From: *Bruno Marchal* <marc...@ulb.ac.be <mailto:marc...@ulb.ac.be>>
On 6 Jul 2018, at 14:18, Bruce Kellett <bhkell...@optusnet.com.au
<mailto:bhkell...@optusnet.com.au>> wrote:
From: *Bruno Marchal* <marc...@ulb.ac.be <mailto:marc...@ulb.ac.be>>
On 5 Jul 2018, at 17:20, Lawrence Crowell
<goldenfieldquaterni...@gmail.com
<mailto:goldenfieldquaterni...@gmail.com>> wrote:
John Bell proved that any objective theory giving experimental
predictions identical to those of quantum theory is necessarily
nonlocal.
Assuming a unique reality. I prefer the term “inseparable”, because
“non-locality” is often interpreted the existence of FTL influence
(even if they cannot be used to transmit information), but such FTL
influence seems to me suspicious. Some might disagree, but I have
not yet seen a proof that any FTL subsists when we abandon the
collapse postulate. Bell assumes that experiments gives univocal
results.
You might not have seen a proof that non-locality remains when we
abandon the collapse postulate, but that does not mean that no such
proof can be given.
Consider the following scenario. Alice and Bob are given a large
number of entangled pairs, which they measure when they are at large
spacelike separation. Each measurement is made at some angle, and
gives a '1' for 'up' or 'passed', and '0' for the opposite result.
Both record the sequence of such results that they obtain in their
individual lab books, together with the corresponding polarizer
orientations. Their lab books then contain a random sequence of say
N, '1's and '0's. There are 2^N possible such sequences in the
many-worlds case, but since each observer keeps the same lab book for
the whole sequence, each series of measurements is necessarily made
in the same one world.
I am not sure I understand the idea of being in the same world when
space-like separated.
Who said anything like that? They end up in the same world when they
meet. Or do you disagree with that as well?
Each time one of them makes a measurement, they are localising
themselves in different worlds. The pair state only entails that their
measurement will fit accordingly,
How? You are just assuming the non-local result that you are claiming is
local. You are not consistent.
but Alice will meet the Bobs she is correlated with, and vice versa.
It does not make sense to say that Alice will meet the original Bob,
or something like that.
Who is the original Bob? You are starting to sound like John Clark in
refusing to accept the consequences of duplication. In your duplication
thought experiments (as in step 3 of the UDA) you talk about each
duplicate keeping a diary and recording W or M as appropriate. After a
long sequence of duplications, each resulting copy will have a diary
with a long sequence of Ws and Ms at random. This is exactly what is
happening with the lab books in my example above. One copy of Alice
meets with one copy of Bob. But when they meet, they are in the same
world, and their lab books record the experiences of that particular
realization of the long chain of Alices and Bobs. You should remember
that there are 2^N such chains of experiences, and after the 2^N runs of
the experiment, when any Alice copy meets the corresponding Bob copy,
the same argument holds-- they are in the same world, and their lab
books record the sequence of results that the obtained in the world that
they happen to inhabit.
Basically, this is because the worlds are disjoint, and the observers
and/or lab books cannot move between worlds.
Any measurement entails new differentiation.
When Alice and Bob meet up at the end of the run of N trials,
Each of Alice and Bob will meet only the Bob and Alice prescribed by
the result of their measurement. You need to look at the entire wave
function.
Why? An Alice copy meets a Bob copy and they compare notes. Any time
this happens the results in their lab books must confirm the quantum
correlations. Or do you not agree with this? The trick is to understand
how this happens. You are not giving an explanation -- you are relying
on some unspecified magic!
they take their lab books with them. When they meet they are clearly
in the same Everettian branch.
“They” is ambiguous here.
Think about it and the ambiguity will disappear. "They" are any of the
2^N copies of Alice and Bob. (But the copies are, themselves correlated.)
And since their lab books cannot have jumped between branches, the
sequence of results that they each bring must also have all been
recorded in this same one branch. So when they come to use their data
to calculate the correlations between the measurements on their
individual particles of the entangled pairs, they are in exactly the
same situation as they would be if they had assumed a collapse model
from the outset.
It is like they find themselves in the relevant partition of the
mutilverse, but as there has not been any collapse, nothing has needed
to propagate after than light. The non-locality, or better inseparability,
You are choking on words. Non-locality just means that the state is not
separable, so what happens at one end affects the other end. If there is
a FTL interaction, that is a non-local hidden variable. But as Saibal
points out, there is no proof that there must be hidden variables --
there is just a proof that there is non-locality, whether or not you
want to explain that with hidden variables.
just assures that whatever differentiation will occur locally, they
will have the correlated spin,
That is what the non-locality or non-separability gives you. I think you
are just choking on the word "non-local". It doesn't have to mean FTL,
you know -- it could just be magic. Or space and time are not what we
think them to be.
but at no point are we assured that Alice meet something like the
original Bob. The differentiation of the universe develops locally.
Once Alice and Bob are space-light separated, they will never meet
again after they made local measurement.
What utter and complete twaddle. You are spacelike separated from your
wife when you each get up in the morning. Do you really think that this
means that you can never meet her again?
Each will meet only the corresponding (correlated) person, but there
is no reason we can identify them in any single word.
So your wife is a different person each time you meet her? You are lost
in word salad again.
The correlations they observe are necessarily single-world correlations.
That comes true after their measurement. But the world have
differentiated before.
So the conditions of Bell's theorem are exactly satisfied,
I don’t think so. All outcomes are realised (assuming the singlet
state, and measurement in any direction). Each Bob and Alice have
localised themselves in the corresponding branches, and will met only
their corresponding partners, due to the local further separation
obtained by their local measurement. That is inseparability. It does
not require simultaneous action at a distance.
Who said it did? Non-separability is the same as non-locality. Neither
implies FTL transfer of a physical messenger.
and since the correlations violate the Bell inequalities, their
experiment has demonstrated the impossibility of a local hidden
variable account.
I agree with this. That is indeed why a world or an entire history is
more like a global “hidden variable”, making sense of those
correlation in a local way, with differentiation occurring locally,
but always ensuing the existence of the correlation.
You just claim that it is local because you have not understood what is
really going on. Playing with words, yet again.
They have demonstrated that the quantum correlations require
non-locality, even with Everett's many-worlds, just as Bell proved.
I can be OK with this conclusion, unless you imply that in Everett
there is still something travelling faster than light.
Where do I claim such a thing? You should remember that Maudlin wrote a
whole book on the reconciliation of non-locality with special
relativity. Everett's many worlds does not achieve this reconciliation.
And all this happens whether they assume many-worlds or a collapse model.
The collapse, if taken in its usual non local (instantaneous) sense,
that Einstein criticised already in 1927, would need to act FTL to
explain the correlation. But that is not needed in Everett. The states
are relative to each others, and further measurement are themselves
propagating locally, ensuring than all the many Alices and Bobs will
have their spins correlated, but they are no more necessarily related
to the original Bob and Alice in any univocal way.
It is always accepted that information is transmitted by classical means
at less than the speed of light. Decoherence means that entanglements
spread at the speed of light or less. This is true, that is why the
correlations of non-separable, non-local states are hard to understand.
Many worlds does not resolve the issue. Everett has not solved anything,
as my account of the experiment proves beyond doubt.
Bruce
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