On 4/22/2018 6:50 PM, Bruce Kellett wrote:
From: *Brent Meeker* <[email protected]>
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.
I think one way to look at it is, if a hidden variable explanation
requires that the hidden variable be FTL, then the phenomenon is non-local.
If find it hard to see how Bruno can say whether or not his theory is
non-local since he has not derived any concept of space, time, and a
Lorentz metric. I would think that would be a minimum before you could
claim a theory had not FTL signaling.
Brent
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