On 06 Jun 2017, at 01:23, Bruce Kellett wrote:
On 5/06/2017 8:42 pm, Bruno Marchal wrote:
On 05 Jun 2017, at 05:52, Bruce Kellett wrote:
On 5/06/2017 12:19 pm, Bruce Kellett wrote:
On 4/06/2017 10:05 pm, Bruno Marchal wrote:
On 02 Jun 2017, at 03:01, Bruce Kellett wrote:
Your claim appears to be that Bell's theorem is not valid in MWI.
Bell's theorem is valid. His inequality does not even assume QM,
but just locality.
I agree, but that is not what you were implying above. It seems
that now you agree that the Bell inequalities assume only locality.
And a mono-universe, or a conservation of identity of Alice and Bob
from the beginning to the end of the experience. But that is no
more the case in the MWI. Everett explains already this when he
introduces what will be called decoherence. Decpherence is local.
Believe it or not, those things are not relevant to the derivation
of Bell's results. Besides, you simply contradict yourself -- you
said exactly the opposite a line or two ago.
?
But these inequalities are violated by experiment.
Yes.
That can only mean that the assumption of locality was wrong --
whatever interpretation of QM you adopt.
It means that locality and mono-universe cannot be maintained at
once. But non-locality is not proved, unless you believe that Alice
and Bob remains the same unique person all along, which is
necessarlly not the case in the MWI of the EPR-Bell situation.
They split, but they retain identity in each branch.
Yes, and?
I think that this important part of recent exchanges might have
got lost in the welter of to-and-fro.
Bruno accepts:
1. Bell's theorem (and the associated inequalities) are valid in
MWI.
2. Bell's theorem assumes only locality (not even QM -- it is
valid in classical physics also).
Locality, and identity preservation (or mono-universe, or
counterfactual definiteness: all go away with Everett).
No, they do not. And these are not essential for Bell's derivation
anyway.
Locality is assume for the Bell's inequality, and the mono-universe,
or counterfactual definiteness is implicit, in both EPR and Bell.
3. The Aspect et al., and subsequent, experiments demonstrate that
the Bell inequalities are violated.
Yes, but only from the points of view of one branch. But when we
look how the singlet state is handled in the MWI, the correlation
are apparent, but the results obtained by space-separated person
does not need to be correlated, in some absolute sense, they need
to be correlated with anyone interacting with both of them later,
so that in all branches, it will look like if there has been an
action at a distance, but all influence and information flows, and
splitting, go at a speed lower than light.
It might have escaped your attention, but Alice and Bob need not be
spacelike separated when they do their measurements on the spin
singlet. For example, Bob could be at all times in Alice's forward
light cone, so they are always in the same world -- as Alice splits
with her measurement, Bob splits along with her,
In Alice branch. But Bob split Alice in his branch, and they do not
need to be the same. There are few chance they could be the same.
decoherence is local and spread at sub-light-speed.
so that when he does his measurement he is in the same world as
Alice with her result as recorded in her lab book.
The two pairs of Alice and Bob can see that. There would be a non-
local influence if there was only one pair, but they "doubly" split,
to be short.
A lot of your prevarications over EPR stem from a failure to realize
that spacelike separations are not essential
Indeed.
-- the proof is valid for *any* separation.
Absolutely.
Usually, spacelike is assumed only to assure the independence of the
measurements made by Alice and Bob. But as long as they are truly
independent, they can be at any separation.
OK.
It seems to follow with the force of simple logic that:
4. Experiment shows that QM is non-local, even in MWI.
Bruno appears to reject this conclusion. I conclude that Bruno's
position is incoherent.
I will again ask you to tell me what is wrong with Michael Clive
Price explanation ... except that the web page is not available.
I have been through this before. I looked at Price again this
morning and was frankly appalled at the stupidity of what I saw.
Have you a link. The webpage seems to be no more available at
https://www.hedweb.com/manworld.htm
Let me summarize briefly what he did. He has a very cumbersome
notation, but I will attempt to simplify as far as is possible. I
will use '+' and '-' as spin states, rather than his 'left', 'right'.
He write the initial wave function as for the case when you and I
agree in advance to have aligned polarizers:
|psi_1> = }me, electrons,you> = |me>(|+-> - |-+>)|you>
= |me, +,-,you> - |me,-,+,you>
He says that at this point no measurements have been made, and
neither observer is split. But his fundamental mistake is already
present.
A little test for you: what is wrong with the above set of equations
from a no-collapse pov?
skipping some tedium, he then gets
|psi_3> = |me[+],+,-,you[-]> - |me[-],-,+,you[+]>
where the notation me[+] etc means I have measured '+', you[-] means
you have measured '-'.
He then claims that the QM results of perfect anticorrelation in the
case of parallel polarizers has been recovered without any non-local
interaction!
Spoiler -- in order to write the final line for |psi_1> he has
already assumed collapse, when I measure '+', you are presented
*only* with '-', so of course you get the right result -- he has
built that non-locality in from the start.
?
From the start shows that it is local.
He then considers the case of polarizers initially set at right
angles, but he makes exactly the same mistake so I will not go
through that case here.
I am not alone skeptical about inferring that the violation of the
Bell inequalities shows action at a distance. What is wrong in
Deutsch and Hayden? What is wrong in Rubin (Rubin, M.A. Found Phys
Lett (2001) 14: 301. doi:10.1023/A:1012357515678), or in Maudlin's
book?
They don't all necessarily make the same mistake as Price, but they
all make equally silly mistakes, and build in the non-locality
without realizing it.
It is build-in locally.
Last year I analysed the argument by Tipler (arxiv:quant-ph/
0003146v1) in detail and showed where he made exactly this mistake
of building the non-locality in without realizing it. I recently re-
posted here a summary of these discussions from last year. A similar
analysis that I posted here last year of the papers by Rubin found
similar mistakes.
I suggest you write a paper. Something like "MWI is still non-local".
It seems obvious that both Bell and EPR assumes the identity of the
observers, who prepare the singlet state and measure the
correlation, but this is simply made false in the MWI.
The trouble here is that you have simply presented an argument from
authority,
?
and have used some less than fully competent authorities.
*that* is an argument from authority. I find them clear. I don't see
any important mistake in their treatment. I don't even see any sense
in your reply to the recent explanation by Saibal Mitra.
I suggest you look at the recent review of Bell non-locality that I
mentioned before.: Brunner et al., (arxiv:1303.2849). I attach the
important sections from this paper here for your convenience.
I read it sometimes ago. An excellent paper. But it does not address
our MWI issue at all.
I particularly draw your attention to the footnote on page 2 of my
excerpt:
"It is relatively frequent to see a paper claiming to "disprove"
Bell's theorem or that a mistake in the derivation of Bell
inequalities has been found.
This does not apply to me or people in this list. We find no error in
Bell's derivation. Nor in Aspect experience, which definitely prove
the existence of "local non locality", but with the many worlds, that
necessarily apparent non-locality does not involve any light speed
influence.
However, once one accepts the above definition of the independence
of the measurements, it is a quite trivial mathematical theorem that
this definition is incompatible with certain quantum predictions.
Only by adding the unicity of the "universe" or of the "Alice-Bob
couple", or of "counterfactual definiteness", or of a instantaneous
collapse, etc.
Such papers are thus either using (possibly unaware) a different
definition of locality or they are erroneous."
I must admit that I prefer the opinion of competent physicists such
as the author of this review over flaky derivations such as those by
Price, Tipler, Rubin, and those who follow them
No one here think that Bell's made any mistake. But QM's violation of
Bell's inequality, confirmed by Aspect (and the whole of experimental
QM) shows only that Bell's inequality are violated, and assuming the
absence of genuine physical infuence at a distance, this imposes the MW.
Brunner does not aboard that question. He does not cite Everett, or
the MW at all, and seems to assume the usual QM with some implicit
collapse.
He mention Kent (who disbelieve in the MW) and says only
"Similarly, how do we know precisely
when a measurement is completed without making some
assumptions on the collapse of the wavefunction (Kent,
2005)? This last issue was addressed in (Salart et al.,
2008) where the violation of Bell inequalities for events
that are space-like separated according to a simple model
of gravitational collapse has been reported."
Bruno
Bruce
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