Long-distance correlation, information, and faster-than-light interaction

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  • #76
Ken G
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Well, personally, I'm not for nonlocal realism because I don't like the LET assumption or non-forward causality. Basically my view is nonlocal nonrealist.
What would you call an approach that says this:
A system is a physical setup that is established by a preparation and has the function of associating any hypothetical measurements we can describe with a set of probability distributions, including two-point correlations (and higher). Quantum mechanics provides us with the instructions for connecting the preparations to the probabilities.

To me, that description is all we need to use the system concept in science. I see that it does not necessarily invoke realism in the sense of independent parts owning their own unique probabilities that can only be changed by "influences", we only invoke that picture when it is actually working for us (which is, as I said, when the influences are subluminal and can themselves be observed by intercepting them), so that gets called "nonrealist." But it's the "realist" approach that seems unrealistic to me. In any event, what I just said never encounters any issue with nonlocality, either in terms of Bell's theorem in EPR systems, or in terms of fermionic indistinguishability in white dwarfs.
 
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  • #77
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What would you call an approach that says this:
A system is a physical setup that is established by a preparation and has the function of associating any hypothetical measurements we can describe with a set of probability distributions, including two-point correlations (and higher). Quantum mechanics provides us with the instructions for connecting the preparations to the probabilities.
I don't know if that's local.

In any event, what I just said never encounters any issue with nonlocality, either in terms of Bell's theorem in EPR systems, or in terms of fermionic indistinguishability in white dwarfs.
It can still be nonlocal by hidden assumption.
 
  • #78
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Well, personally, I'm not for nonlocal realism because I don't like the LET assumption or non-forward causality. Basically my view is nonlocal nonrealist. I just can't grasp your argument: I don't know about that. Surely I am intellectually limited because otherwise I could acknowledge you or rebuttal further. If I could see a fully laid-out exposition on a paper, or a completely different explanation (maybe written by someone else, since your style doesn't seem to be able to convey the concept to me), that'd help.
Chapter 6 of Quantum Theory Concepts and Methods by Asher Peres is well argued. It is here, free.
 
  • #79
Ken G
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I don't know if that's local.

It can still be nonlocal by hidden assumption.
The system could be called "nonlocal" in the sense that it is holistic, like a field can be regarded as holistic. But it's not the system that gets called nonlocal in a lot of places (including on this forum, which I was reacting to initially), it is the influences that do. That's what I object to-- if you regard the system I'm talking about as a "nonlocal system", I don't have any objection. I object to the phrase "nonlocal influences", because that implies we have a propagation of an influence that moves faster than light and even back in time. My approach has none of that.
 
  • #80
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The system could be called "nonlocal" in the sense that it is holistic, like a field can be regarded as holistic. But it's not the system that gets called nonlocal, it is the influences that do. That's what I object to-- if you regard the system I'm talking about as a "nonlocal system", I don't have any objection. I object to the phrase "nonlocal influences", because that implies we have a propagation of an influence that moves faster than light and even back in time. My approach has none of that.
What would you say about the book Mentz linked? Is that your position?
 
  • #81
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What would you say about the book Mentz linked? Is that your position?
Peres argues that in EPR it is impossible to define an observer independent concept of 'before' and 'after.' Using this concept along with particle oriented ideas leads to non-understanding. There's a lot more than that in the chapter.
 
  • #82
Ken G
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What would you say about the book Mentz linked? Is that your position?
I haven't read the book, but from a few short snapshots, I'd say the author is quite careful not to add anything more than is necessary, so I think we are largely in agreement. For example, he says:
"A quantum system is a useful abstraction, which frequently appears in the literature, but does not really exist in nature. In general, a quantum system is defined by an equivalence class of preparations. (Recall that “preparations” and “tests” are the primitive notions of quantum theory. Their meaning is the set of instructions to be followed by an experimenter.)"

So there is no requirement that the system be comprised of "independent parts", that is no kind of fundamental attribute of a system in his approach, and that's what I am advocating as well. He goes on:
"We can then define a state as follows: A state is characterized by the probabilities of the various outcomes of every conceivable test."
So yes, that's what I'm talking about. It's the set of tests that matter, not the set of "real parts". The concept of a part is quite optional, it only works in some situations (though granted, quite a lot).
 
  • #83
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Then I'll try to work through that chapter. Thanks both.
 
  • #84
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Explain how a delayed choice quantum eraser "changes" a correlation.
After a delay of several ns a random event occurs that effects the results of an entangled partner that already hit another screen and was detected to have which path information "preserved" showed a which path pattern. When the path detection was erased the interference pattern was observed.
The correlations are specified by the preparation.
The correlations occur only when a random obfuscation occurs later.
 
  • #85
Ken G
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After a delay of several ns a random event occurs that effects the results of an entangled partner that already hit another screen and was detected to have which path information "preserved" showed a which path pattern. When the path detection was erased the interference pattern was observed.

The correlations occur only when a random obfuscation occurs later.
There is no need to assert when the correlations occur, if we simply say that the system comprises of the preparation and all the probabilities associated with all the possible measurements. There is no "change", there is only a decision about what measurements to do. It makes no difference when those decisions were made, there are no changes-- unless we insist on believing in the concept of independent probabilities for the "parts." That's the same idea that gives us retrocausality, I say stick to only what we need, and both the idea that there are "influences", and also the idea that "correlations change", go away immediately.
 
  • #86
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In the latter case, how do you consider it with respect to different inertial frames? In a frame where Bob measures first, it's Bob influencing FTL and Alice's side is being influenced, and vice-versa?
Yes, and the cute thing about QM is that it's all the same how you look at it. It's some initially random common property that gets changed to match/oppose the setting of the "first" detector, then at the "second" detector it gets changed again for the second particle with probability depending on the first value... Statistically it works out to be equivalent in either order.

If you insist on using deterministic simulation with a set value for the property even before the measurements, you do have different evolution depending on how you look at it, but still both possibilities are ok, so throw a coin or otherwise "predetermine" which one you want to use the same way you predetermined the initial hidden property. In the end it just doesn't matter.
 
  • #87
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If you insist on using deterministic simulation with a set value for the property even before the measurements
Interesting you say deterministic. What's the relationship between hidden variables / realism (I reckon that they're the same) and determinism? Which one implies the other?
 
  • #88
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Interesting you say deterministic. What's the relationship between hidden variables / realism (I reckon that they're the same) and determinism? Which one implies the other?
For any randomness in a model you can just as easily substitute a supposedly deterministic mechanism dependant on unknown "hidden variables". If you should is another question entirely - if they stay hidden forever, Occam's razor may have something to say about them.
[edit: got confused that we're in the other thread from which we got split earlier, and posted some nonsense bits originally. now removed]
 
  • #89
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There is no need to assert when the correlations occur, if we simply say that the system comprises of the preparation and all the probabilities associated with all the possible measurements.
In this case there is obvious reason and sufficient data recorded to determine when interference occurs and when classical trajectories occur, based on the geometry of the preparation alone.
It makes no difference when those decisions were made, there are no changes-- unless we insist on believing in the concept of independent probabilities for the "parts."
How do you relate a detection at D0 which occurs first with particle or wave behavior depending on the entangled partner being detected later either in one slit or both?
 
  • #90
Ken G
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In this case there is obvious reason and sufficient data recorded to determine when interference occurs and when classical trajectories occur, based on the geometry of the preparation alone.
Then please say when the correlations occurred, and how that means any correlation ever "changed" in a delayed choice experiment. It seems to me you are simply applying the philosophy of local realism plus nonlocal influences, but no data forces that interpretation upon us. My interpretation is that correlations do not exist until sufficient measurements have occurred that could demonstrate those correlations. Using that simple rule, they never "change" in any experiment, unless you do a different experiment, in which case it makes perfect sense it could produce different correlations. All we need to understand is why a different experiment produces different correlations, and quantum mechanics already does that for us, in a way that is perfectly consistent so does not "change."
How do you relate a detection at D0 which occurs first with particle or wave behavior depending on the entangled partner being detected later either in one slit or both?
It makes no difference to me at all when the detections occur. All physics does is connect a set of measurements to their expected probability distributions and correlations, where you first have to specify both the preparation and what measurements are being made. I don't care at all when the measurements get made, that plays no role in the physics of entanglement experiments, and affects our expectations for those outcomes not at all. That' simply a fact about these experiments. So all that is happening in delayed choice is, we go in with an expectation that the times when we do the measurements should matter, but we find out they don't, so we retrofit this idea that something had to "change" to get that to be true. But the blame is all on all our initial expectation that when we did the measurements should matter, even though we have no mechanism in mind by which it possibly could matter. We should simply recognize our initial expectation was wrong, and drop it. It's all a holdover from local realism, if we didn't go in with that philosophy in the first place, we wouldn't need to retrofit it.
 
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  • #91
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fermionic indistinguishability in white dwarfs.
Ironically, Ken G, when I think of EPR I can sort of understand what you're saying, but the white dwarf example is holding me back. Actually, are you sure it is a valid example? If the information of an emptied state on one side doesn't travel at least light-like to the other side so that you can fill it, you risk having an overfilled star for some inertial frames (i.e. in some frames the emptying occurs after the filling).
 
  • #92
Ken G
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Ironically, Ken G, when I think of EPR I can sort of understand what you're saying, but the white dwarf example is holding me back. Actually, are you sure it is a valid example? If the information of an emptied state on one side doesn't travel at least light-like to the other side so that you can fill it, you risk having an overfilled star for some inertial frames (i.e. in some frames the emptying occurs after the filling).
What I mean about white dwarfs is that if you look at interactions between particles, say you want to understand the heat conduction, you find that electrons deep in the Fermi sea don't scatter at all-- because the state they would have to scatter into is already "occupied." That's the language we use to talk about what is happening, but it's not a very good language, because it is deeply steeped in a form of local realism that can lead us astray in other applications. We imagine that star is full of different electrons, each with their own momentum state, and we say that the one electron can't go into a momentum state where there is already another one, but the whole reason that can't happen is because the electrons don't have identities like that! So the language is internally inconsistent, though common and somewhat innocuous if not taken too literally.

What is actually true is that if you look at the combined wavefunction of all the electrons, there simply are not unique individual electron states-- you can decompose into a concept of individual electron states in a host of different ways, akin to choosing a different basis for a single-particle wavefunction. So it's just not true that there is "already an electron in that momentum state", that is merely one way to translate the combined wavefunction into language that sounds like it kind of makes sense, but should not be taken literally. For example, it should not be taken so literally as to imagine that when we try to scatter an electron and find we cannot, somehow that "other electron" that is "already in that momentum state" produces some "nonlocal influence" that "prevents" our electron from scattering. None of that language is supported by the quantum mechanics that is determined by the total wavefunction of all the electrons, which does not distinguish any individual electrons at all. The experiment that is trying to scatter an electron could determine properties like the momentum of whatever electron is being culled out in that way, and only then is there "that electron" with "that momentum", but no experiment is doing that for any "other individual electron", so we shouldn't even talk about each "other individual electron" like it was a real thing there. When you avoid that, you avoid the whole concept of "influences" between electrons, you simply don't think of the system as being comprised of individual independent electrons-- it is a whole system that contains some number of electrons, none of which are distinguished and none of which have individual momenta.
 
  • #94
DrChinese
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... Caution: All the wisdom says that one cannot exceed the classical limit with a simulation, so don't be too hopeful.
Mentz114,

I don't think we should be covering this subject in this thread (or any thread actually). The CHSH formula is subject to the same objections I had above for the listed page. There is not going to be a breakthrough here, you and I both know Bell is a limitation. If someone want to make assertions that violate generally accepted science, those should be published elsewhere rather than debated here.

The simulation is fine as a basic tutorial. It does not make any unusual claims. It is only what is being said here that I have an issue with.
 
  • #95
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Mentz114,

I don't think we should be covering this subject in this thread (or any thread actually). The CHSH formula is subject to the same objections I had above for the listed page. There is not going to be a breakthrough here, you and I both know Bell is a limitation. If someone want to make assertions that violate generally accepted science, those should be published elsewhere rather than debated here.

The simulation is fine as a basic tutorial. It does not make any unusual claims. It is only what is being said here that I have an issue with.
I agree. What I posted should have been a private message.
 
  • #96
DrChinese
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I agree. What I posted should have been a private message.
I think that makes more sense to follow up on. Thanks.

-DrC
 
  • #97
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What I mean about white dwarfs is that if you look at interactions between particles, say you want to understand the heat conduction, you find that electrons deep in the Fermi sea don't scatter at all-- because the state they would have to scatter into is already "occupied." That's the language we use to talk about what is happening, but it's not a very good language, because it is deeply steeped in a form of local realism that can lead us astray in other applications. We imagine that star is full of different electrons, each with their own momentum state, and we say that the one electron can't go into a momentum state where there is already another one, but the whole reason that can't happen is because the electrons don't have identities like that! So the language is internally inconsistent, though common and somewhat innocuous if not taken too literally.

What is actually true is that if you look at the combined wavefunction of all the electrons, there simply are not unique individual electron states-- you can decompose into a concept of individual electron states in a host of different ways, akin to choosing a different basis for a single-particle wavefunction. So it's just not true that there is "already an electron in that momentum state", that is merely one way to translate the combined wavefunction into language that sounds like it kind of makes sense, but should not be taken literally. For example, it should not be taken so literally as to imagine that when we try to scatter an electron and find we cannot, somehow that "other electron" that is "already in that momentum state" produces some "nonlocal influence" that "prevents" our electron from scattering. None of that language is supported by the quantum mechanics that is determined by the total wavefunction of all the electrons, which does not distinguish any individual electrons at all. The experiment that is trying to scatter an electron could determine properties like the momentum of whatever electron is being culled out in that way, and only then is there "that electron" with "that momentum", but no experiment is doing that for any "other individual electron", so we shouldn't even talk about each "other individual electron" like it was a real thing there. When you avoid that, you avoid the whole concept of "influences" between electrons, you simply don't think of the system as being comprised of individual independent electrons-- it is a whole system that contains some number of electrons, none of which are distinguished and none of which have individual momenta.
Of course I agree with all this. But what would you say about my issue specifically? Surely, the information that a state has been emptied must travel timelike or lightlike to avoid Pauli principle violation for some inertial frames, in case someone wants to fill that state on the other side of the star. But then aren't we falling back to localized parts for a star that should be a holistic object?
 
  • #98
Ken G
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Surely, the information that a state has been emptied must travel timelike or lightlike to avoid Pauli principle violation for some inertial frames, in case someone wants to fill that state on the other side of the star. But then aren't we falling back to localized parts for a star that should be a holistic object?
It sounds like you are asking, if a cosmic ray enters a white dwarf and knocks an electron clear out from deep in the Fermi sea, what is the wavefunction of the white dwarf? If the interaction is very quick, the cosmic ray could establish very accurately the energy of the electron that emerges, so let's say the electron and cosmic ray both come out with a very definite energy. Then at first it will look a wavefunction with a missing momentum state, much like an atom with a hole deep in one of its energy levels. But the location from where the electron came from would be uncertain, perhaps anywhere in the star. The wavefunction will then evolve via the Schroedinger equation, which has some interaction terms that will eventually fill that hole and release some energy, perhaps a photon. But it will take a long time for that transition to happen, I would guess at least the light travel time across the star, but perhaps much longer. I expect the situation would be like ionizing an electron from a deep shell in an atom-- the light crossing time is much shorter than the timescale for a transition to actually occur, so the issue doesn't even come up.
 
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