B Why is it assumed communication through entanglement would be FTL?

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No, relativity does not become incorrect if you add preferred reference frame to it. Only consensus interpretation of relativity becomes invalid.
I disagree. There is no sensible definition of "preferred" that would be self-consistent.
 

zonde

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He does quite the opposite.
Really? "Correlations have physical reality; that which they correlate does not." does not mean correlations fall from the sky but quite the opposite?
We have communication problem.

So? Who claimed that it does?

...

If I had to bet money, I would always go for non-realism/contextuality or whatever you would like to call it.
You claim the very thing in the same post. You say non-realism/contextuality allows violation of Bell inequality without non-locality. Unless you attribute non-realism/contextuality to detections themselves you are proved wrong by Eberhard.
 

zonde

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I disagree. There is no sensible definition of "preferred" that would be self-consistent.
I'm not sure I understand. You are a bit too short for me to get what you mean.
 
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No one is questioning relativity per se. Experiments support it.
Yes, experiments support it, but I disagree that relativity is not being questioned, at least unconsciously.

(And as a point worth mentioning, the equations of relativity are time symmetric anyway. So that could potentially explain the appearance of quantum nonlocality, although that is speculation at this time.)
But qft is a strictly local theory. I have an entire textbook devoted to that very thing, "Local Quantum Physics: Fields, particles, Algebras," Rudoplf Haag.

But there seems to be a gray area when it comes to entangled systems: they have spatial extent but act as if that extent doesn't constrain the system as might be otherwise expected.
And in the typical Bell type experiment, the detectors are separated from the source by null intervals, since obviously, photons live on the light cone. So between the source and each detector, the distance is zero, regardless of the spatial seperation between the detectors. That seems to be a missed point.

And on the quantum side: ordering of the following makes no discernible difference to the outcome in any reference frame:

a. Pair A and B entangled.
b. A measured.
c. B measured.
I really have no idea what you are trying to say here.
 
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I'm not sure I understand. You are a bit too short for me to get what you mean.
Define a "preferred frame" that is consistent with relativity. The only definition of "preferred" that makes sense is one in which the measured values are some "true values" to which other frames can be referenced and not lead to contradictions.
 

zonde

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I'm using it in the same sense as in probability theory. See Chapter 6 of Streater's "Lost Causes in Theoretical Physics". He has a good explanation of it.
I gave definition of "sample space": In probability theory, the sample space of an experiment or random trial is the set of all possible outcomes or results of that experiment.
Is that the definition you are using?

It seems that the book you gave was freely available online some time ago but it is no more. But "sample space" is such a basic concept in probability theory that it should not require specific book to give it's definition. Googling gave plenty explanations of this concept and they were all consistent with definition I gave.
 

zonde

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Define a "preferred frame" that is consistent with relativity. The only definition of "preferred" that makes sense is one in which the measured values are some "true values" to which other frames can be referenced and not lead to contradictions.
You can take any inertial reference frame in SR and label it as preferred. Then label any measured values in that reference frame as "true values" and measured values in other inertial reference frames as "apparent values". No inconsistency with SR arises because nothing in the math of SR was changed by that labeling.
Lorentz transformation ensures any "apparent values" in other reference frames can be transformed into "true values" of preferred reference frame.
That labeling just introduces asymmetry on top of SR that SR by itself does not have.
 
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As far as I can tell nobody actually knows how quantum entanglement really works,[...]
Sorry, this thread is predicated on a falsehood. Entanglement is a known and understood consequence of many particle quantum mechanics. And it does not enable communication of any kind, faster or slower than the speed of light.
 

Demystifier

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But that cannot be correct. The measurements are spacelike separated, so they are made simultaneously. Neither measurement can occur "before" the other and create a cause and effect relationship because they cannot be time ordered. You would have to reject relativity. Apparently, what people must be arguing about is whether relativity is correct.
You would have to reject fundamental relativity, but you would not have to reject effective relativity valid only under restricted set of circumstance. This restricted set of circumstances must involve all experiments performed so far, but one cannot exclude the possibility that some future experiments, perhaps experiments with the next generation of particle colliders, will show violations of relativity.
 

Cthugha

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Really? "Correlations have physical reality; that which they correlate does not." does not mean correlations fall from the sky but quite the opposite?
We have communication problem.
I am really and honestly puzzled as I really think this is trivial. If you have two quantities that are connected via some kind of uncertainty relation, you can prepare states, in which one of these properties is well defined, while the other is not (or both are defined to some extent). If I prepare a state of known position, momentum is ill-defined and vice versa. For entangled states, entanglement and coherence in the entangled property are another pair of properties connected via an uncertainty relation. Entanglement boils down to the presence of non-classical correlations of some chosen property. Coherence boils down to sharply defined values of this property. So if I prepare a state such, that the individual properties (e.g. the spin of two photons) are well defined, well-defined non-classical correlations cannot exist. If I prepare a state with well-defined non-classical correlations, well-defined values of the individual properties cannot exist. Correlations are an element of reality in the latter case, while the correlated quantities are not. In order to achieve this, you simply have to prepare an entangled state (or equivalently prepare well-defined non-classical correlations - and to emphasize it again: ONLY the correlations, not the individual spin/energy/OAM/polarization/whatever values). You need to explicitly prepare the state as such. I do not see why explicit preparation of such a state would correspond to falling from the sky.



You claim the very thing in the same post. You say non-realism/contextuality allows violation of Bell inequality without non-locality. Unless you attribute non-realism/contextuality to detections themselves you are proved wrong by Eberhard.
I disagree. Eberhard explicitly develops several concepts of locality in his paper(s) (Bell's Theorem and the Different Concepts of Locality, Il Nuovo Cimento 46, 392 (1978)). Concepts 1-3 more or less lead to deterministic or probabilistic LHV theories and are at odds with quantum theory. Notion 4 is the simple relativistic notion of locality, which means no FTL-signaling and is not at odds with quantum theory as explicitly pointed out by Eberhard. This is the relevant notion of locality in this case.
 

DarMM

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I gave definition of "sample space": In probability theory, the sample space of an experiment or random trial is the set of all possible outcomes or results of that experiment.
Is that the definition you are using?

It seems that the book you gave was freely available online some time ago but it is no more. But "sample space" is such a basic concept in probability theory that it should not require specific book to give it's definition. Googling gave plenty explanations of this concept and they were all consistent with definition I gave.
I'm not using Streater to give a definition of sample spaces, as I said I am using the normal definition in Probability theory as Wikipedia gives. He just gives a good example of how QM uses multiple sample spaces.
 
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relativity does not become incorrect if you add preferred reference frame to it. Only consensus interpretation of relativity becomes invalid.
Discussion of such alternate interpretations of relativity is out of bounds here at PF. Please do not refer to it.
 

vanhees71

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I do not see how the assumption that QFT as outlined by @vanhees71 is classically local causal is warranted. Essentially, it boils down to Mermin's tongue-in-cheek statement (American Journal of Physics 66, 753-767 (1998) , https://arxiv.org/abs/quant-ph/9801057):
Just to make it very clear: I do not state that QFT is classically local. That contradicts the plethora of Bell tests that prove QFT to be not compatible with local deterministic HV models. Mermin's article, of course, is a gem!
 
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Cthugha

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Just to make it very clear: I do not state that QFT is classically local.
Also just to make it clear: I intended to say that it is not warranted to assume that you think that QFT is classically local. I did not intend to say that your position is unwarranted because you think that QFT is classically local. My post might have been easy to misinterpret. ;)
 
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To the contrary it's ruled out by the very construction of this QFT as made explicit in Weinberg's books much more explicitly than in most other QFT textbooks.
It's not quite true its ruled out - things just come out more rationally and elegantly if you rule it out. It like the aether in LET. I can't rule LET out - all I can do is give a very elegant explanation of SR based on symmetry rather than an inherently undetectable aether. Things get more and more difficult the deeper you go into areas like GR and QFT. Its scientific tact really that's actually at the base of what's in many of our theories. I know its not entirely satisfactory things in science are like that. I believe the cluster decomposition property as explained by Weinberg makes things harder if you allow the concept of locality to apply to correlations. Of course you can do it - but one must ask why?

Thanks
Bill
 

vanhees71

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What do you mean by "the concept of locality to apply to correlations"? For me it doesn't make sense to say "correlations are local or nonlocal". If you refer to entanglement, it's very clear that it describes correlations between far-distant parts of quantum systems, but this has nothing to do with locality or nonlocality of interactions, i.e., microcausality. There's no constradiction between microcausality and inseparability. Entangled states are no problem within standard relativistic QFT. It's even the rule rather than the exception since already the Bose or Fermi nature of indistingushbable particles implies usually entangled states: Product states are rare since you need to symmetrize or antisymmetrize them, and that's all done automatically using the field operators to describe the states.
 

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