I Objective Wave Function and Non-locality

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In interpretations where the wave function represents something real, like Many worlds, Copenhagen with objective wave function and spontaneous objective collapses. I'd like to understand which of them has true non-locality.

First. Is Many Worlds not having true non-locality due to the randomness output in Alice and Bob observations? Does this mean there is no non-locality in principle?

To rephrase it. In Copenhagen with objective collapse. The non-local correlations use random encryption (meaning nature randomizes the outputs so you can't use it to send message faster than light). Likewise, does Many worlds also use randomness encryption? Meaning there is true non-local correlations only you can't use it to travel faster than light? Or is Many worlds correlations just classical (but is the reasoning its classical because the observers are arbitrarily choosing and matching the random outputs of A and B sound?).
 
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I'd like to understand which of them has true non-locality.
What do you mean by "true non-locality"?

Non-locality in the sense of violating the Bell inequalities (which is the common definition of that term) is an experimental fact; it is not dependent on which interpretation of QM you adopt. If that's what you mean by "true non-locality", then all QM interpretations have it, since they all have to be consistent with the known experimental facts.

If you mean something else by "true non-locality", then what is it?
 
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What do you mean by "true non-locality"?

Non-locality in the sense of violating the Bell inequalities (which is the common definition of that term) is an experimental fact; it is not dependent on which interpretation of QM you adopt. If that's what you mean by "true non-locality", then all QM interpretations have it, since they all have to be consistent with the known experimental facts.

If you mean something else by "true non-locality", then what is it?
Yes. I mean "true non-locality" in the sense of violating the Bell inequalities. But in Many worlds, non-locality is explained away as just classical correlations (since all outputs exist), is it not?
 
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in Many worlds, non-locality is explained away as just classical correlations (since all outputs exist), is it not?
"All outputs exist" is not the same as "outputs exhibiting correlations that violate the Bell inequalities exist in the same branch of the wave function". The latter is what we observe experimentally (interpreting experimental results according to the MWI). Even though all outputs exist in the MWI, it's still a highly non-trivial problem to show that the outputs in a particular branch of the wave function will match up the way they need to to exhibit the experimentally verified violations of the Bell inequalities. I don't think "classical correlations" is sufficient for that.
 
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I mean "true non-locality" in the sense of violating the Bell inequalities.
Then, as I said, all QM interpretations have "true non-locality", because violations of the Bell inequalities are established experimentally, and all QM interpretations must be consistent with the experimental facts.
 
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Then, as I said, all QM interpretations have "true non-locality", because violations of the Bell inequalities are established experimentally, and all QM interpretations must be consistent with the experimental facts.
Is the Bell inequalities non-local correlations 100% correlated?
I often heard it was just statistical. For example.

If Alice uses spin up, Bob gets spin down 10 billion light years away. So are the results:

Alice U D U U D U D D U
Bob D U D D U D U U D

Do Alice and Bob always get opposite correlations in the spin in their data? Or is there sometimes both UP or both Down? The Aspect Experiments seem to be quite complicated. Is there not any non-local experiments where you produce 100% direct correlations like the above? Although I know it can't use to transmit signal because of the randomness nature.
 
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Is the Bell inequalities non-local correlations 100% correlated?
For measurements made in appropriate directions, yes. But the full violation of the Bell inequalities requires collecting statistics for many different combinations of measurement directions.

Do Alice and Bob always get opposite correlations in the spin in their data?
If they both measure spin in the same direction, yes. But, again, the full violation of the Bell inequalities requires collecting statistics on a large number of measurements for many different combinations of measurement directions.
 
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For measurements made in appropriate directions, yes. But the full violation of the Bell inequalities requires collecting statistics for many different combinations of measurement directions.



If they both measure spin in the same direction, yes. But, again, the full violation of the Bell inequalities requires collecting statistics on a large number of measurements for many different combinations of measurement directions.
Why doesn't it work for only say 9 statistics in one direction only (or other observable equivalent to one direction only)? Why must it be thousands or more data?

Alice U D U U D U D D U
Bob D U D D U D U U D
 
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Why doesn't what work? What's the problem?
Can experiments be done between Earth and Saturn where the outputs were only like:


Alice U D U U D U D D U
Bob D U D D U D U U D
 
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Can experiments be done between Earth and Saturn where the outputs were only like
In principle, sure; just make sure Alice and Bob are both measuring spin in exactly the same direction. In practice, ensuring that is a highly non-trivial problem for measurements on Earth and Saturn. But in principle you could do it.

Again, what's the problem? I don't understand what you're having trouble with.
 
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In principle, sure; just make sure Alice and Bob are both measuring spin in exactly the same direction. In practice, ensuring that is a highly non-trivial problem for measurements on Earth and Saturn. But in principle you could do it.

Again, what's the problem? I don't understand what you're having trouble with.
Actual experiments always look at thousands or millions of data, and there didn't seem to be simply direct matches. That is why many still seem to believe the correlations was because of the initial setup (like Fedex sending you opposite blue and red socks in the pair at random and the correlations already occurred at the start).

Any reference for such experiments where Alice and Bob both measuring spin in the same directions. And results were simple like only:

Alice U D U U D U D D U
Bob D U D D U D U U D
 
The Bell inequality shows that no "local realist" hidden variable theory can reproduce what we actually - and demonstrably - measure in an EPR type experiment.

Since the Many Worlds interpretation and the Copenhagen interpretation are not hidden variable theories, the Bell inequality says nothing of them.

In quantum mechanics, the wave function explores all possible paths, and forms an interference pattern on the screen or in the mind of the observer.

Quantum mechanics is "nonlocal" in the sense that the wave function of a single particle explores all paths. In the double slit experiment, a single photon maps the paths through both slits. The interference on the screen is a "local" phenomenon.

Are classical water waves a "local" or "nonlocal" process? In newtonian mechanics, they definitely are a purely "local" process. There is no spooky action at a distance. In quantum mechanics we learn that even a single quantum can form a complex wave pattern, which would not happen in newtonian mechanics.
 
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Actual experiments always look at thousands or millions of data, and there didn't seem to be simply direct matches.
There are other ways of setting up experiments for which violations of locality do not require statistics but are simple direct observations: basically, the analogue of Bell's locality assumption for such cases implies that certain results should never occur, so observing those results disproves locality directly. AFAIK these always require more than two particles. An example is described in this classic GHZ paper:


Note that this paper also discusses how, for the case of two entangled spins measured in the same directions, a classical, local model can be constructed that explains the results; in other words, these measurements by themselves do not violate the Bell inequalities. That is why you need, for this case, to make many measurements with varying combinations of directions and do statistics on them to show Bell inequality violations.

Any reference for such experiments where Alice and Bob both measuring spin in the same directions.
I don't know that experimenters would consider this worth documenting, since nobody doubts the outcome and it doesn't give any useful information about Bell inequality violations. To show Bell inequality violations with a pair of entangled particles, as I've already said, you need to make measurements over many combinations of directions.
 

Nugatory

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Any reference for such experiments where Alice and Bob both measuring spin in the same directions. And results were simple like only:

Alice U D U U D U D D U
Bob D U D D U D U U D
I don't know that experimenters would consider this worth documenting, since nobody doubts the outcome and it doesn't give any useful information about Bell inequality violations.
Indeed, the result is so accepted and so confirmed that even back in the 1970s it was part of the undergraduate physics lab at my college. Of course the proposition being tested was not whether the quantum mechanical prediction was correct, it was whether the student was competent to set up the experiment and get repeatable results. I did get repeatable results, as did many tens of other students, and they were the expected opposite correlations.

And as @PeterDonis says, no one is going to bother writing up a result that’s so routine that was used as a lab exercise decades ago.
 

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The Bell inequality shows that no "local realist" hidden variable theory can reproduce what we actually - and demonstrably - measure in an EPR type experiment.
Yes, but it would be better to say that no local realist theory can reproduce the predictions of quantum mechanics. Phrased that way, it should be clear that
Since the Many Worlds interpretation and the Copenhagen interpretation are not hidden variable theories, the Bell inequality says nothing of them.
does not follow.
No interpretation of quantum mechanics is a hidden variable theory. The significance of Bell’s theorem is that QM cannot be incomplete in the EPR sense and this has nothing to do with your choice of interpretation.
 
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There are other ways of setting up experiments for which violations of locality do not require statistics but are simple direct observations: basically, the analogue of Bell's locality assumption for such cases implies that certain results should never occur, so observing those results disproves locality directly. AFAIK these always require more than two particles. An example is described in this classic GHZ paper:


Note that this paper also discusses how, for the case of two entangled spins measured in the same directions, a classical, local model can be constructed that explains the results; in other words, these measurements by themselves do not violate the Bell inequalities. That is why you need, for this case, to make many measurements with varying combinations of directions and do statistics on them to show Bell inequality violations.



I don't know that experimenters would consider this worth documenting, since nobody doubts the outcome and it doesn't give any useful information about Bell inequality violations. To show Bell inequality violations with a pair of entangled particles, as I've already said, you need to make measurements over many combinations of directions.
But why do professional physicists like Vanheez71 still seemed to believe the correlations occurred during the initial setup, like the blue and red socks pair delivered by Fedex? What part have I misunderstood. For example. He wrote this:

"It does not instantly the other entangled photon. Entanglement describes correlations which have been prepared at the moment where the photons were created, it does not describe an action at a distance when a measurement on one of these photons is made. The interactions are, according to the very fundamental construction of relativistic QFTs of which QED is the paradigmatic example, local and also only those QFTs are successful which obey the microcausality principle and thus ensure that there are no faster-than-light information transmitting signals possible."

Reference https://www.physicsforums.com/threads/confused-by-nonlocal-models-and-relativity.973876/#post-6198475
 
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why do professional physicists like Vanheez71 still seemed to believe the correlations occurred during the initial setup
He doesn't say the correlations occurred at the initial setup; he says they were prepared at the initial setup. He appears to prefer that language because he wants to emphasize microcausality in QFT (i.e., the fact that operators at spacelike separations commute). Nothing he says is inconsistent with the fact that QM/QFT predicts violations of the Bell inequalities.
 
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He doesn't say the correlations occurred at the initial setup; he says they were prepared at the initial setup. He appears to prefer that language because he wants to emphasize microcausality in QFT (i.e., the fact that operators at spacelike separations commute). Nothing he says is inconsistent with the fact that QM/QFT predicts violations of the Bell inequalities.
In QFT. The phase of the wave function of different locations can't be the same. They differ and this produced gauge freedom and gauge fields existed because of locality. Is this related to microcausality in QFT?

Can't the inherent non-locality in QM be in conflict with the locality requirement in QFT that should produced the forces of nature (gauge forces)?
 
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In QFT. The phase of the wave function of different locations can't be the same.
QFT isn't based on wave functions. It's based on quantum fields. I'm not sure what you're referring to here.

They differ and this produced gauge freedom and gauge fields existed because of locality.
I'm not sure what you're referring to here either. Giving some references for where you're getting all this from would help.

Is this related to microcausality in QFT?
Microcausality, as I said, means that field operators at spacelike separated events commute. I'm not sure how that relates to the other things you're saying.

Can't the inherent non-locality in QM be in conflict with the locality requirement in QFT that should produced the forces of nature (gauge forces)?
What locality requirement in QFT are you talking about? If you're using "locality" to mean "microcausality", then no, since microcausal QFT predicts the same Bell inequality violations that are observed.
 
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QFT isn't based on wave functions. It's based on quantum fields. I'm not sure what you're referring to here.



I'm not sure what you're referring to here either. Giving some references for where you're getting all this from would help.
I was assuming local symmetry in QFT has to do with locality. I think it has to do with the internal symmetry space and this isn't connected to actual space? The author of Deep Down Things stated that gauge symmetry and EM fields came about because the phase of wave function be can't correlated in different points in space, so the EM gauge fields are due to the counterterms that should make the equation still be invariant under global symmetry.

"Many powerful theories in physics are described by
Lagrangians that are invariant under some symmetry transformation groups. When they are invariant under a transformation identically performed at
every point in the spacetime in which the physical processes occur, they are said to have a
global symmetry. Local symmetry, the cornerstone of gauge theories, is a stronger constraint. In fact, a global symmetry is just a local symmetry whose group's parameters are fixed in spacetime (the same way a constant value can be understood as a function of a certain parameter, the output of which is always the same)."

Microcausality, as I said, means that field operators at spacelike separated events commute. I'm not sure how that relates to the other things you're saying.



What locality requirement in QFT are you talking about? If you're using "locality" to mean "microcausality", then no, since microcausal QFT predicts the same Bell inequality violations that are observed.
 
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I was assuming local symmetry in QFT has to do with locality.
It is related to microcausality, since local gauge symmetries affect the behavior of the field operators. But as I've already said, microcausality is not in conflict with violations of the Bell inequalities.
 
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It is related to microcausality, since local gauge symmetries affect the behavior of the field operators. But as I've already said, microcausality is not in conflict with violations of the Bell inequalities.
So
gauge transformations alter the wave function, but only in the mathematics, not in the physics, so gauge transformations are not in any way related to non-local correlations?
 
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Let's return to microcausality (a concept I only learnt today). I just checked atyy definition of it (#159).


"Since this is important. Let me restate what the correct meaning of microcausality is. Microcausality is a sufficient condition to prevent superluminal signalling.

If spacelike observables do not commute, then measuring one will change the probabilities at a distant location, enabling superluminal signalling. So spacelike observables must commute. In the Heisenberg picture, the observables evolve with time. Then the cluster decomposition is a condition that ensures that even under time evolution, spacelike operators continue to commute.

The important point is that "no superluminal signalling" is not the same as "classical relativistic causality".

So QFT doesn't allow any superluminal signaling. Does this mean QM also doesn't allow it (if there is no randomness in the observable)? But what is the microcausal equivalent in QM that forbids it?

I guess this is different concept to gauge transformation which doesn't affect the physics at spacetime points.?
 

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In Copenhagen with objective collapse.......
The "Copenhagens" see the wave function collapse in a complete different way. For example, Carl Friedrich von Weizsäcker in “The Structure of Physics”:

“The ψ-function is defined as knowledge. The reduction of the wave packet is not a dynamical evolution of the ψ-function in accordance with the Schrödinger equation. Rather, it is identical to the event in which an observer recognizes a fact. It does not happen so long as only the measured object and measurement apparatus interact, nor so long as the apparatus has not been read out after the measurement interaction ends; it is the gain of knowledge associated with reading.” [italics in original]
 

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