A Paradox: Do LHV Theories Need the HUP?

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ZapperZ said:
You need to address how your "reality" jive with the Stony Brook/Delft experiments. Till then, I truly believe all of this is meaningless.
... we intend to "clarify" what we don't understand via verification of that understanding through a series of experiments. There's nothing to indicate what you claim to be a separate "underlying reality" is valid. Believing in such a thing dispite the lack of evidence isn't "modern physics".
Zz.
The idea that conservation laws having nothing to do with entanglement doesn't make sense to me at this point. Maybe someday it will. In any case I don't want to take the thread any further off topic. If i get a chance to prepare some researched statements or questions on it, then I'll post them in a new thread. Until then, I'll take your word for it.

I also don't want to argue here about whether or not reality exists independent of measurement, although it seems pretty clear to me that it does, and that that's not what Bell tests are testing. It also seems clear to me that physics doesn't yet know everything that there is to know about a whole lot of experimental phenomena (quantum entanglement being just one example).

'Underlying reality' just refers to what isn't yet known about reality (like the behavior of the light incident on polarizers in a Bell test). Of course, we don't know yet what it is that we don't know (at least I don't :-) ) -- but what you seem to be saying is that it's ok to assume that we already know everything that there is to know about reality.

The experimental evidence isn't just the foundation for what can be said to be known, it's also an indicator that what is known isn't the whole story.
 

ZapperZ

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Sherlock said:
The idea that conservation laws having nothing to do with entanglement doesn't make sense to me at this point. Maybe someday it will. In any case I don't want to take the thread any further off topic. If i get a chance to prepare some researched statements or questions on it, then I'll post them in a new thread. Until then, I'll take your word for it.
No, I'm saying the conservation laws when applied to classical system is DIFFERENT than that in the EPR type experiment. The system is under A SUPERPOSITION of states that have no particular state until it is measured! Once it is measured, then the "conservation" law that caused the entanglement in the first place kicks in! When this occurs, one can easily ask "So what's the difference between this and a classical system where we have conservation of angular momentum, for example?". This is there the measurement of non-commuting components will show the difference!

I also don't want to argue here about whether or not reality exists independent of measurement, although it seems pretty clear to me that it does, and that that's not what Bell tests are testing. It also seems clear to me that physics doesn't yet know everything that there is to know about a whole lot of experimental phenomena (quantum entanglement being just one example).
'Underlying reality' just refers to what isn't yet known about reality (like the behavior of the light incident on polarizers in a Bell test). Of course, we don't know yet what it is that we don't know (at least I don't :-) ) -- but what you seem to be saying is that it's ok to assume that we already know everything that there is to know about reality.
The experimental evidence isn't just the foundation for what can be said to be known, it's also an indicator that what is known isn't the whole story.
I NEVER said we know all there is to know. I simply refuse to play the speculation game, which is what you're doing. If we do that, I can speculate anything I want without bothering for any physical justification. So how would this solve anything and answer anything? I could speculate that some unseen angels are actually directing the polarization of each photon passing through the polarizers, and it would be no different than what you're doing. Would this make it better?

I'd rather stick to what we already know, and continue to LOOK for any violation of things we thought we understood already! I find those to be a more definitive indication of new and unknown things.

Zz.
 
ZapperZ said:
... I'm saying the conservation laws when applied to classical system is DIFFERENT than that in the EPR type experiment. The system is under A SUPERPOSITION of states that have no particular state until it is measured! Once it is measured, then the "conservation" law that caused the entanglement in the first place kicks in! When this occurs, one can easily ask "So what's the difference between this and a classical system where we have conservation of angular momentum, for example?". This is there the measurement of non-commuting components will show the difference!
Ok, that's different than what you first wrote when I asked if you were saying that conservation laws have nothing to do with entanglement -- and you said yes. I still don't think I agree with the way you've put it above -- but that can be a topic for another thread.
ZapperZ said:
I NEVER said we know all there is to know. I simply refuse to play the speculation game, which is what you're doing.
Where have I speculated? What I said, in effect, is that the results of optical Bell tests might be taken as an indication that we don't know all there is to know about the underlying reality, which in the case of optical Bell tests would include, among other things, what is commonly referred to as the electromagnetic field (and particular phenomena related to this field such as photons and polarization).
ZapperZ said:
If we do that, I can speculate anything I want without bothering for any physical justification. So how would this solve anything and answer anything? I could speculate that some unseen angels are actually directing the polarization of each photon passing through the polarizers, and it would be no different than what you're doing.
Let's see what I'm doing:
I asked, "Might one conclude that the *observables* are not in one to one correspondence with the underlying reality, and therefore that the qm form and experimental tests aren't revealing that nonlocal phenomena exist (or that they don't exist)?"

And you answered, "No, one might not. By making such a statement, you are already making a HUGE assumption that there is (i) an underlying reality and that (ii) it is inaccessible via ANY measurement since, after all, it is, then we would have detected a deviation from QM's predictions.

And I clarified (or so I thought) what I was talking about in post 18 of this thread -- and none of it involved speculating about the underlying reality itself. (We can do that in another thread also.) The speculation is about the interpretation of Bell's analysis of lhv supplements to qm and the physical meaning of experimental violations of Bell inequalities.

By the way, the idea that the wavefunction isn't in one to one correspondence with reality came from my quantum theory textbook, which teaches a probability interpretation.
ZapperZ said:
I'd rather stick to what we already know, and continue to LOOK for any violation of things we thought we understood already! I find those to be a more definitive indication of new and unknown things.
Yes of course. That's probably the most productive way that discoveries have been, and are going to continue to be, made -- along with advances in the hardware.

But, I'm not speculating specifically about hitherto unknown physical phenomena. This is just one possible avenue of inquiry regarding the physical meaning of Bell tests, and the essence of quantum entanglement. But such speculation will probably turn out to be unnecessary, imo -- since the essence of entanglement has been known since Schroedinger first talked about it, and partly due to this it can be inferred that Bell's theorem and Bell tests aren't telling us anything about local realism.
 
vanesch said:
Sherlock, I never understood fully what you wanted to say when you wrote about Bell.
Neither did I. :-) I just have this nagging feeling that people are focusing on irrelevant stuff wrt some statements that are made regarding the meaning of Bell's theorem and experimental violations of inequalities. The basic datum in Bell tests is nonseparable. So is the instrumental variable. They can't be analyzed into component parts. Bell discussed situations in which lhv supplements to qm would be ok. So the results don't rule out all lhv supplements for all situations -- just a certain form wrt entangled states.
vanesch said:
Since quantum theory doesn't deal explicitly with this mechanism, then the form of qm calculations can't be taken to mean that there exist nonlocal phenomena in nature.
That's the way I've learned to think about it, but some people seem to think that the qm formalism itself indicates the existence of nonlocal phenomena in nature.
vanesch said:
It is not the form of the calculations, it is the outcomes! What Bell proved, is that a certain class of theories, which have very reasonable assumptions (and which Einstein supposed were underlying QM), WILL NEVER BE ABLE TO GET THE SAME RESULTS OUT AS QM.
This was the big surprise of Bell's theorem. We're not saying that the QM *calculations* are somehow non-local, we're saying that the predictions that come out of it cannot be also the result of a theory that satisfies the premises of Bell's theorem.
It depends on what premises are being considered. There are more assumptions involved than local realism. In any case, we certainly can't conclude from Bell tests that there's no physical reality between emitters and detectors. We can, however, be pretty sure that extant models aren't in one to one correspondence with what is happening between emitters and detectors. The idea that entanglement is *created* via common source or common interaction is a compelling one -- and, as far as my limited experience is concerned, this idea is the one held by most physicists.

As for locality, is it possible that the joint result can be nonseparable and still be partly dependent on a local common source or interaction? It would seem so. The joint result also depends on the variable joint setting of polarizers, but this joint setting is one nonanalyzable thing -- so when you change the setting of one polarizer, no matter the extent of their spatial separation, then the value of the variable that reveals predictable rates of coincidental detection is instantaneously changed. And, as far as I can tell, the instrumental variable is the only variable involved in determining coincidental detection (if whatever is happening between emitters and detectors is varying, it doesn't affect the results) -- so the search for a *hidden* variable wrt Bell test situations would seem to be wrongheaded.

Thus, the statement that Bell test *results* disallow lhv supplements is a bit off. Bell test *setups* disallow lhv supplements -- because such variables as might exist between emitters and detectors are simply not relevant to the results, and including them in the calculation screws it up.
 
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vanesch

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Sherlock said:
It depends on what premises are being considered. There are more assumptions involved than local realism.
Which ones ?
 

ZapperZ

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Sherlock said:
Ok, that's different than what you first wrote when I asked if you were saying that conservation laws have nothing to do with entanglement -- and you said yes. I still don't think I agree with the way you've put it above -- but that can be a topic for another thread.
Well, no. Let's address it here.

Case 1: A blob with no net angular momentum. At some time, it fragmented into two pieces that flew off in opposite directions. When they are very far away, I measure Piece 1 and found its angular momentum. I immediately can say that I know the angular momentum of Piece 2.

Case 2: 2 entangled particle flew off in opposite direction. I perform an EPR-type measurement. Upon measurement of one of the paritcles, the entangled property of the other is immediately set.

Do you think those two cases are identical?

Let's see what I'm doing:
I asked, "Might one conclude that the *observables* are not in one to one correspondence with the underlying reality, and therefore that the qm form and experimental tests aren't revealing that nonlocal phenomena exist (or that they don't exist)?"
And you answered, "No, one might not. By making such a statement, you are already making a HUGE assumption that there is (i) an underlying reality and that (ii) it is inaccessible via ANY measurement since, after all, it is, then we would have detected a deviation from QM's predictions.
And I clarified (or so I thought) what I was talking about in post 18 of this thread -- and none of it involved speculating about the underlying reality itself. (We can do that in another thread also.) The speculation is about the interpretation of Bell's analysis of lhv supplements to qm and the physical meaning of experimental violations of Bell inequalities.
But what evidence do you have to be able to say:

".... *observables* are not in one to one correspondence with the underlying reality..."?

You made two explicit assumptions here: (i) the observables do not have a one-to-one correspondence with some reality and (ii) that there is an "underlying reality" that is DIFFERENT than what these observables are producing.

When you produce no such evidence, and we currently do not have one, I categorize that as speculation.

But, I'm not speculating specifically about hitherto unknown physical phenomena. This is just one possible avenue of inquiry regarding the physical meaning of Bell tests, and the essence of quantum entanglement. But such speculation will probably turn out to be unnecessary, imo -- since the essence of entanglement has been known since Schroedinger first talked about it, and partly due to this it can be inferred that Bell's theorem and Bell tests aren't telling us anything about local realism.
Then you owe the community a favor by rebutting all those papers that continually claim violation of local realism. If you believe in such a thing, then you have an obligation to correct the situation. Write a rebuttal with your arguments to make sure such a claim is never made.

Zz.
 
vanesch said:
Which ones ?
One is the assumption that the hidden variables are relevant to the outcome. Bell tests are measuring entanglement. The entanglement is assumed to not vary from entangled pair to entangled pair. Experimenters tweak and calibrate everything to minimize untoward effects. Any variations in the entanglement should be in the results.

Another is the assumption that the separable formulation is applicable to entangled states, which apparently it isn't.
 

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Sherlock said:
One is the assumption that the hidden variables are relevant to the outcome. Bell tests are measuring entanglement. The entanglement is assumed to not vary from entangled pair to entangled pair. Experimenters tweak and calibrate everything to minimize untoward effects. Any variations in the entanglement should be in the results.
But this is making a slanderous implication that experimenters deliberately manipulate their instruments so as to only produce results that will only verify a prevailing idea. Are you willing to stand by such an accusation?

Zz.
 

vanesch

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Sherlock said:
One is the assumption that the hidden variables are relevant to the outcome.
What does it mean, "relevant" ?
The idea of a local realist theory is that the local hidden variables is THE ONLY MECHANISM of inducing correlations. You are free NOT to use that mechanism but then OF COURSE you will not find any correlation, so you better use it.
Bell tests are measuring entanglement.
No, entanglement is a theoretical property proper to quantum theory, while Bell's inequalities apply to ANOTHER class of theories which does NOT have the mechanism of entanglement, but only of (hidden or not) local quantities which can have a common origin (or not).

Bell's tests are testing CORRELATIONS of measurements (that's not the same as "entanglement").

Local quantities which have common origin produce correlations between outcomes, and quantum entanglement produces correlations between outcomes. Bell's inequalities put limits on the amount of correlation you can obtain with the first kind of mechanism, and quantum entanglement can produce correlations which go beyond these limits (exactly because it is not a "common origin of local quantities" kind of mechanism)
Another is the assumption that the separable formulation is applicable to entangled states, which apparently it isn't.
But that's exactly what "local realism" means, no ? That each individual particle has a well-defined state which will from that point on, determine all what can be measured on that particle. This well-defined state can have a common origin with another particle (that's the "common origin of hidden variables" = state of the local particle) but that's the only allowed mechanism to generate correlations.

Clearly the quantum description doesn't allow that: the quantum state of an entangled pair does NOT split into two individual states of the individual particles (and THAT is the exact reason that quantum theory is NOT a local realist theory, and hence does not have to satisfy Bell's inequalities - which indeed it doesn't). But the Local Realist wants to see an individual state assigned to each particle, which will entirely determine what will be measured.
Bell's theorem just states that such a theory must satisfy certain conditions on the correlations that can be observed (Bell's inequalities), which are NOT satisfied by QM, and hence that such a theory can never be the "underlying mechanism" of QM.
 
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DrChinese

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Sherlock said:
Thus, the statement that Bell test *results* disallow lhv supplements is a bit off. Bell test *setups* disallow lhv supplements -- because such variables as might exist between emitters and detectors are simply not relevant to the results, and including them in the calculation screws it up.
This is definitely not the case. Plenty of experiments have been performed in which the detector settings are changed mid-flight. As a a result, there is no possibility that any kind of equilibrium state (between the emitter and the detector) affects the outcome.
 

ttn

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DrChinese said:
I like what you have to say about "if locality is true, there must exist a certain kind of hidden variables which determine the outcomes on each side". That is a great way to frame the argument about local reality. And I agree about the importance of getting EPR and Bell straight, which is why I try to stay close to their words on the subject where possible.
But I disagree about Bell not finding something wrong with EPR. There is something wrong with EPR, and Bell showed it to us!
The thing I said that you liked ("if locality is true, there must exist a certain kind of hidden variables which determine the outcomes on each side") is simply a summary of the EPR conclusion. So I don't see how/why you claim to like this statement, given that you think "there is something wrong with EPR."



EPR said both of the following:
a) Either QM is incomplete, or there is not simultaneous reality to non-commuting observables.
In other words, either QM is incomplete, or it is complete. (Either the two quantities whose simultaneous values are restricted by the HUP are both simultaneously real, or they aren't. If they are real, orthodox QM is incomplete; if they aren't simultaneously real, QM is complete, at least in so far as those two variables are concerned.)

b) They believed that there IS simultaneous reality to non-commuting observables because a more complete specification of the system is possible.
Yes, they argued that there "IS simultaneous reality to non-commuting observables"... but not merely because they felt a more complete specification of the system is possible. That's not a *reason* to believe in the simultaneous reality of those two properties, it's just another way of saying that one believes in the simultaneous reality of those two properties. To believe that a more complete specification of the system is possible is simply to deny the completeness doctrine, specifically, to deny the orthodox view that the uncertainty principle is ontological rather than epistemic.

The *actual reason* EPR believed in the simultaneous reality of these properties is because he saw that to *not* believe in them would violate the assumption of reality. If you think that x and p are not simultaneously real for the distant particle, then, in order to explain the perfect correlations between the particle here and the one there, you *must* posit some kind of non-local mechanism by which the measurement here *causes* the particle there to assume the appropriate, correlated value for the property in question. Completeness entails non-locality (given the predicted correlations). Or equivalently, locality entails in-completeness. Specifically, locality entails that both x and p for that distant particle were already (simultaneously) definite/real before any measurement was made here.


a) was supported by the logic presented. Clearly, b) was not rigorously supported and was an ad hoc assumption. Some people never accepted b) anyway, so it may not be material to them. Maybe that is your opinion too.
Hogwash. You can't be analyzing what's wrong with their argument until you've understood it.


On the other hand, some people did accept b) - but Bell saw a problem with that. He formulated his paper ("On the EPR Paradox") mathematicially assuming there WAS simultaneous reality to such observables, and found that was incompatible with QM itself.
Fine, but you miss what's essential if you just say "some people did accept b)". That's true, but what's important is that people accepted "b)" (which as I noted above is just equivalent to your "a)") *because it was a requirement of locality. The whole point of EPR in so far as it relates to Bell's Theorem is this: given the perfect correlations predicted by QM, the *only* way to respect locality is to deny the completeness doctrine and add local hidden variables to account (locally, duh) for the outcomes. That's the only way you can possibly have a local theory.

Bell later proved that even this way of trying to have a local theory couldn't work. You can't reproduce the QM predictions with a hidden variable theory that respects locality. But this isn't "too bad for hidden variables". To say that is to simply *forget* why we should have believed in hidden variables in the first place, namely: because having them is the only way to have a local theory. That's what EPR showed.

Think of it this way: EPR and Bell both showed that a certain theory (or class of theories) was non-local. EPR pointed out the orthodox QM was nonlocal, and noted what, in principle, would have to be done to construct a local theory. Bell showed that the kind of theory needed to respect locality in the face of the EPR argument, also doesn't work -- such a theory cannot be made to agree with QM/experiment. Conclusion: locality is false. The only way to save it doesn't work.



Hardly a result that EPR envisioned.
No doubt. But that doesn't mean their *argument* is wrong. It means one of the premises of that argument turns out to be untenable. This is elementary logic. "Locality" and "Locality --> InCompleteness" are not the same proposition. EPR accepted the first and proved the second, and hence believed in the conclusion "InCompletness".

Later, Bell proved that this conclusion "InCompleteness" plus the original "Locality" premise entails a contradiction with experiment. So, just considering Bell, either "Incompleteness" or "Locality" must fail. But if "InCompleteness" fails, that means we are upholding the orthodox Completeness doctrine, and that means, per EPR, that we are denying "Locality". So... again... you can't save "locality" by denying "InCompleteness". The two arguments together prove absolutely that "Locality" is false.

(Well, not absolutely... You could always say something completely bonkers, like that we're all deluded when we think that the experiments in question even had definite outcomes. Of course, that would be even crazier than clinging to the various "loopholes" in those experiments that are clung to by fans of local hidden variable theories.)

I would definitely say that EPR took locality as an axiom.
You mean, they took it as a premise in their argument for "InCompleteness"? No doubt. Yet you seem ignorant of the role this premise actually played in the argument.


Bell definitely did not, as he was explicit in this regard.
Excuse me? Are you saying that you don't think "Locality" is a premise of Bell's Theorem? The whole point of the theorem is to show that a certain class of LOCAL theories are inconsistent with the QM predictions.

Or maybe you just mean that, in a broader sense, Bell was willing to consider that possibly "Locality" might be false. That's certain true, although he only thought that because his theorem, combined with the EPR argument, *proved* that "Locality" is untenable.
 
ZapperZ said:
But this is making a slanderous implication that experimenters deliberately manipulate their instruments so as to only produce results that will only verify a prevailing idea. Are you willing to stand by such an accusation?
Zz.
Oh come now ZapperZ. Look at what I wrote.

"Bell tests are measuring entanglement. The entanglement is assumed to not vary from entangled pair to entangled pair. Experimenters tweak and calibrate everything to minimize untoward effects. Any variations in the entanglement should be in the results."

What this means is that experimenters do everything they possibly can to ensure that the hardware is working as it should, and that the entanglement preparation is actually producing entangled results.

What are you saying ... that experimenters who conduct Bell tests are *not* trying to produce entangled pairs?
 

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Sherlock said:
Oh come now ZapperZ. Look at what I wrote.
"Bell tests are measuring entanglement. The entanglement is assumed to not vary from entangled pair to entangled pair. Experimenters tweak and calibrate everything to minimize untoward effects. Any variations in the entanglement should be in the results."
What this means is that experimenters do everything they possibly can to ensure that the hardware is working as it should, and that the entanglement preparation is actually producing entangled results.
What are you saying ... that experimenters who conduct Bell tests are *not* trying to produce entangled pairs?
I didn't say anything. I was asking if you're implying that experimenters tweak their equipment to only produce what they think it should. That was how I understood from reading what you said.

And oh, unless I again misunderstood what you are saying, the measurement itself produce only a detection of the component of the entangled variable - it does NOT measure "entanglement". It is only when you look at the whole collection of the measured data can one deduce such such correlation. Other experiments are a lot more convincing in detecting entanglement than the EPR-type experiments (example: the defeat of the diffraction limit by entangled particles).

I'm confused as to why entanglement is the issue here. Even those who are sticking with local realism isn't debating the reality of entanglement. EPR wasn't addressing such a thing, but the non-local nature of entanglement.

Zz.
 

DrChinese

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ttn said:
The thing I said that you liked ("if locality is true, there must exist a certain kind of hidden variables which determine the outcomes on each side") is simply a summary of the EPR conclusion. So I don't see how/why you claim to like this statement, given that you think "there is something wrong with EPR."
In other words, either QM is incomplete, or it is complete. (Either the two quantities whose simultaneous values are restricted by the HUP are both simultaneously real, or they aren't. If they are real, orthodox QM is incomplete; if they aren't simultaneously real, QM is complete, at least in so far as those two variables are concerned.)
Yes, they argued that there "IS simultaneous reality to non-commuting observables"... but not merely because they felt a more complete specification of the system is possible. That's not a *reason* to believe in the simultaneous reality of those two properties, it's just another way of saying that one believes in the simultaneous reality of those two properties. To believe that a more complete specification of the system is possible is simply to deny the completeness doctrine, specifically, to deny the orthodox view that the uncertainty principle is ontological rather than epistemic.
The *actual reason* EPR believed in the simultaneous reality of these properties is because he saw that to *not* believe in them would violate the assumption of reality. If you think that x and p are not simultaneously real for the distant particle, then, in order to explain the perfect correlations between the particle here and the one there, you *must* posit some kind of non-local mechanism by which the measurement here *causes* the particle there to assume the appropriate, correlated value for the property in question. Completeness entails non-locality (given the predicted correlations). Or equivalently, locality entails in-completeness. Specifically, locality entails that both x and p for that distant particle were already (simultaneously) definite/real before any measurement was made here.
Hogwash. You can't be analyzing what's wrong with their argument until you've understood it.

Fine, but you miss what's essential if you just say "some people did accept b)". That's true, but what's important is that people accepted "b)" (which as I noted above is just equivalent to your "a)") *because it was a requirement of locality. The whole point of EPR in so far as it relates to Bell's Theorem is this: given the perfect correlations predicted by QM, the *only* way to respect locality is to deny the completeness doctrine and add local hidden variables to account (locally, duh) for the outcomes. That's the only way you can possibly have a local theory.
Bell later proved that even this way of trying to have a local theory couldn't work. You can't reproduce the QM predictions with a hidden variable theory that respects locality. But this isn't "too bad for hidden variables". To say that is to simply *forget* why we should have believed in hidden variables in the first place, namely: because having them is the only way to have a local theory. That's what EPR showed.
Think of it this way: EPR and Bell both showed that a certain theory (or class of theories) was non-local. EPR pointed out the orthodox QM was nonlocal, and noted what, in principle, would have to be done to construct a local theory. Bell showed that the kind of theory needed to respect locality in the face of the EPR argument, also doesn't work -- such a theory cannot be made to agree with QM/experiment. Conclusion: locality is false. The only way to save it doesn't work.
No doubt. But that doesn't mean their *argument* is wrong. It means one of the premises of that argument turns out to be untenable. This is elementary logic. "Locality" and "Locality --> InCompleteness" are not the same proposition. EPR accepted the first and proved the second, and hence believed in the conclusion "InCompletness".
Later, Bell proved that this conclusion "InCompleteness" plus the original "Locality" premise entails a contradiction with experiment. So, just considering Bell, either "Incompleteness" or "Locality" must fail. But if "InCompleteness" fails, that means we are upholding the orthodox Completeness doctrine, and that means, per EPR, that we are denying "Locality". So... again... you can't save "locality" by denying "InCompleteness". The two arguments together prove absolutely that "Locality" is false.
(Well, not absolutely... You could always say something completely bonkers, like that we're all deluded when we think that the experiments in question even had definite outcomes. Of course, that would be even crazier than clinging to the various "loopholes" in those experiments that are clung to by fans of local hidden variable theories.)
You mean, they took it as a premise in their argument for "InCompleteness"? No doubt. Yet you seem ignorant of the role this premise actually played in the argument.
Excuse me? Are you saying that you don't think "Locality" is a premise of Bell's Theorem? The whole point of the theorem is to show that a certain class of LOCAL theories are inconsistent with the QM predictions.
Or maybe you just mean that, in a broader sense, Bell was willing to consider that possibly "Locality" might be false. That's certain true, although he only thought that because his theorem, combined with the EPR argument, *proved* that "Locality" is untenable.
I can't really debate what you are saying, as I don't really get your points.

EPR was not about locality: it really isn't discussed and is more a tacit assumption ("... after which time we suppose that there is no longer any interaction between the two parts..."). It was about the completeness of QM. They felt they demonstrated that QM was incomplete. I don't feel they demonstrated that. I feel they DID demonstrate that if QM was complete, then there is not simultaneous reality to non-commuting observables. Do we disagree about this? Or maybe you read the last 2 paragraphs of EPR differently than I do.

Bell discusses locality explicitly ("... the signal involved must propagate instantaneously" ...), as I say. His primary argument, however, has nothing to do with locality and everything to do with reality (i.e. hidden variables) - that is what the math is all about ("It follows that c is another unit vector..."). After then proving that hidden variables are inconsistent with the predictions of QM, he concludes that hidden variables can only be "rescued" within a non-local theory. Do we disagree on this?

It is patently false that EPR+Bell excludes locality. It is also possible that reality is both local and observer dependent (i.e. there is no unit vector c). I am merely stating that QM, per Copenhagen, can be interpreted several different ways and often is.
 
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ZapperZ said:
I didn't say anything. I was asking if you're implying that experimenters tweak their equipment to only produce what they think it should. That was how I understood from reading what you said.
And oh, unless I again misunderstood what you are saying, the measurement itself produce only a detection of the component of the entangled variable - it does NOT measure "entanglement". It is only when you look at the whole collection of the measured data can one deduce such such correlation. Other experiments are a lot more convincing in detecting entanglement than the EPR-type experiments (example: the defeat of the diffraction limit by entangled particles).
I'm confused as to why entanglement is the issue here. Even those who are sticking with local realism isn't debating the reality of entanglement. EPR wasn't addressing such a thing, but the non-local nature of entanglement.
Zz.
So called "Bell states" are maximally entangled states. Bell inequalities are entanglement 'witnesses'. Violations of Bell inequalities are an indication of the presence of entanglement. Hence, my statement that Bell tests measure entanglement, which, if experimenters have prepared things correctly, is not a *variable* property. The only thing that is varying in the Bell tests (that has anything to do with the outcome) is the joint setting of the polarizers.

Therefore, the application of a hidden variable supplement to the qm formulation is just a *mis*application -- since the variable properties of entangled pairs don't affect the rate of coincidental detection.

The relevant hidden parameter is the entanglement itself, and it isn't varying from pair to pair. So, we can still assume that the entanglement is locally produced at emission.
 
Originally Posted by Sherlock
Thus, the statement that Bell test *results* disallow lhv supplements is a bit off. Bell test *setups* disallow lhv supplements -- because such variables as might exist between emitters and detectors are simply not relevant to the results, and including them in the calculation screws it up.
DrChinese said:
This is definitely not the case. Plenty of experiments have been performed in which the detector settings are changed mid-flight. As a a result, there is no possibility that any kind of equilibrium state (between the emitter and the detector) affects the outcome.
I wasn't talking about any kind of equilibrium state between emitter and detector. Just that the exact properties of each entangled pair will vary somewhat from pair to pair. But this variability will not affect the outcome, as long as the pairs being jointly analyzed are entangled.

Change the joint setting as much as you want while the particles are in flight. It's still that case that for any given pair that's analyzed in a given coincidence interval there is one and only one joint setting that is analyzing the pair -- and a given joint setting that is analyzing entangled pairs will produce a certain predictable rate of coincidental detections.

The point of my statement is that whatever variability there is from pair to pair, this variability isn't relevant to the results. This is one thing that varying the joint analyzer setting randomly while the particles are in flight ensures. But whether the analyzers are varied randomly while the particles are in flight or not, you get the same average results for a given setting -- which is a pretty solid indication that the variable properties of the incident disturbances aren't what is producing the variable results.

So, I take the *entanglement*, per se, as a nonvarying property of the jointly analyzed particles, and then the only variable in the setup is the joint analyzer setting. Hence my statement that the *setup* itself is what disallows a hidden variable supplement to the qm formulation.
 

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DrChinese said:
It is patently false that EPR+Bell excludes locality. It is also possible that reality is both local and observer dependent (i.e. there is no unit vector c). I am merely stating that QM, per Copenhagen, can be interpreted several different ways and often is.
:approve:
But nevertheless, it makes us make the rather unconfortable choice between non-locality, and (observed) reality is observer-dependent.
Bohmians (such as, I presume, ttn) and make the first choice, MWIers (like me) make the second choice. Copenhageners do something in between...
 

vanesch

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Sherlock said:
Therefore, the application of a hidden variable supplement to the qm formulation is just a *mis*application -- since the variable properties of entangled pairs don't affect the rate of coincidental detection.
The relevant hidden parameter is the entanglement itself, and it isn't varying from pair to pair. So, we can still assume that the entanglement is locally produced at emission.
Yes, of course entanglement is locally produced at emission, that is entirely correct. I'm sorry, and no offense, but I think you never got the "click" of what Bell is all about.
Entanglement is a special kind of state description entirely proper to the formalism of quantum theory, and simply means that the way states of systems are described are, eh, well, entangled, meaning, you cannot disentangle the state description as "the state of A" and "the state of B". I don't know if you realise how revolutionary a concept that is. Never this occured before in physics. In classical physics, if you have two systems which are in DIFFERENT LOCATIONS, it is always possible to describe the state of the TOTAL SYSTEM as "the state of A" and "the state of B". Of course there can be interactions between these states, and of course these states can have a "common origin" even if they are not interacting, because the systems interacted before.
But "the state of the sun-Betelgeuse system" can always be written as "the state of the sun" and "the state of Betelgeuse". All you can measure about the sun will depend ONLY on "the state of the sun" and all you can measure about Betelgeuse will depend only on "the state of Betelgeuse".
THAT DOESN'T MEAN that there cannot be correlations between both. Indeed, it could be that the sun and Betelgeuse had some interaction long ago, and the correlations of our measurements only measure that "common part" induced by that interaction long ago.
But, as Bell showed, when you have such a separate "this is the state of A" and "this is the state of B" state description EVEN IF THERE WAS A COMMON PART, the correlations you can obtain in such a way have to satisfy certain properties.
This is what is violated in QM. And it is (after the fact) not surprising because the states in QM are NOT of the form "the state of A" and "the state of B". There is only the state of AB.
Again, don't think that this is "simply because they have common origin". Classical systems can have "common origin", but that doesn't deny a reality both to the "state of A" and "the state of B". Measurements on A and B will show statistical correlations, but these correlations WILL SATISFY CERTAIN RULES (like Bell's inequalities). Quantum entanglement goes further and DENIES the existence of a state of A and a state of B.
But people feel uneasy about the fact that there is "no state of A" and "no state of B", and only a "state of AB", because we're used to thinking that what is right here, should have its own 'reality' (state). That's what local reality is all about.
Now it could be that you find it very normal for something NOT to have a state of its own (a reality of its own) just because it is confined to some space. You might have the intuition that "reality is holistic". But it is a very special way of thinking in physics, because VERY MANY USEFUL laws are based upon the locality principle. This is why people like Einstein thought that QM had an UNDERLYING theory which had identical predictions, but which had a (totally different) state description for A and for B. He called those state descriptions "hidden variables".
But Bell's theorem shows that this is not possible.
What is possible is that A and B DO have an individual state description ON THE CONDITION THAT they keep constantly in immediate interaction over long distances. Then, of course, the measurement of one can immediately change the STATE of the other, thus mimicking the quantum result. This is Bohmian mechanics.
 

DrChinese

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vanesch said:
:approve:
But nevertheless, it makes us make the rather unconfortable choice between non-locality, and (observed) reality is observer-dependent.
Bohmians (such as, I presume, ttn) and make the first choice, MWIers (like me) make the second choice. Copenhageners do something in between...
I hoped you would like my little defense of reality. :smile:

I think all the interpretations are uncomfortable at least in some way. And that keeps us on our collective toes!
 

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Sherlock said:
So called "Bell states" are maximally entangled states. Bell inequalities are entanglement 'witnesses'. Violations of Bell inequalities are an indication of the presence of entanglement. Hence, my statement that Bell tests measure entanglement, which, if experimenters have prepared things correctly, is not a *variable* property. The only thing that is varying in the Bell tests (that has anything to do with the outcome) is the joint setting of the polarizers.

Therefore, the application of a hidden variable supplement to the qm formulation is just a *mis*application -- since the variable properties of entangled pairs don't affect the rate of coincidental detection.

The relevant hidden parameter is the entanglement itself, and it isn't varying from pair to pair. So, we can still assume that the entanglement is locally produced at emission.
Then I will await your rebuttal letters to all those experiments that claim that their experiments negate local realism.

Zz.
 

DrChinese

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vanesch said:
Quantum entanglement goes further and DENIES the existence of a state of A and a state of B.

What is possible is that A and B DO have an individual state description ON THE CONDITION THAT they keep constantly in immediate interaction over long distances. Then, of course, the measurement of one can immediately change the STATE of the other, thus mimicking the quantum result. This is Bohmian mechanics.
As you say, a lot of people get tripped up over this point.

If there are non-local hidden variables (in Bell's terms: there is a unit vector c), then there must be a hitherto unknown non-local mechanism in existence as well. After all, you are taking a classical idea - that there are separate A and B systems rather than a combined AB system - and trying to make it give identical predictions to QM. You will need the new hypothetical non-local mechanism to make this happen. Of course, this mechanism must elsewhere be dormant! (Since this mechanism is not explained by QM, QM would necessarily be incomplete if this actually exists. It would also violate special relativity.)

If you believe QM, there is no need (requirement) to postulate Bell's unit vector c. That is because the superposition of states explains it already. So in that limited respect, QM is complete. And since the superposition principle explains everything observed, there is no need for a new non-local mechanism.

(Of course, here I am talking of local and non-local in terms of Bell locality.)
 

ttn

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DrChinese said:
I can't really debate what you are saying, as I don't really get your points.
Was something unclear? Or you just don't believe me, despite the logic being clear, or what? I honestly don't understand how you could fail to "get" what I'm saying.
EPR was not about locality: it really isn't discussed and is more a tacit assumption ("... after which time we suppose that there is no longer any interaction between the two parts..."). It was about the completeness of QM. They felt they demonstrated that QM was incomplete. I don't feel they demonstrated that. I feel they DID demonstrate that if QM was complete, then there is not simultaneous reality to non-commuting observables. Do we disagree about this? Or maybe you read the last 2 paragraphs of EPR differently than I do.
Yes, we disagree. First of all, what you are saying makes no sense. "EPR was not about locality... it was about the completeness of QM." The whole point of the EPR argument is to show that ***if*** you assume that QM is complete, the theory violates locality. Completeness entails non-locality.
You say that EPR did succeed in demonstrating that "if QM was complete, then there is not simultaneous reality to non-commuting observables." Do you even understand what you are saying here? There can be no "demonstration" of this statement. It's simply a tautology. The formalism of QM simply doesn't *permit* one to write down a state that attributes simultaneous definite values to non-commuting observables. So if the theory is complete, then those observables really fail to have simultaneous definite values (as opposed to: they have values, but we don't know them).
So, it seems that you think that all EPR proved was an empty tautology -- that if QM is incomplete, then it's incomplete. But that just means you have failed to grasp the actual EPR argument. Now, in your defense, it seems you are overly focused on the EPR paper itself. And that does lead to confusion about this point, because the EPR paper was written by Podolsky and Einstein was rather pissed off about how confusing it was. But we needn't be bothered by any of this, and we needn't remain unnecessarily confused. Einstein made quite clear, many times, later in his life, what the real point of the EPR paper was supposed to have been.
Bell discusses locality explicitly ("... the signal involved must propagate instantaneously" ...), as I say. His primary argument, however, has nothing to do with locality and everything to do with reality (i.e. hidden variables) - that is what the math is all about ("It follows that c is another unit vector..."). After then proving that hidden variables are inconsistent with the predictions of QM, he concludes that hidden variables can only be "rescued" within a non-local theory. Do we disagree on this?
Yes, I don't think you understand Bell's derivation. Look at his paper, "On the EPR Paradox." Right after equation (1) he states openly: "The vital assumption is that the result B ... does not depend on the setting a ... nor A on b." (Check out the footnote, too.) This is the locality assumption. It's only because of the locality assumption that we *forbid* A to depend on b, and vice versa. And without that assumption, the derivation (obviously) does not go through. Bell's inequality is a bound that applies to *local* theories only. So... how you can say "his primary argument...has nothing to do with locality", I have no idea. Again, all I can conclude is that you simply don't understand either EPR or Bell's Theorem. *Both* of these *crucially* involve locality.
It is patently false that EPR+Bell excludes locality.
I can see now how you might think so, since evidently you think that neither EPR or Bell's Theorem has anything to do with locality in the first place!
It is also possible that reality is both local and observer dependent (i.e. there is no unit vector c).
You will have to flesh this out. What, exactly, is "observer dependent"? And please note: if your "observer dependence" includes a dependence of the A-side outcome on the B-side observation, your theory is *nonlocal*. Which means it isn't "both local and observer dependent."
Or was your point that regular old Copenhagen-ish QM is "both local and observer dependent"? That is just patently false. Orthodox QM violates Bell's Locality condition (not the inequality, but the definition of locality that he applies to hidden variable theories in the derivation of the inequality) -- i.e., "regular old Copenhagen QM" is nonlocal. So it isn't an example of a theory that is "local and observer dependent."
In fact, I challenge you to provide any example of a theory that is local, predicts that experiments have definite outcomes, and consistent with the QM predictions. You can include "observer dependence" or anything else you want in the theory as long as it is consistent with Bell Locality. I will be delighted to put my money where my mouth is and make this interesting, if you are game.
 

ttn

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DrChinese said:
If there are non-local hidden variables (in Bell's terms: there is a unit vector c), ...
Huh? If you mean the "unit vector c" that appears in Bell's derivation of his inequality, that is not a non-local hidden variable. First off, the lowercase letters in Bell's derivation (in almost all his papers) refer to unit vectors denoting the orientation of Stern-Gerlach devices. The hidden variables are the "lambdas" or, if you prefer (and in the deterministic case), the spin component values A(a,lambda), B(b,lambda), etc.
But my real issue is that, even leaving aside the question of whether you meant lowercase "c" or uppercase "c" or *whatever*, it just doesn't make any sense to talk about non-local hidden variables and cite *anything* from Bell's derivation. The derivation is always talking *exclusively* about *local* theories. If there were non-local hidden variables involved in Bell's derivation of the inequality, it wouldn't exactly be a bound on local hidden variable theories, now would it?
(Since this mechanism is not explained by QM, QM would necessarily be incomplete if this actually exists. It would also violate special relativity.)
But orthodox QM already violates special relativity. The collapse postulate tells us that the quantum state of distant systems can change simultaneously with a nearby measurement. And that "simultaneously" is meaningless according to SR.
If you believe QM, there is no need (requirement) to postulate Bell's unit vector c.
If you believe in locality, you *must* postulate these local hidden variables to account for the correlations.
That is because the superposition of states explains it already. So in that limited respect, QM is complete. And since the superposition principle explains everything observed, there is no need for a new non-local mechanism.
Yes, orthodox QM explains the correlations, but it explains them using a non-local mechanism. The collapse postulate violates relativity.
Of course, you can get around this conclusion if you deny the orthodox completeness doctrine. ...or so it seems until you learn about Bell's Theorem.
 

DrChinese

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ttn said:
If you believe in locality, you *must* postulate these local hidden variables to account for the correlations.

Yes, orthodox QM explains the correlations, but it explains them using a non-local mechanism. The collapse postulate violates relativity.
I guess I could insult you back, but that wouldn't be nice. :smile:

First, assuming "unit vector c" is equivalent to assuming a hidden variable. You can believe in locality and reject the existence of "unit vector c". This is basic logic and is fully compatible with Bell's Theorem. Despite the fact that Bell mentions locality, he did not include it explicitly in his mathematical proof as he did the hidden variable assumption.

Second, QM is as local or non-local as you interpret it to be. This too is basic and why folks argue about Copenhagen vs. Many Worlds vs. Bohmian Mechanics vs. Transactional Interpretation etc. Many scientists feel that QM's collapse postulate is not "non-local" in the sense that no signalling mechanism is violated. So I disagree with you that relativity is violated.
 

ttn

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DrChinese said:
I guess I could insult you back, but that wouldn't be nice. :smile:
If you think I've misunderstood something, then by all means "insult" me by pointing it out. I won't take it as an insult (as I hope you don't), but as an opportunity to get to the truth on this important issue.
First, assuming "unit vector c" is equivalent to assuming a hidden variable.
Maybe you should clarify your notation, but if it means what I think it means, then you are confused: the unit vector c isn't a hidden variable, it's the direction a certain person measures the spin of a certain particle along. The hidden variable is the "lambda" (Bell's notation) which determines what the *outcome* of that measurement will be.
You can believe in locality and reject the existence of "unit vector c".
Your claim was that the "unit vector c" was a *non-local* hidden variable. Even leaving aside the question of whether it's a hidden variable at all, it isn't non-local. No non-local hidden variables appear in Bell's derivation.
This is basic logic and is fully compatible with Bell's Theorem. Despite the fact that Bell mentions locality, he did not include it explicitly in his mathematical proof as he did the hidden variable assumption.
Look, with all due respect, you have just missed something that is incredibly important. Bell *did* include locality in the proof. See equation (1) of "On the EPR paradox", and the subsequent sentence. Or better yet, see one of his later papers where he makes all of this increasingly clear. I urge you, for example, to read the paper "La nouvelle cuisine" (chapter 24 of the new 2nd edition of Speakable and Unspeakable). Just to give you a taste, here are the titles of the sections from this paper: Introduction, What cannot go faster than light?, Local beables, No signals faster than light, Local commutativity, Who could ask for anything more, Principle of local causality, Ordinary quantum mechanics is not locally causal, Locally explicable correlations, Quantum mechanics cannot be embedded in a locally causal theory, But still we cannot signal faster than light, Conclusion.
Don't you think that suggests that locality was a rather important issue for Bell? But don't believe me -- read the paper.
Second, QM is as local or non-local as you interpret it to be. This too is basic and why folks argue about Copenhagen vs. Many Worlds vs. Bohmian Mechanics vs. Transactional Interpretation etc. Many scientists feel that QM's collapse postulate is not "non-local" in the sense that no signalling mechanism is violated. So I disagree with you that relativity is violated.
I'm sorry, but isn't this just silly naked subjectivism? Would you say that Bohmian Mechanics "is as local or non-local as you interpret it to be"? No way! Bohmian Mechanics is a definite theory, defined by certain equations. If you want to find out if the theory is local or non-local, you just examine the theory carefully and see how it works and assess whether or not it includes non-local physics. There's nothing subjective about this. You don't just close your eyes and inhale incense and wait for a mystical experience to tell you whether it's nonlocal. You just look. And in the case of Bohmian Mechanics, there is no controversy or ambiguity: it's non-local. Specifically, it violates Bell Locality (though it is consistent with "signal locality" -- you can't transmit a message faster than light).
It's the same story with orthodox Copenhagen QM. There's nothing open or unclear. Whether the theory is local or nonlocal isn't a matter of feelings or mystical revelation. You just look at the theory and test: is it consistent with Bell locality? Is it consistent with signal locality? The answers are respectively: no, yes. Same as Bohmian Mechanics. So if you think Bohm's theory violates relativity, you have to think that orthodox QM also violates relativity (on pain of inconsistency). There is nothing to debate here -- just something that needs to be grasped clearly.
 

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