On 10 May 2016, at 02:10, Bruce Kellett wrote:
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.
It seems to me that you have shown non-separability (that some call
non-locality). You have no shown that in any branches where the couple
Alice and Bob can talk to each other some genuine physical influence
at a distance has existed.
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.
?
Bell does not mention collapse, nor EPR, because it is the assumption
by default.
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.
It is convenient FAPP, OK. But only when a physical collapse is
supposed to be done, can we say that the inseparability entails action-
at-a-distance. Without collapse, I see only contagion of
superpositions propagating at sub-light speed.
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.
I have to do that to recover the quantum indeterminacy and non-
locality as first person (plural) phenomenologies. It is the whole
point of Everett: to explain the appearance of a collapse when they
are none.
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".
OK, but then he shows that to get the indeterminacy (and non-locality)
we have to look at the person's diaries or memories (like in the WM-
classical-duplication case) to explain the appearance of the collapse,
indeterminacy etc.
All terms are real in the wave, but only one (class-equivalence of)
term will seem observable by all persons described in each term.
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.
But Alice and Bob have been obliged to interact in some past to make
their particle entangled. Of course they could use entanglement
swapping or more indirect way, but in all case the singlet state if
brought by local interactions at some point.
All possible branches are present, and all are equally "real" if you
follow the basic Everettian paradigm.
No problem.
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.
OK.
That information could come into the wave function only non-locally.
Why? (see my remark above).
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.
No problem with equation 9. The universal wave described a global
structure, but the relative outcome in each term of the wave have a
statistics which depends on that global structure, making singlet
state describing non separable particles without having to add action
at a distance when Alice or Bob make measurement, whatever terms they
are in, as long as they are in the same terms (so to speak). I just
don't follow you critics of Price, Tipler, Brown & Timpson, etc.
Would you say that the universal deployment is non local. The
statistics on any measurement depends on an infinite of computations
at once. There is no action at a distance, because the statistics
comes only from our non capability of localizing ourself in the many
computations, and our incapacity to "feel" the UD-times (which is the
natural number usual ordering of the UD steps).
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 --
OK.
there is no "selection" of branches involved.
In the 3p outsider picture? OK. But when Alice obtain a result, she
selects, from her person pov, a specific term of the wave. Obviously,
the statistics on such relative outcomes involves an apparent selection.
Look at the equation again. Where do you see more than the four
branches I have discussed?
Once Alice makes her choice, there is no problem keeping those four
terms, but if Alice and Bob are on different lightcome, it makes no
sense to say that the outcome will be correlated, and they will be
correlated only with the corresponding people they can meet, and the
particular result will get that first person plural contagion of the
superposition. It seems this is clear from Brown and Timpson
explanation.
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.
Literally, it is even more local, as it is a unitary transformation.
Just a rotation in a complex space. The non-locality appearance needs
the appearances of the outcomes, and thus of the relative outcomes
obtained and described in the diaries.
...........
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 then you should show me one term of the wave with some action at a
distance. But by defining a world by a structure closed for
interaction, that becomes impossible, unless you abstract the
"parallel terms" away.
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.
May be write a paper yourself. If you are right, you are on something.
I might reread Bell to show you the precise place where the collapse
is used implicitly. Not to much time right now.
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?
Certainly not me, unless they forget instrumentalism at the pause-
café, and make metaphysical statement which do not follow the theory.
They are interested in results, after all: reality can look after
itself!
Sure, but our conversation is not on practical thing, but on the
nature of reality.
Bruno
Bruce
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