From: *Bruno Marchal* <marc...@ulb.ac.be <mailto:marc...@ulb.ac.be>>
On 16 Apr 2018, at 05:33, Bruce Kellett <bhkell...@optusnet.com.au
<mailto:bhkell...@optusnet.com.au>> wrote:
From: *Bruno Marchal* <marc...@ulb.ac.be <mailto:marc...@ulb.ac.be>>
On 11 Apr 2018, at 14:19, Bruce Kellett <bhkell...@optusnet.com.au
<mailto:bhkell...@optusnet.com.au>> wrote:
From: *Bruno Marchal* <marc...@ulb.ac.be <mailto:marc...@ulb.ac.be>>
If you believe in influence at a distance, you are the one needing
to show the evidence of that extra-ordinary fact.
The fact is demonstrated by the experiments that test Bell
inequalities on the singlet state.
Not at all. This proves the existence of influence at a distance
when we suppose that a measurement gives an outcome, but in QM
without collapse, a measurement gives all outcomes, with varying
relative probabilities.
The measurement on one of the spin-half particles in the singlet
state has only two possible outcomes. As is often said in discussions
of non-locality in Everettian QM, 'measurements that are not made do
not have outcomes!’
Contextually yes. Because Alice and Bob, in most experience, have a
common protocol, in most thought experience, but we need to take into
account at the start all Alice and Bob to see that there will be non
influence at distance, but only sharable self-localisation issues.
That is interesting. What you are saying, quite clearly, is that the
starting point is for Alice and Bob to rule out the possibility of
non-locality. That is not a very open-minded or scientific stance.
Surely the point of the thought experiments is to investigate whether or
not the data can be accounted for in a purely local way. To assume from
the start that there must be a purely local explanation is unscientific
because you have already closed your mind to the possibility of
non-locality.
You did not. You have even considered a singlet state like if it
involves 4 parallel universes, when it involves infinitely many.
See more in the archive.
The singlet state involves only four possible combinations of
experimental results
We have discussed this, and I have never agree with this. The
singlet state (in classical non GR QM) describes at all times an
infinity of combinations of experimental result.
This is false. Even in Everettian QM there are only two possible
outcomes for each spin measurement: this leads to two distinct worlds
for each particle of the pair. Hence only 4 possible parallel
universes. Where do you get the idea that there are infinitely many
parallel universes? This is not part of Everettian QM, or any other
model of QM.
From Deutsch and many others, but you can deduce it from Everett long
text. Just take the universal wave seriously.
I am taking the wave function very seriously for this simple (but
closed, isolated) system of the singlet state formed from two spin
one-half particles. The wave function is very simple:
|psi> = (|+>|-> + |->|+>),
within normalization factors. The first ket refers to particle 1 and the
second ket to particle 2. Note that this wave function is intrinsically
non-local in that it has no dependence on the spatial separation of the
particles -- space and time are not relevant for this structure. The
temporal evolution of this state is simply the free particle propagation
of the two particles to arbitrary separations (at least until one or the
other interacts with something else).
Note that the standard expansion requires a set of basis vectors. I have
written these symbolically as a |+> or |-> basis, but there is freedom
in the choice of spatial direction for these basis vectors. But note
particularly that the spin measurement is made in the basis chosen by
the experimenter (by orienting his/her magnet). The outcome of the
measurement is + or -, not one of the possible infinite set of possible
basis vector orientations. The orientation is not measured, it is chose
by the experimenter. So that is one potential source of an infinite set
of worlds eliminated right away. The singlet is a superposition of two
states, + and -: it is not a superposition of possible basis vectors. If
you think about it for a little, the formalism of QM does not allow the
state to be written in any way that could suggest that.
I don't know what Everett says in his long text, but if it is any
different from the above, then it is not standard quantum mechanics.
Deutsch is a different case. He has a very strange notion about what
constitutes different worlds in QM. Standard QM and Everett's
interpretation are very clear: different worlds arise by the process of
decoherence which diagonalizes the density matrix. The net effect is
that worlds are, by definition, non interacting (contra Deutsch's ideas).
But even if you can manufacture an infinity of universes, you still
have not shown how this removes the non-locality inherent in the
quantum formalism.
You have not shown non locality.
I have demonstrated non-locality in the Everettian context many times.
The simplest demonstration was in the timelike separation of Alice and
Bob's measurements. It is in the archives if you don't recall the
details. The argument then is that any local influence that would
explain the timelike separated measurements must also work for spacelike
separated measurements, and that is not possible.
In the Everett, the locality is preserved by the fact that you need
interaction/measurement at some point, and the superstition get
“contagious” only at the speed of light, something zurek explained
well in his account of decoherence.
This is what you suggested above -- your view is that locality is
maintained by refusing to accept the possibility of non-locality. Sorry,
but that does not wash, scientifically or logically.
Locality is also trivial if you look at each time to the entire
multiverse phase space structure. I don’t see how you perceive any
influences at a distance.
You perceive them by doing the Bell-type experiments. Remember that
quantum mechanics is ultimately defined in Hilbert space, and questions
of spatial/temporal separation do not arise there, so it is all local in
Hilbert space. The problem is that converting from Hilbert space (and/or
configuration space) to normal 3-dimensional space with a distinct time
variable, gives rise to some conceptual difficulties. Unless you can
come to terms with these conceptual difficulties, you will never
understand quantum mechanics. One of these conceptual difficulties is
that in normal space-time, quantum mechanics is intrinsically non-local.
Bruce
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