From: *Brent Meeker* <meeke...@verizon.net <mailto:meeke...@verizon.net>>
On 4/22/2018 9:19 AM, Bruno Marchal wrote:
It follows from both QM and Comp. If Alice and Bob are
space-separated, I cannot even makes sense of how you can measure
correlations, given that once they are separated, whatever result
they got, will be shared with different Alice and Bob in different
branch. I am not even sure we can define what could be an action
at a distance in the quantum formalism. The notion does not even
makes sense when we assume special relativity. The only reason to
believe this is the habit to think that there is only one bob and
one Alice, which makes no sense once separated, unless they are
correlated with a third observer, but then, again by looking at
the wave without collapse, there will be no action at a distance.
The no locality is only an appearance due to the fact that we
belong to infinities of histories, and cannot known which one we
are in.
It depends on what you mean by "action at a distance". The theory you
are depending on for these pronouncements entails that, on a MW
picture, some of the possible worlds have probabilities that go to
zero as a result of an interaction at Alice or at Bob. So an
interaction at one of them changes the probabilities at the other.
For Bruno, it seems that "non-locality" means "action at a distance",
where he interprets that to mean that there is some superluminal
transfer of information, by tachyons or some such. And he is quite right
to say that there is no such interaction or dynamics in quantum theory.
Because if "non-locality" meant some superluminal transfer of
information, by particles or something else, then that would be giving a
*local* explanation of non-locality, which is a contradiction. So
non-locality can never mean "action at a distance", it can only mean
that the theory is such that the state is not separable, and changing
one end automatically changes the other, just as pushing one side of a
billiard ball moves the other side as well. (Ignoring the problems of a
relativistic explanation of extended physical objects. This is not a
particularly good analogy, but it is the best I can think of at short
notice!) In quantum mechanics, there can be no "mechanical" explanation
of the non-locality inherent in the non-separable state. That is why we
call it "non-locality" rather than "action at a distance".
I acknowledge that there are linguistic problems here, but that is just
the nature of quantum mechanics, and we have to live with it. Trying to
"explain" this fact further is bound to fail, because there is no deeper
explanation.
Bruce
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