On 05 Jun 2017, at 04:19, Bruce Kellett wrote:
On 4/06/2017 10:05 pm, Bruno Marchal wrote:
On 02 Jun 2017, at 03:01, Bruce Kellett wrote:
In QM, with or without collapse, decoherence and the transition
from the pure state to a mixture gives a definite measurement
result.
In particular branches only. When looking at the whole wave
including the observers, decoherence explain why it *looks*, to all
observers in the different branches, that mixed states have been
obtained, but that is not the case in the global description.
The transition to the mixed state is essential for one to get a
definite experimental result.
I agree. But this needs to happen from the subject point of view only.
If Alice looks at a dead+alive cat, she split/differentiate in seeing
a cat alive or a cat dead, although in the big picture she put herself
in the superposition seeing the cat alive = seeing the cat dead. OK?
Physicists realized a long time ago that the pure unreduced state of
the MWI does not work. The difficulty is known as the 'basis
problem'. If you retain the full superposition of the pure state,
there is no preferred basis, and expanding this superposition in
terms of different bases gives different -- usually nonsensical --
physical results. It is only when you reduce to a mixed state that
the basis is fixed, and results are definite.
This will happen in the relevant basis, and *that* happens whatever
base is chosen for the universal wave.
This does not mean, as you appear to think, that you have lost the
other branches. All the branches of the MWI are still present,
except that now there is a different definite measurement result in
each branch.
That is exactly what I meant.
Without collapse, different branches get different results, but
once obtained, these results are fixed, and are not affected by
whether Alice and Bob exchange information or not.
I agree. That is used in the fact that in EPR like situation, when
Alice and Bob are space-time separated, what we have is "only" an
infinity of Alices and Bobs,
This is wrong. There is no "infinity of Alices and Bobs".
?
There is one Alice-Bob pair for each possible spin direction, or
polarizer angle. Contrary to what the notation of the singlet state
suggest, there is no preferential polarizer angle.
all with their spin correlated, and when Alice makes her
measurement, at any angle, she will know Bob's possible result,
Bobs possible results, as far as Alice knows, is 50/50 for '+' or '-'.
without needing any action at a distance. She just localize
herself, and her corresponding Bob, in which branch they belong.
That is not correct. You keep saying it, but you offer no proof or
mechanism whereby such a thing could happen.
I did, and Saibal Mitra have point on the same reasoning done, many
times. I have referred to Steven Price, also.
There is no influence at a distance, although we would need it to
talk of token unique Alice and Bob in case there would be only one
universe.
That is a total misunderstanding as well. All branches might exist
(two for Alice in this case, one where she got '+' and one where she
got '-'), but we need consider only one typical branch to get the
general result -- that is how things are done in physics when you
have superpositions.
When they are space separated, and make both measurement in non
perpendicular angle, I don't see how we could talk of any typical
branch. In that case thay might find any random result, even
uncorrelated one. But they will never be aware of this, as they will
compare the results later only with the part of the multiverse on
which their respective (uncorrelated a priori) outcome will contagiate
to the Alice (resp. Bob) they will be able to communicate with.
So, it is only because you maintain the identity of Alice and Bob
throughout the experience, that you can interpret the violation of the
inequality as an influence at a distance. That does not make sense
once you keep track of the "slow" spreading of the superposition on
their respective environment, in all relevant branches we started with.
Bruno
........
But there is non-locality -- non-local influence -- in all
interpretations since it is inherent in the quantum formalism.
I don't see any non-locality in the MWI. EPR, Bell, assumes always
one Alice and Bob, and as Everett shows, decoherence explains the
manitenance of coherent first person plural description, and the
absence of collapse prevent any non-local influence.
That is not the case either. Bell does not assume a necessary
collapse. Bell's theorem is a mathematical theorem, it is true
whatever interpretation of QM you adopt. You seem to be suggesting
(and you are more explicit in this suggestion elsewhere) that Bell's
theorem is invalid for MWI.
1. That is not true -- Bell's theorem is valid in all interpretations.
2. Even if you did find an error in Bell's theorem, all that that
would get you is the possibility of a local hidden-variable account
of the correlations. If you think such a local hidden variable
account is possible, then give it - in full mathematical detail -
and we might begin to think that you know what you are talking about.
You cannot get away by reversing the onus of proof. Bell's theorem
is independent of whether or not a collapse is assumed,
To interpret the experimental violation, you need to identify the
Alice and Bob you talk about. But EPR and Bell talk of Alice and
Bob like if they were in a definite universe all along the
experience, when that is never the case. They do assume implicitly
one physical universe.
Not true. See above. Even if this were the case so that the theorem
was not valid in MWI, that actually does not get you anywhere -- non-
locality would still exist, except that now you could give a local
hidden variable account. I see no sign that you are actually doing
this.
so if you want to argue that MWI removes the non-locality proved
by Bell, then the onus of proof is very much on you: you have to
demonstrate how this can be possible.
It is a trivial consequence of the linear differential shroedinger
equation. Or of the fact that the evolution is a rotation (unitary)
in Hilbert space.
Rubbish. If you remain with the linear Schrödinger equation, you
cannot get definite results for experiments. Once you have definite
results, as you need to calculate correlations, the non-locality is
evident.
You say that Bell's theorem relies on the unicity of outcomes. By
this, I presume you mean that Bell assumes counterfactual
definiteness (in the usual terminology). If by counterfactual
definiteness you mean that a measurement gives a definite (though
unknown in advance) result, even if that measurement is not
performed.
This does not make sense in the MWI. If I measure a alive+dead cat,
it is only a "first person illusion, 1p" that such an experience
gives a definite result. In the 3p picture, both results must be
said to obtained.
Again, you have a fundamental confusion between the 3p account of
the situation and the 'bird' view from outside space and time. There
is no 'person or persons' who have the bird view. The 3p account
still gives just one definite result for each observer -- even
though these results may be different on different branches. Both
results are obtained only on the "1-plural" view, if I have
understood what that piece of jargon means.
Then I accept that counterfactual definiteness is assumed in
quantum mechanics. Without such an assumption, the whole notion of
an expectation value would collapse.
Not in the 1-views. It continues to make sense, and the contagion
of superposition (the linearity of the tensor product) even
prolongate the 1-views into 1-plural views, that gives the
siplitting/differentiation/decoherence, which needs only to
propagate at the speed of light or below.
That is very confused.
So if you abandon counterfactual definiteness, you have a
different theory -- you have abandoned standard QM, and you then
have to explain how you can get and use expectation values.
We got them in the memories of the person's involved. Not from
looking at the whole universal wave. It is again like with
computationalism.
Your claim appears to be that Bell's theorem is not valid in MWI.
Bell's theorem is valid. His inequality does not even assume QM,
but just locality.
I agree, but that is not what you were implying above. It seems that
now you agree that the Bell inequalities assume only locality. But
these inequalities are violated by experiment. That can only mean
that the assumption of locality was wrong -- whatever interpretation
of QM you adopt.
It is violate when we do the experience, like Aspect, and this
shows non-locality in our branch,
When you make measurements, you get definite results. When you
measure an intrinsically non-local entity such as the singlet state
of two electrons, you get non-local results.
but when looking at the big picture, we see that this non-locality
has a local origin.
No, it has its origin in the fact that the state Alice and Bob are
measuring is intrinsically non-local.
It would need an action at a distance to destroy the alternante
branches alwailable to Bob,
Not at all. There are only ever two branches available to Bob and
Alice, and thus only four possible combinations of results, each
combination occurs in a single world, and the non-locality is
evident in every such world.
but without collapse, non-locality is a local, branch-owned,
phenomenon. I take Bells theorem + Aspect as a quasi definite proof
that if there is one universe, then there are many universes.
That can work only if you insist on locality, and that MWI preserves
locality. Both these assumptions are wrong.
This is nonsense. Bell's theorem is a theorem of quantum
mechanics, and it is therefore valid in all interpretations of
that theory.
Yes, in all interpretation of quantum mechanics, the relevant
branches violate the inequality, but they do that without involving
an action at a distance when we look at the entire wave. It is
phenomenological.
If it is not valid in MWI, then Many-Worlds is a different theory,
and not just an interpretation of standard QM.
It is valid in the MWI, but interpreted differently than in a mono-
universe interpretation which requires non local action at a
distance to get the same non-locality.
(with or without hidden variables).
That is what making a measurement means. It is what happens in all
interpretations. It makes no sense to deny counterfactual
definiteness -- that is not QM.
It is QM without collapse, and using the simple mechanist FPI.
You must have some strange understanding of what counterfactual
definiteness means.
Alice and Bob get *all* (always correlated) answers, but when
light-separated, it make no sense to compare them. They can only
make comparison with the person accessible in their light cone,
where the contagious superposition spread out.
I presume you mean "space-like separated". Alice and Bob do their
measurements;
The infinities of Alices and Bobs do their measurements.
they get their results and write them in their lab books. They
meet years later and compare lab books. Are you trying to suggest
that they do not have definite answers in their lab books before
then?
The infinities of Alices and Bobs get their infinities of definite
results.
In MWI (with two-outcome experiments), there is a copy of Alice
that writes '+' in her lab book, and a copy who writes '-' (for a
given orientation theta). Similarly for Bob. There are, therefore,
only four possibilities when they meet: '++', '+-', '--', and '-
+'. The non-locality is necessary to set the probabilities for
each of these four possible combinations of results. If you want
to eliminate the non-locality, you have to give a non-magical way
of establishing the necessary probabilities. You have never been
able to do this.
QM does that, and without collapse, I don't see how any influence
leaking at the speed of light need to be introduced.
Remember that in a sequence of such experiments, the probabilities
for '+' and '-' are 50/50 for both Alice and Bob.
OK.
The joint probabilities, or correlations, depend on the relative
orientations of their polarizers.
Right.
It is information about this relative orientation that must be
conveyed non-locally for the correlations to come out correctly
when they meet.
Why? That would be the case if you think that it is the same Bob
and Alice all along the experiences, but that cannot be the case.
It is not sufficient for them simply to exchange this information
later, because their results at particular orientations are
already fixed when they meet.
I don't see this. If the angle is some theta different from 0° or
90° they will both split/differentiate, and whoever they will meet
later will be the correspond partner with the correct correlation,
obtained by the decoherence local to their respective branch. In
this case, it is clear that it does not make sense to attribute to
"Alice and Bob" the same identity than the initial one.
You are just resorting to unexplained magic again. The Alice and Bob
that meet to compare results are simply the original Alice and Bob
that got particular results -- in MWI there are two copies of Alice
and two of Bob, with just four possible combinations. The
correlations must be correct for each branch of the combined wave
function (each of the possible combinations). There are no
"infinities of Alices and Bobs". That is not in the quantum
formalism. I think it is clear that you are working in some theory
that is different from quantum mechanics, but are unwilling to spell
out what that theory is.
Bruce
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