From: BRUNO MARCHAL <[email protected]>
On 16 Apr 2018, at 05:33, Bruce Kellett <[email protected]>
wrote:
From: BRUNO MARCHAL <[email protected]>
On 11 Apr 2018, at 14:19, Bruce Kellett <[email protected]>
wrote:
From: BRUNO MARCHAL <[email protected]>
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
