the same predictions and explanations of observations in all cases.
If Bob measures his spin component of a two spin 1/2 particles in the
singlet state, then his measurement result is going to localize him in
the branch were Alice will make the opposite measurement if her
polarizer setting is the same. The fact that Alice measurement can be
said to be predetermined by what a space-like separated Bob has found
is not true in the MWI, because you have two Bob's in two branches.
Alice's results are then totally random even if Bob makes his
measurement and he can tell what Alice will find, because of the
existence of the other branch where he found a different result.
That is a bit confused. If the polarizer settings are the same, Bob can
predict Alice's result whatever result he gets -- in both branches in
the MWI.
In a single universe theory, this implies non-locality, because of the
absence of local hidden variables. If local hidden variable were to
exist then you could say that Alice and Bob where to find whatever
they found anyway only due to their local interactions with the spins
and polarizers. But with that ruled out, whatever Alice will find is
information that just popped into existence when Bob made his measurement.
What is ruled out by Bell's theorem is that local hidden variables can
account for all possible correlations between the observations, Bell's
theorem does not rule out the possibility of a local hidden variable
explanation in special cases, like that of polarizers set at the same angle.
This is then not similar to a common cause effect where Bob and Alice
are given a box containing either a white or black ball such that if
Bob gets a box containing the white ball, Alice gets the box
containing the black ball and vice versa. This can then be only be
analogous to a local hidden variable case.
The example you have chosen, where the polarizer orientations are preset
to be the same, is not the general case, and that particular case can be
compared to the classical situation of balls in boxes, (or Bertlmann's
socks in the example Bell, gave to illustrate this). The point of
Bertlmann's socks is that the general singlet entangled state is not in
the least like the classical common cause effect, and cannot be
explained in this way.
In the MWI, the analogy with the black and white balls continues to
hold because we have two branches available where the boxes can be
swapped. Now, it does get more complicated when you consider different
polarizer settings, but then you need to specify exactly how these
settings are made. They can be predetermined, or they can be the
result of other quantum measurements, in which case further branchings
need to be considered.
You are right, things do get more complicated when you consider
arbitrary polarizer settings. The settings are made at random at
spacelike separations. No further branches are introduced by this. The
settings are not predetermined, and the possibility of superdeterminism,
such that the supposed arbitrary settings are determined by same distant
common cause, has been ruled out by setting the polarizers at the remote
locations according to fluctuations of light from galaxies at opposite
sides of the sky, such that those galaxies can never have been in causal
contact.
As far as the quantum formalism for this situation is concerned, the way
in which the polarizer settings are chose is not relevant -- it suffices
to assume that these settings are random and independent. So there are
no further relevant branches introduced.
If you are to convince me that MWI is a fully local theory, then you
have to account for the violations of Bell inequalities in the general
case in terms of the quantum formalism itself, without introducing
irrelevant extras. Do the maths and come back when you have the proof.
Bruce
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