On 09 May 2016, at 15:46, Bruce Kellett wrote:
On 9/05/2016 10:45 pm, Bruno Marchal wrote:
On 09 May 2016, at 04:12, Bruce Kellett wrote:
On 9/05/2016 1:39 am, Bruno Marchal wrote:
Thanks Scerir. Very interesting.
On 08 May 2016, at 09:58, 'scerir' via Everything List wrote:
https://arxiv.org/abs/1501.03521
'Bell on Bell's theorem: The changing face of nonlocality'
Authors: Harvey R. Brown, Christopher G. Timpson
there are several interesting points here
ch. 9 - Locality in the Everett picture
ch. 9.1 EPR and Bell correlations in the Everettian setting
Nice.
I think that what we are trying to explain to Bruce is well
summed up in their section 9.1.2 (the Everett description of the
singlet state, case of non-align polarizer).
I have already discussed this in my reply to Saibal. The basic
point I would make again is that the splitting of the universal
wave function into separate "worlds" is an interpretive gloss that
does not actually alter anything in the theory.
As long as you don't separate them too much, as the wave describe a
pure state remaining pure all the time, the "world" are the
phenomenological views as seen by each doppelgangers involved. If
not, you lose the possible interferences in principle possible by
quantum memory erasure.
Quantum erasure is possible only in limited, tightly controlled
circumstances. Generally, decoherence into many environmental
degrees of freedom is irreversible FAPP.
Absolutely, but the arguments here is not concerned with the PP (the
Practical Purpose in the "FAPP" (For all Parctical purposes).
Furthermore, 'who knows what about whatever' is also an
irrelevance as far as the universal wave function is concerned. If
you are going to work in the many worlds paradigm, then everything
ultimately stems from the unitary evolution of the universal wave
function -- all else is just interpretive gloss, of no fundamental
significance.
This is the case for the discussion in section 9.1.2 of the paper
by Brown and Timpson. Their equation (9) contains all the relevant
results that set the universal wave function -- the additional
third measurement (or measurement-like interaction) leading to
equation (10) is, therefore, irrelevant. All that happens in eq.
(10) is an exchange of information -- but it is an exchange of
information that is already present in the universal wave
function, no new information is created at this point. Just like
opening the box on Schrödinger's cat, which is either alive or
dead long before, looking changes nothing. Eq. (10) is,
similarly, just an interpretive gloss of no fundamental
significance. The important point here is that everything is set
in the universal wave function before Alice and Bob meet. The
relative angle of the respective polarizers is set in the wave
function long before the light cones of Alice and Bob overlap, so
that relative angle is determined non-locally.
The universal wave function is not a local object --
I am not sure what does this mean. The SWE is linear which is a
case of extreme locality I would say.
Linearity does not entail locality. Where did you get that notion
from?
If you can show me a linear transformation which emulates something
non local (and not just phenomenological), I would be interested. To
have the non-locality from Bell, what we are arguing is that you need
the collapse, which is not linear.
You can study the book by Pour-El, which contributed to my idea that
linear transformation preserve local influence. My intution stem from
the fact that "linear" is simpler than computable". Of course we use
real numbers, and so "computable" itself does not admit standard
definition on which everyone agree, so I will not insist on this.
the unitary evolution does not have any implicit notion of locality.
?
Locality is a human convention, and the universal wave function is
under no compulsion to take any notice of human conventions or
preferences.
The question is only: does Alice's measurement change something
instantaneously and physically at a distance? Obviously, this is
not a question of convention.
No, it is not a convention, and the violation of Bell and similar
inequalities shows that such non-local action is present
That is what you are asked to justify.
-- the measurement at particle 2 is not independent of what happened
to particle 1.
There is a recent review of Bell non-locality by Brunner et al. (RMP
86 (2014) pp. 419-478) which takes non-locality as an established
physical result. This would be the position of most working
physicists.
Because most working physicist believe there is a unique single
physical reality.
Argument of majority are not argument at all, also.
I see clearly that such action at a distance has to occur in all QM
with a physical collapse assumption, as Einstein saw already in
1927 at the Solvay Congress, and EPR-BELL-Bohm made testable. But
if the collapse is a first person view entangled with the particle
in the singlet state, I don't see any action at a distance
occurring, even if it looks like that for the person involved. I
don't get your critic of Brown and Timpson (9.1.2 in https://arxiv.org/abs/1501.03521
).
The move from eq. (9) of section 9.1.2 to eq. (10) is unnecessary.
In eq (9) all the local measurements are complete; both Alice and
Bob have split into A(+) and A(-), and B(+) and B(-), for seeing +
or - results respectively. And these measurements were done with
particular polarizer orientations, so by the time we can write eq.
(9), the relative polarizer orientation, and the set of all possible
results, are fixed. Alice and Bob might still be spacelike
separated, and Alice may not have been split according to Bob's
results, but that does not matter. Everything that is needed for
that splitting (if it occurs only after the light cones intersect)
is in place -- nothing new is added when the light comes overlap and
Alice and Bob exchange information about their results.
Brown and Timpson state: "Following this third measurement-
interaction [leading to eq. (10)], which can only take place in the
overlap of the future light cones of the measurements at A and B, a
definite outcome for the spin measurement in one region finally
obtains, relative to a definite outcome for the measurement in the
other. That is, we can only think of the correlations between
measurement outcomes on the two sides of the experiment actually
obtaining in the overlap of the future light-cones of the
measurement events -- they do not obtain before then and -- a
fortiori -- they do not obtain instantaneously."
But the universal wave function contains all the information about
outcomes and correlations long before the light cones overlap --
that overlapping does not create any new information. The
information might not be available to Alice and Bob before overlap,
but learning about something does not create that thing -- the
information gained generally pre-exists (Schrödinger's cat is either
dead or alive, long before we open the box!). So the correlations
are implicit in the universal wave function as soon as Alice and
Bob's measurements are complete -- the wave function does not have
to wait till Alice and Bob separately know each other's results. So
contra Brown and Timpson, the correlations do exist before the
future light cones of the measurement events overlap. They might not
be known before then, but they certainly exist before then because
nothing happens at the exchange of information between Alice and Bob
that can cause the particular relative polarizer orientation to
suddenly spring into existence, and the consequent probabilities for
each of the four possible worlds to suddenly materialize; on the
contrary, the relative orientation and the probabilities are built
into the universal wave function by the non-local interaction
between the two separated measurement events.
In the MWI literature, too much is made of the fact that Bob's
results are indeterminate for Alice until she hears from him by a
classical channel.
Just until she made a measurement. Once she does it, the measurement
to all Bobs she can ever met are determinate, and the measurement of
the Bob she could never met, remains, relatively to us, indeterminate.
As before, merely learning about something does not cause it to
spring into existence -- Bob has split into disjoint branches for
his possible measurement results, and each of Bob's branches is
duplicated in all of Alice's branches. That is what the linear
evolution of the wave function tells us -- that is the result of eq.
(9) in the paper.
No problem.
Alice and Bob may only self-locate on one of these branches after
information exchange, but that does not create these potential worlds.
Indeed, but that select the branches in which Alice and Bob find the
correlations predicted by QM, violating the inequality, without any
spooky action at a distance.
Brown and Simpson are close to my feeling (say), which is that
Bell's inequality violation testing does not test locality, but the
MWI itself.
I think that is why you are so resistant to seeing that there is a
better account than that given by Brown and Timpson, and many
others. The reality of non-local effects does not necessarily spell
the end of the MWI
Nobody said that. It signs only the end of Einstein's relativity
theory. And of rationality, I might guess. But we are used to humans
abandoning rationality when they don't grasp something.
If Nature is not local, that might be an evidence more that we are
dreaming, and thus an argument more for computationalism, but yet I
don't see any non-locality once we interpret the wave "literally". The
MW *is* the main loophole in the implication "Bell's violation ===>
non local action at a distance".
-- since MWI is only an interpretation of quantum mechanics,
Not in my book. MWI = SWE. Copenhagen = SWE + collapse, or MWI minus
all worlds but one.
and gives exactly the same results as any other interpretation.
I am not sure of this. If there is a physical collapse, some type of
quantum erasure seems impossible to me.
It is the same only FAPP (which is not concerning us).
Non-locality no more kills off MWI than non-locality kills off any
other interpretation, collapse or non-collapse. I have given a
perfectly coherent account of non-locality within the Everettian
paradigm.
Correct, but your interpretation of the MWI adds something to the SWE.
In fact, I have shown that such non-locality is indeed necessary
even in that paradigm.
That is what we were hoping you would show, but I am not sure you did.
The conventional MWI argument does not actually account for the
correlations at all -- because it does not explain how the relative
orientation of the polarizers enters the evolution of the universal
wave function.
?
I would say by Alice and Bob using their free-will, or by Aspect's
technic to randomize locally the choice. Again, I miss your argument
here.
To say that there is a real physical action at a distance is gross
(even if 100% of the physicists would say so). Bell's inequality
violation shows that this happens, very clearly, when we believe in
the unicity of outcomes of measurement, OK. But in the MWI, some work
needs to be done (at least) to convince me. I don't even find a paper
on the subject, only paper which shows that MWI is local (some more
rigorous than other). Do you have a reference of a paper showing that
Bell's inequality violation entails non locality in the MWI? I would
like to take a look on it, if it exists.
Many physicists just never think about the many-worlds, and use QM as
an instrument prediction only, and get shwoekd by Bell's result,
without ever pondering about the fact that all outcomes are realized.
That is why many believes in locality, they have just never study
Everett.
Bruno
Bruce
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