On 10/05/2016 2:22 am, Bruno Marchal wrote:
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:
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.
No, you don't need collapse, that is the whole thrust of my argument. I
have demonstrated non-locality within the Everettian approach without
collapse. Bell's original argument didn't mention collapse, and the
argument that his theorem fails because he assumed definite outcomes
from measurements is actually without substance: no such assumption is
required by Bell. All you have to do to convince yourself of this is to
think of my illustration of working in momentum space in order to deal
with the superpositions involved in the description of a particle by a
wave packet. If there are superpositions involved in the outcome of
experiments, it is sufficient for calculational purposes to consider
just one typical member of the superposition. The superposition can be
recovered later by a convolution integral over the initial distribution.
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.
And I have done so.
-- 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.
Again, you place far too much emphasis on what the participants in the
experiment can know at any instant. The point of Everett was to treat
the universal wave function seriously -- if something was in the wave
function it could be treated as "real". Everett saw that all terms in
the superposition were in the wave function, so he treated them all as
equally "real".
In the case under discussion, after Alice and Bob have completed their
measurements, but before their light cones overlap and they have had a
chance to communicate, everything about the situation is contained in
the universal wave function -- the respective outcomes are all described
by the unitary evolution of the wave function from the initial state.
All possible branches are present, and all are equally "real" if you
follow the basic Everettian paradigm. So what Alice and Bob think about
things can have no possible bearing on the objective facts as described
by the wave function. And that objective wave function contains all four
possible combinations of outcomes, with weights determined by the
relative orientation of the polarizers. That information could come into
the wave function only non-locally. If you have an alternative
explanation as to how the universal wave function could contain all this
information before Alice and Bob meet, then tell me. If you don't think
this information is in the wave function, then tell me why eq. (9) of
Brown and Timpson is wrong.
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.
The QM correlations are realized on all branches -- there is no
"selection" of branches involved. Look at the equation again. Where do
you see more than the four branches I have discussed?
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".
It is by interpreting the wave function "literally", rather than
selectively as you do, that we see the evidence of non-locality.
...........
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 the uniqueness of outcomes is not a requirement for the
demonstration of locality to go through. That is what I have been
arguing all along.
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.
I have not seen anything published along these lines. That does not mean
that no such papers exist -- I have not really been keeping up with all
the literature in recent years. But I do know that many remain
unconvinced by the many worlds argument, and it is clear that Bell's
theorem does not ultimately depend on any assumption of collapse,
despite claims to the contrary.
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.
Many physicists are instrumentalists at heart -- and who is to say that
they are wrong? They are interested in results, after all: reality can
look after itself!
Bruce
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