Is QM Inherently Non-local in EPR and Bell Discussions?

[Sherlock]

"Bell Locality" is a particular definition of what it means for a theory to be local. So the obvious way to use it is to see if a theory satisfies it -- i.e., to see if a given theory is local.
...
Look, it's really simple. Forget for the moment about hidden variables and everything else. Suppose you accept "Bell Locality" as a definition of what it means for a theory to be local. Just look at orthodox QM and ask: is it Bell Local? Answer: no. Now you think: is there some way I could fiddle with QM (e.g., by adding hidden variables) in order to construct a Bell Local theory (making sure of course to stay true to the QM predictions since those are verified by experiment)? Answer: no. (Bell's theorem.) So no Bell Local theory can agree with experiment, whether it has hidden variables or not. There's no empirically viable theory that is Bell Local. So nature violates Bell Locality. QED.

One thing I don't get is why/how OQM violating Bell Locality, (which I'll accept), necessarily leads to nature being nonlocal. It's clear that local hidden variable theories must satisfy Bell Locality. But hidden variables are based on the classical mechanical idea that a one to one mapping between theory and nature is possible. OQM prohibits such a one to one mapping (for reasons that I'm just beginning to understand).


Insofar as classical mechanics is valid, then the principle of locality holds. However, In the range of quantum phenomena, classical mechanics breaks down, and, if the principles of quantum theory are correct, there's not even the possibility of the sort of relationship between theory and nature that hidden variables require.


So what does it mean to say that OQM is Bell Nonlocal -- that is, when the Bell Locality test, P{A|a-hat} = P{A|a-hat, B, b-hat}, is applied to OQM, and OQM is thereby discerned to be 'nonlocal', then what's the physical meaning of the term 'nonlocal' in this context?


If you interpret it to mean that A and B are causally affecting each other in real space and time, then I would say that that interpretation isn't necessarily correct.


Something you seem to have glossed over, or just aren't considering (even though you're well aware of it) is that changes in the wave function of the AB system when either A or B has registered a detection are happening in an imaginary space.


The probability of A after B acquires a definite value is given by the unitary-space analogue of the square of the cosine of the angle between b-hat and a-hat. This is an underivable formal assumption of quantum theory (the basis for which, in the development of OQM, I'm also just learning).


Whether this axiom's physical (predictive) utility corresponds in any way to physically nonlocal phenomena is unknown (at least to me). So, pending a definitive assessment of that, the assumption that the natural universe obeys the principle of locality remains -- and the physical meaning of calling OQM a nonlocal theory based solely on the Bell Locality test remains unclear.

It was known before Bell that hidden variable theories were incompatible with qm, wasn't it?

Um, seeing as the statement is completely false, no, it wasn't known before Bell (nor after).

I asked the question because in the section of Bohm's Quantum Theory text entitled Proof that Quantum Theory is Inconsistent with Hidden Variables he says, "We conclude then that no theory of mechanically determined hidden variables can lead to all the results of the quantum theory." (The italics are Bohm's.)
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I've gotten some of the original papers contributing to the development of quantum theory. Hopefully these will give me a better idea of whether, and in what sense, to call qm a 'local' or a 'nonlocal' theory.

 

[ttn]

One thing I don't get is why/how OQM violating Bell Locality, (which I'll accept), necessarily leads to nature being nonlocal.

By itself, it doesn't. It merely leads to the conclusion that if you construct a local theory by adding hidden variables to QM, maybe there's a good chance that local theory will be consistent with experiment... in which case you'd be in a good position to argue that nature is local.

The problem is that Bell's theorem tells us we can't have any such local hvt that agrees with experiment.

Here's the logic:
1. OQM is nonlocal
2. In order to make a local theory, you have to add a certain specific kind of hidden variables to OQM
3. Theories containing those specific kinds of hv's cannot agree with experiment

Conclusion: nature is nonlocal.

So what does it mean to say that OQM is Bell Nonlocal -- that is, when the Bell Locality test, P{A|a-hat} = P{A|a-hat, B, b-hat}, is applied to OQM, and OQM is thereby discerned to be 'nonlocal', then what's the physical meaning of the term 'nonlocal' in this context?

All alone, that point has no physical significance. It just says: if OQM is *true*, then nature would be nonlocal. But of course we don't know, really, whether OQM provides a correct description of nature, i.e., is true.

This is the point I was trying to make earlier, that our knowledge of nature "goes through" theories. We can assess straightforwardly whether or not a given theory is local (e.g., by asking if it respects Bell Locality). But in order to say anything about nature, we need to know whether a given theory is *right*. And that's notoriously *hard*, especially when the theories in question are all the various (proposed and as-yet unimagined!) possible interpretations of QM! But this is what's so beautiful about Bell's theorem. It wipes out a whole broad class of theories as definitely not viable. And we also know that that wiped out class is *all the possible local theories*! It's only because of that amazing generality that we can say anything about nature from all of this.

Whether this axiom's physical (predictive) utility corresponds in any way to physically nonlocal phenomena is unknown (at least to me).

Look, OQM is just nonlocal. You don't have to figure out the one true interpretation of QM to know that. OQM boldly asserts that the wf alone provides a complete description of reality. This entails non-local action at a distance during measurements, when the wf collapses. You're absolutely right that OQM might turn out to be a *wrong* theory of nature. I'm nearly 100% sure it's wrong. But whether it's right or wrong doesn't matter here. The claim is just that *if* it's right, then nature would be nonlocal. The world that OQM describes is nonlocal, whether that is our world or not.

I asked the question because in the section of Bohm's Quantum Theory text entitled Proof that Quantum Theory is Inconsistent with Hidden Variables he says, "We conclude then that no theory of mechanically determined hidden variables can lead to all the results of the quantum theory." (The italics are Bohm's.)

That book was published the year before Bohm discovered Bohm's alternative hidden variable quantum theory. So it doesn't exactly represent his mature views.

Maybe you knew that, but if not, I can see why you're very confused by it.

 

[DrChinese]

Here's the logic:
1. OQM is nonlocal
2. In order to make a local theory, you have to add a certain specific kind of hidden variables to OQM
3. Theories containing those specific kinds of hv's cannot agree with experiment
Conclusion: nature is nonlocal.

OR...

1. Start with something that is not commonly accepted (1, as should be obvious from discussions in this thread);

2. Add a deduction that is invalid (2, no one has really added any variables to QM to get a specifically local version);

3. Then throw in a seemingly important fact that actually does not apply here (3, non-locality is not a logical deduction from the negation of local realism);

4. Voila, you have the conclusion you wanted in the first place.

 

[ttn]

OR...
1. Start with something that is not commonly accepted (1, as should be obvious from discussions in this thread);

As I said before, I'm more concerned with what's true than what's "commonly accepted."

 

[Hurkyl]

The problem I have with your posts, ttn, is you seem to carefully speak about the specific condition of "Bell locality", and then you eventually start using "nonlocal" without the qualifier, making it seem that you're talking about the general concept of locality, rather than specifically meaning to say that something is "Bell-nonlocal".

 

[ttn]

The problem I have with your posts, ttn, is you seem to carefully speak about the specific condition of "Bell locality", and then you eventually start using "nonlocal" without the qualifier, making it seem that you're talking about the general concept of locality, rather than specifically meaning to say that something is "Bell-nonlocal".

Good point. It's true, I don't always specify, and you're right that usually I mean Bell Locality. But the problem, really, is that there is no such thing as "the general concept of locality." There are several distinct senses of the term "locality" and that is part (but only part) of what contributes to the never ending confusion over this topic. For example, there is the issue of "signal locality" which has to do with whether a theory permits signals to be sent superluminally. (Both OQM and Bohmian Mechanics are signal local.) And there is the issue of "Bell Locality" (which Bell proposed as a way to test if a theory respects what he called "local causality", obviously a more stringent condition than signal locality since it is possible for a theory to violate Bell Locality and yet to be signal-local). And there are at least a half dozen other senses of "locality" that people have talked about (e.g., whether matter can move faster than light, whether information can move faster than light, etc...).

So the conclusion I've been lobbying for here lately is this: nature isn't Bell Local. And the reasoning for this is what I outlined earlier: Orthodox QM violates Bell Locality, and it's clear exactly what you'd need to have a Bell Local theory which accounts for the perfect correlations when alice and bob measure in the same direction. But then Bell's theorem proves that a local hvt like that can't agree with experiment. So there's no Bell Local theory that can agree with experiment.

Hope that clarifies.

 

[Sherlock]

... OQM is just nonlocal.

I agree that a straightforward application of Bell's Locality test to OQM allows that it's a nonlocal theory, in some sense -- however the sense in which it's a nonlocal theory needs be explicitly qualified. Accordingly, I suggest that Bell Nonlocality is an artifact of limitations on theoretical representations of probings of the micro-reality of quantum level events.


The form of OQM is constrained by the principles of the theory. In a local universe, these principles don't allow a hidden variable formulation of the theory. In a local universe, OQM can't, in principle, be an explicitly local theory.


Any explicitly local theory of micro-reality would have to be a hidden variable theory. However, if we live in a universe constrained by the principle of locality, and in which the principles of OQM are valid, then no explicitly local theory of micro-reality is possible.


OQM's Bell Nonlocality is due to its necessary incompleteness as a description of nature. Whether there is actually any physical nonlocality happening in nature is an open question.

 

[Rade]

But the problem, really, is that there is no such thing as "the general concept of locality." There are several distinct senses of the term "locality" and that is part (but only part) of what contributes to the never ending confusion over this topic.

I have a question for ttn. On another thread I asked if the interaction between the neutron and proton in the deuteron [NP] could be tested via Bell Tests, and the answer I came away with was no, Bell Tests can say nothing about the local reality of the deuteron because it (Bell Test) does not apply to entities bound by strong force--would this be correct ? Thanks for any comments.

 

[ttn]

I have a question for ttn. On another thread I asked if the interaction between the neutron and proton in the deuteron [NP] could be tested via Bell Tests, and the answer I came away with was no, Bell Tests can say nothing about the local reality of the deuteron because it (Bell Test) does not apply to entities bound by strong force--would this be correct ? Thanks for any comments.

I don't really get the question. The standard setup to test Bell's Inequalities involves two spin- or polarization-entangled particles, like two spin-1/2 particles or two photons in a singlet state and propagating in opposite directions. A deuteron is a bound state where the two particles are right next to each other. So even if there were the correct type of spin entanglement, you'd never be able to make the two measurements at space-like separation. So you'd never be able to disentangle the *obvious* kinds of interactions (like the strong force) from the less obvious interaction that a Bell Inequality test is really looking for: namely, the weird nonlocal entanglement interaction by which the measurement on one particle subtly affects the other guy's spin.

Or maybe I'm just not understanding the point of the question.

 

[ttn]

OQM's Bell Nonlocality is due to its necessary incompleteness as a description of nature. Whether there is actually any physical nonlocality happening in nature is an open question.

That's one possible view, yes. If you take the wf as a complete description, OQM violates Bell Locality. So either the wf really is a complete description (and nature is nonlocal) or the wf *isn't* a complete description (and we can't yet say anything about nature). Sherlock and Einstein suggest the second option.

Unfortunately, Bell showed that that option fails. The additional variables you have to add to create a Bell Local explanation for the perfect correlations, turn out to have implications for non-identical magnet settings that conflict with experiment. So the Bell Nonlocality of OQM cannot be gotten rid of by adding Bell Local hidden variables. That's Bell's Theorem.

And that means you're going to be stuck with a Bell Nonlocal theory no matter what. (Well, as long as you don't reject the idea of definite outcomes for the measurements on both sides.) If you want to say that the wf alone provides a complete description, you can say that, and you have OQM -- a Bell Nonlocal theory. Or if you want to deny completeness and add some hidden variables (say, to solve the measurement problem) then the resulting theory will have to be Bell Nonlocal -- e.g., Bohmian Mechanics.

 

[Rade]

I don't really get the question. The standard setup to test Bell's Inequalities involves two spin- or polarization-entangled particles, like two spin-1/2 particles or two photons in a singlet state and propagating in opposite directions. A deuteron is a bound state where the two particles are right next to each other. So even if there were the correct type of spin entanglement, you'd never be able to make the two measurements at space-like separation. So you'd never be able to disentangle the *obvious* kinds of interactions (like the strong force) from the less obvious interaction that a Bell Inequality test is really looking for: namely, the weird nonlocal entanglement interaction by which the measurement on one particle subtly affects the other guy's spin.
Or maybe I'm just not understanding the point of the question.

Thank you, you answered my question, e.g., the deuteron [NP] by definition is a type of local reality entity with two particles having spin (e.g., isospin) that exists prior to my observation because it does not have as you say a "weird nonlocal entanglement interaction" where the isospin can be tested by a Bell type Test. Thus you have defined a limit on what Bell Tests can and cannot say about local realism, which is what I was looking for, unless I have confused what you did say.

 

[Sherlock]

That's one possible view, yes. If you take the wf as a complete description, OQM violates Bell Locality. So either the wf really is a complete description (and nature is nonlocal) or the wf *isn't* a complete description (and we can't yet say anything about nature). Sherlock and Einstein suggest the second option.
Unfortunately, Bell showed that that option fails.

I don't start by assuming the completeness or incompleteness of OQM. I start by assuming that its principles are essentially correct, and that nature is local.

The principles of OQM exclude the formulation of either a complete theory or an explicitly local theory, because both of these require a hidden variable formulation, and hidden variables are excluded by the principles of OQM in a universe constrained by the principle of locality.

Thus, the Bell Nonlocality of OQM, or of any theory (about a local universe) that mimics the predictions of OQM, is an *artifact* of nature's (and therefore theoretical) constraints as, I think, are essentially correctly laid out in relativity theory and OQM.

... the Bell Nonlocality of OQM cannot be gotten rid of by adding Bell Local hidden variables.

I agree.

That's Bell's Theorem.

That's also OQM, the principles of which lead to the same conclusion.

And that means you're going to be stuck with a Bell Nonlocal theory no matter what.

I agree.

(Well, as long as you don't reject the idea of definite outcomes for the measurements on both sides.)

We have Bell Nonlocality as long as (in a theory constrained by the principle of locality) we reject the idea of ascertaining definite physical states of individual particles independent of measurement -- and OQM requires us to do just that.

If you want to say that the wf alone provides a complete description, you can say that, and you have OQM ...

The principles of OQM provide no basis for saying that the wave function is a complete description of physical reality. In fact, they lead to the opposite conclusion, because they exclude a hidden variable formulation -- and a hidden variable formulation is what would be required to have a complete description of physical reality.

Or if you want to deny completeness and add some hidden variables (say, to solve the measurement problem) then the resulting theory will have to be Bell Nonlocal ...

The incompleteness of OQM as a description of physical reality, and its accompanying Bell Nonlocality, follows necessarily from the assumptions that nature is local and that the principles of OQM are essentially correct.

... eg., Bohmian Mechanics.

The problem with Bohmian Mechanics is that it is explicitly nonlocal.

Such a theory can be constructed so that it mimics the predictions of OQM. However, we start by assuming that nature is local (for very good reasons) and that assumption hasn't been invalidated by the arguments.

But the fact of the matter is that we don't know if nature is local or nonlocal. So, perhaps OQM and Bohmian Mechanics should both be developed, and if the predictions of Bohmian Mechanics should ever prove to be more accurate than those of OQM, then we've got a good reason for assuming that nature is nonlocal (in some physical sense) and preferring Bohmian Mechanics to OQM.

The way things are at the moment, OQM is to be preferred because the assumption of locality has a better foundation than the assumption of nonlocality.

 

[johnf]

.
But the fact of the matter is that we don't know if nature is local or nonlocal. So, perhaps OQM and Bohmian Mechanics should both be developed, and if the predictions of Bohmian Mechanics should ever prove to be more accurate than those of OQM, then we've got a good reason for assuming that nature is nonlocal (in some physical sense) and preferring Bohmian Mechanics to OQM.
The way things are at the moment, OQM is to be preferred because the assumption of locality has a better foundation than the assumption of nonlocality.

I favour the Bohmian approach and believe that Nature 's processes are inherently non-local. We construct and favour local theories psychologically perhaps, because they seem more primitive, basic and immediate to us.

I would argue that quantum theory is more naturally a macroscopic thermodynamic theory of the large than it is a microscopic theory of the small. Like thermodynamics, which is rigorously accurate only the the limit of infinitely extensive quantities, the most primitive and and logically rigorous and consistent constructs of QM are objects like plane waves and fields of infinite spatio-temporal extent.

I would like to see the measurement process at a microscopic level as a 'yanking' or extraction of a field excitation from a global cosmological entanglement. In performing such a measurement one is choosing to localise a process, to the extent that one's detector is localised. The locality may be thus more a property of our measurements than an inherent property of Nature.

I was led to adopt and prefer this kind of thinking by my Machian and thermodynamic conception and view of such questions as the significance of physical quantities such as inertia and the speed of light.

 

[Sherlock]

I favour the Bohmian approach and believe that Nature 's processes are inherently non-local. We construct and favour local theories psychologically perhaps, because they seem more primitive, basic and immediate to us.

The discussion in this thread has resolved (I think) that i) QM is inherently non-local, and ii) that this formal non-locality is due to constraints on experimental determinations of fundamental quantum processes which prohibit a hidden variable formulation.
Whether Nature's processes are inherently non-local can't be resolved from currently available information.

I would argue that quantum theory is more naturally a macroscopic thermodynamic theory of the large than it is a microscopic theory of the small.

I agree, in a sense, because the theory's basic datum is detector response, and its basic data sets are, necessarily, manifestations of macroscopic reality in the form of irreversible measurement results.

I would like to see the measurement process at a microscopic level as a 'yanking' or extraction of a field excitation from a global cosmological entanglement. In performing such a measurement one is choosing to localise a process, to the extent that one's detector is localised. The locality may be thus more a property of our measurements than an inherent property of Nature.

What do you mean by "cosmological entanglement"?

Locality is a property of any individual measurement, afaik. However, an explicitly local description of correlations between entangled quantum meansurements would require a more complete specification of the histories of the individual quanta than is possible.

Nevertheless, that the entanglement (at the level of the micro or sub-micro quantum reality) has a local origin can be inferred due to the involvement of the conservation laws which don't require that the particles involved be followed through continuous paths in space time.

I was led to adopt and prefer this kind of thinking by my Machian and thermodynamic conception and view of such questions as the significance of physical quantities such as inertia and the speed of light.

This is some interesting stuff for a new thread.

 

[Sherlock]

Thank you, you answered my question, e.g., the deuteron [NP] by definition is a type of local reality entity with two particles having spin (e.g., isospin) that exists prior to my observation because it does not have as you say a "weird nonlocal entanglement interaction" where the isospin can be tested by a Bell type Test. Thus you have defined a limit on what Bell Tests can and cannot say about local realism, which is what I was looking for, unless I have confused what you did say.

*Local realism* refers to what theoretical constructions are possible wrt descriptions of spacelike separated correlations. Bell's Theorem and Bell tests are the 'icing on the cake', so to speak, of the argument (which, pre-Bell, began with an application of the principles of quantum theory to the question) which excludes the possibility of constructing a local realistic theory of the quantum world. This of course doesn't mean that the quantum world isn't conforming to the principle of locality or that it isn't real -- just that certain macroscopic manifestations of it can't be explicitly formally described in local realistic terms.

 

[DrChinese]

The discussion in this thread has resolved (I think) that i) QM is inherently non-local, and ii) that this formal non-locality is due to constraints on experimental determinations of fundamental quantum processes which prohibit a hidden variable formulation.

1. ttn believes oQM is inherently non-local, where we define non-local as violating "Bell Locality". There have been no substantive arguments to support that position, since violation of Bell Inequalities is not generally considered to be a proof of ttn's position. Also, we have concluded that there is no change in outcomes as seen by Bob as a result of anything Alice does. To be fair: among those who accept this definition (non-local means violating Bell Locality), there are many who agree with ttn's views.

2. Generally, oQM IS considered non-local IF you define non-locality as being represented by instantaneous collapse of the wave function. Many do not adhere to this definition, which is why the Bell Locality definition was introduced - and is partly why ttn pushes that definition. If you believe that "instantaneous collapse of the wave function" is evidence of non-locality, then Bell tests might push you more to this view since the Bell tests are pretty conclusive. On the other hand, this was something that oQM has seen as fundamental from before EPR and is therefore nothing new.

For 1. and 2.: Please note that Bell tests do not purport to provide evidence of a violation of locality. Look at any Bell test paper and they will not draw this as a conclusion - they simply say that local hidden variable theories are excluded.

3. There is also the issue of whether "signal locality" is violated by oQM. Generally, this is NOT considered a feature of oQM - nor is it something considered demonstrated by Bell tests (such as Aspect). Many believe that signal locality must be violated to demonstrate that SR is inconsistent with oQM.

Conclusions:

Most scientists do not see a violation of relativity in a violation of a Bell Inequality. I.e. The mainstream position is that Bell's Theorem has no direct absolute bearing on whether oQM is or is not considered "inherently non-local". However, there is a substantial group that hold onto "reality" as a continuing requirement (post Aspect) and therefore find themselves in the non-local realistic camp. (I guess some of those individuals might support Bohmian Mechanics without even realizing it.)

What would a "non-realistic" universe look like? This trips up a lot of people, but according to Bell: this is a universe in which there aren't "answers" for "questions" which aren't asked. I.e. there are no well-defined values for observables independent of the act of measurement. This is fully in keeping with the ideas of oQM. If you accept the idea of the HUP being fundamental - and not a technological limitation due to the resolution of our experimental apparatus - then you already accept the idea of a "non-realistic" universe.

And that universe could be local, and still satisfy Bell's Theorem.

 

[Sherlock]

1. ttn believes oQM is inherently non-local, where we define non-local as violating "Bell Locality". There have been no substantive arguments to support that position, since violation of Bell Inequalities is not generally considered to be a proof of ttn's position. Also, we have concluded that there is no change in outcomes as seen by Bob as a result of anything Alice does. To be fair: among those who accept this definition (non-local means violating Bell Locality), there are many who agree with ttn's views.

I should have qualified my statement. OQM is inherently Bell Non-Local, because Bell Locality, P(A|a) = P(A|a,B,b), is violated, formally, by OQM.

ttn's argument that OQM is Bell Non-Local seems correct to me. The argument doesn't refer to Bell's Theorem or violations of Bell inequalities. It just has to do with relating Bell's formal locality condition to OQM.

ttn also argues that the non-locality of Nature is implied by taking into account EPR, Bell's Theorem, violations of Bell inequalities, the Bell Non-Locality of OQM, and the assumption that OQM is a complete description of the physical reality of quantum systems. However, I think, and I take it that you do too, that there's something wrong with this argument and the conclusion that Nature is, necessarily, non-local.

2. Generally, oQM IS considered non-local IF you define non-locality as being represented by instantaneous collapse of the wave function. Many do not adhere to this definition, which is why the Bell Locality definition was introduced - and is partly why ttn pushes that definition. If you believe that "instantaneous collapse of the wave function" is evidence of non-locality, then Bell tests might push you more to this view since the Bell tests are pretty conclusive. On the other hand, this was something that oQM has seen as fundamental from before EPR and is therefore nothing new.

Without a specific formal test such as Bell Locality, I wouldn't necessarily think of OQM as a non-local theory. OQM, considered by itself, isn't explicitly local or non-local as far as I can tell.

For 1. and 2.: Please note that Bell tests do not purport to provide evidence of a violation of locality. Look at any Bell test paper and they will not draw this as a conclusion - they simply say that local hidden variable theories are excluded.

Ok ... that's the way I'm thinking about it.

3. There is also the issue of whether "signal locality" is violated by oQM. Generally, this is NOT considered a feature of oQM - nor is it something considered demonstrated by Bell tests (such as Aspect). Many believe that signal locality must be violated to demonstrate that SR is inconsistent with oQM.

I don't know what "signal locality" means exactly. I've Googled a bit to find out, but haven't found a precise definition yet.

Anyway, I don't think that anything that's been said in this thread, or Bell tests, or Bell Locality, etc., demonstrate that non-locality is a fact of Nature. It's just that no theory of quantum correlations can be explicitly local, because that would require hidden variables and they're ruled out.

Conclusions:

Most scientists do not see a violation of relativity in a violation of a Bell Inequality. I.e. The mainstream position is that Bell's Theorem has no direct absolute bearing on whether oQM is or is not considered "inherently non-local".

Ok ... that's also the way I'm thinking about it.

However, there is a substantial group that hold onto "reality" as a continuing requirement (post Aspect) and therefore find themselves in the non-local realistic camp. (I guess some of those individuals might support Bohmian Mechanics without even realizing it.)

I'm not a member of that camp. Although I held onto the idea that realistic or hidden variable theories should be possible (even in a local universe) for quite a while. I think the argument that natural processes are constrained by the principle of locality is stronger than the argument that they aren't. So, Bohmian Mechanics, while apparently empirically viable, isn't to be preferred over OQM.

The argument against the construction of realistic or hidden variable theories of quantum processes constrained by the principle of locality is very strong.

What would a "non-realistic" universe look like? This trips up a lot of people, but according to Bell: this is a universe in which there aren't "answers" for "questions" which aren't asked. I.e. there are no well-defined values for observables independent of the act of measurement. This is fully in keeping with the ideas of oQM. If you accept the idea of the HUP being fundamental - and not a technological limitation due to the resolution of our experimental apparatus - then you already accept the idea of a "non-realistic" universe.

I agree that OQM is a theory about experimental determinations of quantum processes, and the uncertainty relations specify a constraint on such experimental determinations. How fundamental the theory is wrt Nature itself is unknown. But I believe that it's as accurate as any fundamental theory can be (assuming that all processes in our universe are constrained by the principle of locality).


There is a small matter of terminology in accepting "the idea of a 'non-realistic' universe". It's confusing (at least to us lay persons) to refer to the universe as "non-realistic". Rather, just saying that there can be no realistic description of quantum processes more clearly communicates what the principles of OQM and Bell's Theorem and Bell tests are revealing wrt the question of the viability of local hidden variable theories.

And that universe could be local, and still satisfy Bell's Theorem.

I don't know what you mean here. The universe isn't what's satisfying (or not) Bell's Theorem, is it? :-)


Anyway, I think we're pretty much on the same page wrt the Bell stuff, aren't we? Am I up to speed yet? I should have qualified my remark about the inherent non-locality of OQM. We've only resolved that it's inherently Bell Non-Local, and that only tells us something about OQM, not about Nature. For, even with its inherent Bell Non-Locality, OQM is still not explicitly non-local in a realistic or hidden variable sense.

 

[ttn]

1. ttn believes oQM is inherently non-local, where we define non-local as violating "Bell Locality". There have been no substantive arguments to support that position, since violation of Bell Inequalities is not generally considered to be a proof of ttn's position. Also, we have concluded that there is no change in outcomes as seen by Bob as a result of anything Alice does. To be fair: among those who accept this definition (non-local means violating Bell Locality), there are many who agree with ttn's views.

You misunderstand the argument. One doesn't need anything as fancy as a theorem to show that OQM violates Bell Locality. One just asks: is it true, in OQM, that the probability of a given event P(A|a,psi) is unchanged when one also conditionalizes on some information pertaining to a space-like separated region? The answer is clearly no. For example, the marginal probability for Alice to measure spin up is 50%, but the conditional probability for this same event when we specify also that Bob got "spin down" is 100% -- even though Bob's getting "spin down" is not in the past light cone of Alice's measurement. So Bell Locality is violated. (Note: this assumes that the wf "psi" is a complete description of the state of the particles prior to the measurements. In fact, that assumption is what I mean by the "O" in OQM.)

2. Generally, oQM IS considered non-local IF you define non-locality as being represented by instantaneous collapse of the wave function. Many do not adhere to this definition, which is why the Bell Locality definition was introduced - and is partly why ttn pushes that definition. If you believe that "instantaneous collapse of the wave function" is evidence of non-locality, then Bell tests might push you more to this view since the Bell tests are pretty conclusive. On the other hand, this was something that oQM has seen as fundamental from before EPR and is therefore nothing new.

If the wf provides a complete description of reality (as Bohr insisted) then there should be no question about this: the collapse of the wf describes a real instantaneous physical change in the state of distant systems, caused by some local measurement. So it's a nonlocal theory. (This is just a looser way of saying what is made precise by the above argument in terms of Bell Locality.)

For 1. and 2.: Please note that Bell tests do not purport to provide evidence of a violation of locality. Look at any Bell test paper and they will not draw this as a conclusion - they simply say that local hidden variable theories are excluded.

That's because that is exactly what *is* excluded by violation of Bell's Theorem. The question is: is there a local theory without hidden variables that can account for the data? OQM certainly isn't one. Can you provide an example?

3. There is also the issue of whether "signal locality" is violated by oQM. Generally, this is NOT considered a feature of oQM - nor is it something considered demonstrated by Bell tests (such as Aspect). Many believe that signal locality must be violated to demonstrate that SR is inconsistent with oQM.

There is no controversy about this. OQM is signal-local. And so, by the way, is Bohmian Mechanics. Which is why it's so annoying when the "many" you refer to here dismiss Bohmian Mechanics by saying it's nonlocal and hence not consistent with relativity. The fact is, Bohm is exactly as consistent with relativity as OQM, on (at least) the two major definitions of locality that are relevant here: Bell Locality and Signal Locality.

Most scientists do not see a violation of relativity in a violation of a Bell Inequality.

Though, for the record, Bell hiimself *did* believe that this was the issue.

I.e. The mainstream position is that Bell's Theorem has no direct absolute bearing on whether oQM is or is not considered "inherently non-local".

Bell's theorem alone isn't sufficient to prove that nature is nonlocal. It proves only that certain local theories aren't empirically viable. The question is: is there some other kind of local theory that is empirically viable? I'm still waiting to hear about an example...

However, there is a substantial group that hold onto "reality" as a continuing requirement (post Aspect) and therefore find themselves in the non-local realistic camp. (I guess some of those individuals might support Bohmian Mechanics without even realizing it.)

You make it sound like it's crazy to hold onto a belief in reality! Given that no local theory is empirically viable, it seems ridiculous to chastize people who prefer a clear, realistic (nonlocal) theory over a vague, ambiguously anti-realistic (nonlocal) theory.

What would a "non-realistic" universe look like? This trips up a lot of people, but according to Bell: this is a universe in which there aren't "answers" for "questions" which aren't asked. I.e. there are no well-defined values for observables independent of the act of measurement. This is fully in keeping with the ideas of oQM. If you accept the idea of the HUP being fundamental - and not a technological limitation due to the resolution of our experimental apparatus - then you already accept the idea of a "non-realistic" universe.
And that universe could be local, and still satisfy Bell's Theorem.

What do you mean "that universe could be local"? Are you saying there's a theory along these lines (one that doesn't attribute definite values to un-asked questions, or whatever) which is consistent with Bell Locality and which is consistent with experiment? What theory is that exactly? How does it work?

 

[ttn]

Anyway, I don't think that anything that's been said in this thread, or Bell tests, or Bell Locality, etc., demonstrate that non-locality is a fact of Nature. It's just that no theory of quantum correlations can be explicitly local, because that would require hidden variables and they're ruled out.

I don't agree with the way you put this in the last sentence, but even leaving that aside what you say here makes no sense. If -- for whatever reason -- "no theory ... can be explicitly local" then that means nature is nonlocal, right? If no viable theory can get along without a certain feature, then that feature is part of nature, right? That's what it *means* to say that the theories can't get along without it -- they can't agree with *experiment* without it, they can't match the *facts* without it.

 

[DrChinese]

1. You make it sound like it's crazy to hold onto a belief in reality! Given that no local theory is empirically viable, it seems ridiculous to chastize people who prefer a clear, realistic (nonlocal) theory over a vague, ambiguously anti-realistic (nonlocal) theory.

2. What do you mean "that universe could be local"? Are you saying there's a theory along these lines (one that doesn't attribute definite values to un-asked questions, or whatever) which is consistent with Bell Locality and which is consistent with experiment? What theory is that exactly? How does it work?

1. I do not in any way think that belief in (non-local) reality is crazy or undesirable. It is a reasonable choice among the 2 basic branches you get once you reject local reality.

2. oQM is as much a "local non-realistic" theory as a "non-local realistic" theory. This conclusion is consistent with Bell's Theorem and experiment. I'm not sure how this point is so difficult to understand. There are 2 basic assumptions which lead to Bell's Inequality: locality (expressed as the factorizing) and reality (expressed by the hypothesis of a third measurement setting c). Since we know Bell's Inequality is violated, at least one of these assumptions is wrong.

There is no experimental differentiation between these. Therefore the one you choose to throw out is purely a matter of personal choice, as I keep pointing out. This position is generally accepted within the physics community, although that is apparently something you don't consider relevant. I do.

 

[ttn]

2. oQM is as much a "local non-realistic" theory as a "non-local realistic" theory. This conclusion is consistent with Bell's Theorem and experiment. I'm not sure how this point is so difficult to understand. There are 2 basic assumptions which lead to Bell's Inequality: locality (expressed as the factorizing) and reality (expressed by the hypothesis of a third measurement setting c). Since we know Bell's Inequality is violated, at least one of these assumptions is wrong.

Ack! How many times do I have to say it? Violations of Bell's Inequalities *alone* are not sufficient to prove the point I'm arguing for. You are absolutely correct that Bell derives the inequalities from several assumptions, most importantly (1) Bell Locality and (2) the existence of certain "hidden variables". So if all one knew was that Bell's inequalities are empirically violated, all one would be able to say is that either (1) or (2) is false. Here we absolutely agree.

But you don't seem to be willing to accept that Orthodox QM is a particular theory, which makes definite commitments about what is and isn't real. In particular, according to OQM, the wf alone constitutes a complete description of reality. There are no hidden variables. This *immediately* means that Bell's Theorem simply does not apply to OQM. OQM isn't a member of the class of theories which Bell's Theorem shows to be inconsistent with experiment. I think here we also agree, but this puzzles me, because I have repeatedly attempted to clarify that this *isn't* why I say OQM violates Bell Locality. Violating Bell Locality and violating Bell's Inequality are *not* the same thing. My argument is *not* that Bell's Inequalities are empirically violated, hence OQM is nonlocal. That would be a totally invalid argument for the reason I just stated: OQM isn't one of the theories to which Bell's Theorem even *applies* -- it isn't one of the theories that Bell's Theorem says have to obey the inequality.

So, one last time: that is *not* my argument. I am *not* saying that OQM is nonlocal because Bell's Inequalities are empirically violated.

I am instead making a rather trivial observation: OQM does not respect the mathematical condition (on conditional probabilities) called "Bell Locality". This is the same one that is used by Bell in the derivation of the inequality, but Bell Locality is just not the same thing as the inequality. And there is no assumption of hidden variables or anything else in the definition of Bell Locality. It's just a simple test for whether a theory includes superluminal/nonlocal causation. And OQM fails that test. It simply does not respect that mathematical condition, period. We don't have to do an experiment to know this. We simply have to define Bell Locality (which I have done repeatedly) and note that this condition is violated by OQM.

OQM is not a Bell Local theory. And if you or other members of the physics community refuse to admit/accept/see that, it's your problem. But it's a fact nonetheless.

Now maybe you want to say: "OK, OQM violates Bell Locality, and there's a theorem that hidden variables theories have to violate Bell Locality in order to be empirically viable, but maybe there's some other Bell Local non-hidden-variable theory that would be empirically viable." If that's your position, I'll ask you to put up or shut up. Show me an example of such a theory and then I'll accept that, after all, Bell Locality is consistent with the experiments.

But if your position is that OQM is a Bell Local theory, you are just WRONG.

 

[DrChinese]

One doesn't need anything as fancy as a theorem to show that OQM violates Bell Locality. One just asks: is it true, in OQM, that the probability of a given event P(A|a,psi) is unchanged when one also conditionalizes on some information pertaining to a space-like separated region? The answer is clearly no. ?

It is a true statement that Bell defined locality in this manner. This also being the conjunction of "parameter independence" (PI) and "outcome independence" (OI).

It is also a true statement that if you considered locality in terms of PI or OI individually, you would not see locality violated. Specifically, nothing Bob does affects P(A|a,psi), which is the PI case. So the question ultimately becomes, what is the meaning and relevance of Bell Locality as defined by Bell? A full analysis of this gets very complex and there are a lot of opinions on the subject. As I said before, there is group of scientists who believe that a) this is a good definition of locality; and b) such locality is violated by oQM (and nature). I readily acknowledge that Bell used this definition in his theorem and that would be a good reason to use it.

So I guess I can see how you come to your perspective. Your logic is: Bell's "Locality" is violated by oQM; it is one of the assumptions leading to Bell's Inequality; Bell's Inequality is violated and therefore viable theories are both non-local and consistent with oQM.

But wait... that exact same argument - in parallel form - is how I come to my conclusion! My logic is: Bell's "Reality" is violated by oQM; it is one of the assumptions leading to Bell's Inequality; Bell's Inequality is violated and therefore viable theories are both non-realistic and consistent with oQM.

So I guess the point I am making is that it is quite difficult to reduce it to your position ALONE when there are other viable options. And besides, your position is NOT the commonly accepted one. I would like those following this thread to walk away with that perspective.

 

[ttn]

So I guess I can see how you come to your perspective. Your logic is: Bell's "Locality" is violated by oQM; it is one of the assumptions leading to Bell's Inequality; Bell's Inequality is violated and therefore viable theories are both non-local and consistent with oQM.

I don't know what you mean at the end with "consistent with OQM". All empirically viable theories violate Bell Locality. That's my claim. And I also claim that that is grounds for saying that Nature violates Bell Locality.

But wait... that exact same argument - in parallel form - is how I come to my conclusion! My logic is: Bell's "Reality" is violated by oQM; it is one of the assumptions leading to Bell's Inequality; Bell's Inequality is violated and therefore viable theories are both non-realistic and consistent with oQM.

But that's not a good argument. It's true that OQM denies what you insist on calling "reality" but I think is better to call "hidden variables". But let's not get hung up on terminology. Given how you want to use the terms: yes, OQM denies Bell's "reality" assumption. Does that tell us anything about *nature* though? Surely not. It tells us only that a certain theory denies that certain observables are built of "elements of reality." But some other theory might include those purported elements of reality! And such a theory might even be empirically viable!

Here's a silly example. Suppose I have a new hidden variable theory in which particles are described by the usual quantum wave function obeying the usual dynamical rules including the collapse postulate. But suppose I say also that all particles have definite real pre-measurement values for the z-component of their spins. Heck, I can even say that they're all spin-up. But then, whenever any kind of measurement is made, the spin values stochastically change into new values in accordance with the usual QM rules. So when Alice measures the z-spin of her particle, say she gets the result "spin up" -- which in my theory I interpret as meaning that the initiall spin-up particle *stayed* spin-up when it interacted with her SG device. But then, at the instant Alice's measurement happens, Bob's distant particle suddenly *switches* from being spin-up to spin-down, and then, later, when Bob measures the z-spin of his particle, he finds it spin down.

Now, nobody should waste their time thinking about this theory. It probably has all sorts of problems and wouldn't really work or isn't well-defined or whatever. But who cares. My point is simply that you can *always* add structure to a theory and arrange to get the same predictions out. In particular, you could add (to OQM) the idea of definite spin component values for unmeasured things. No problem. (It's pointless and cumbersome, maybe, but it's *possible*.) And you can even have these things play a role in what is measured -- and you can even have the theory continue to be consistent with experiment -- so long as you permit just the right kind of non-local interactions to make all the spins suddenly flip into the right positions when measurements are made.

I assume/hope there is no controversy about this. My point is just this: there *are* empirically viable theories that posit more structure to the world than OQM does, i.e., there are empirically viable theories which do *not* violate Bell's "realism" (aka hidden variables) assumption. And that means there are no real grounds whatsoever for saying that nature violates this "realism" assumption.

This is of course to be contrasted with the issue of Bell Locality. There is *no* empirically viable theory which is Bell Local. So you can actually say something about *nature*, namely: Nature isn't Bell Local.

You *cannot* say anything parallel to that about "realism". So it is *not* a matter of personal preference which property (realism or locality) one wants to reject. One *has* to reject locality. Then, as a totally separate issue, one can *choose* whether or not to accept/reject realism. There are empirically viable theories that are non-local and non-realist. There are empirically viable theories that are non-local and realist. So one does have a choice about realism. But there are *no* empirically viable theories that are local and realist. (About that I know we agree... but:) There are also *no* empirically viable theories that are local and non-realist.

So taking this last position is irrational, based on fantasy not evidence or logic. One cannot get around the non-locality by denying "realism"... no matter how many people with PhD's want to pretend this is possible.

So I guess the point I am making is that it is quite difficult to reduce it to your position ALONE when there are other viable options.

This is my whole point. There *aren't* other viable options. Every viable option is non-local. There are options in regard to "realism". But there is no trade-off between realism and locality. One can't trade the non-locality for non-realism. We're stuck with the non-locality no matter what we say about realism.

And besides, your position is NOT the commonly accepted one. I would like those following this thread to walk away with that perspective.

Yup, my position is not the commonly accepted one. That's true.

 

[DrChinese]

...But if your position is that OQM is a Bell Local theory, you are just WRONG.

I can accept that Bell Locality (defined as PI+OI) is inconsistent with oQM. I can accept that Bell included this definition in his formulation of Bell's Theorem. I thank you for helping me to arrive at this common ground.

I think this is just a small part of the story, though. Obviously, there are still a lot of different conclusions to arrive at. For example, local theories are viable as long as they do not require what I might call "Bell Reality". And: loosening the standard for what we deem "local" (so that the OI condition is not required), oQM qualifies as such a local non-realistic theory.

To be fair to your position, we could also say that oQM does not fall within the class of theories affected by Bell's Inequality. Now that we have agreed on this point, what possible connection does any of this have to Bohmian Mechanics? Why would Bell have felt compelled to mention BM IF he already saw oQM as a good non-local theory (as you think is obvious and always has been) ?

I personally think the answer is (and this is somewhat speculative): Bell was not sure if OI made sense as a requirement of locality. After all, he saw oQM as Lorentz invariant! On the other hand, mentioning BM had the advantage of at least indicating that there existed theories other that oQM which might meet the standards of Bell's Theorem.

 

[DrChinese]

1. I don't know what you mean at the end with "consistent with OQM". All empirically viable theories violate Bell Locality. That's my claim. And I also claim that that is grounds for saying that Nature violates Bell Locality.

2. Yup, my position is not the commonly accepted one. That's true.

1. We know that one cannot construct a viable theory which is both local and realistic, and we agree on this point. If it is possible to construct a theory which mimics the predictions of QM but is non-local and realistic - that would be BM - then... it might also be possible to construct a a theory which mimics the predictions of QM but is local and non-realistic. That is simple logic.

I believe that a future version of Bell's Inequality may drop the OI requirement from the definition of locality; thus we would have oQM as this same (and now considered local) non-realistic theory. But this is not generally accepted at this time.

2. The reason I keep bringing this point up is for those who are following this discussion who might find this point relevant. (Like our moderators, for instance. )

 

[Sherlock]

If no viable theory can get along without a certain feature, then that feature is part of nature, right? That's what it *means* to say that the theories can't get along without it -- they can't agree with *experiment* without it, they can't match the *facts* without it.

The feature that quantum theory can't get along without, if it's going to provide a conceptual understanding, and accurate predictions, of quantum correlations in a local universe, is the conservation laws.


The feature that quantum theory would require in order to be explicitly local is the ability to experimentally track the trajectories of entangled particles (if you want to think in terms of particles), or, (if you want to think in terms of wave structures) the ability to experimentally track the evolutions of entangled disturbances. (I'll speak of particles for convenience.)


There's a theoretical constraint on the extent to which such tracking can be done. But quantum theory can still accurately predict the average results of experiments on entangled particles due to the incorporation of the conservation theorems into the theory -- which could be ported to quantum theory from classical theory, because the classical conservation laws don't require following the particles through continuous paths in space time.


From Bohm's text, Quantum Theory:

"Even in an impulsive collision in which we cannot follow the motion continuously, these laws apply for the collision as a whole. Such laws do have meaning even in discontinuous processes. It is an experimental fact that these laws can all be taken over directly into the quantum theory. ...
... Hence, not all classical deterministic laws must be abandoned, but only those requiring a description in terms of continuous processes."


Quantum theory isn't explicitly local because it can't be. It isn't explicitly non-local because it was developed in line with the assumption of locality, and the idea that quantum correlations can be conceptually understood (if not explicitly described) as emerging from relationships (caused by local interactions), due to conservation laws, between particles which have a common source, or which have interacted, or which have been altered by a common torque, etc.


So, there's no reason to scrap the assumption of locality in Nature just because OQM (or any theory of quantum processes if the principles of OQM are essentially correct) can't be formulated as an explicitly local theory.

 

[ttn]

And: loosening the standard for what we deem "local" (so that the OI condition is not required), oQM qualifies as such a local non-realistic theory.

And loosening the standard for what we deem "a peanut butter sandwich", Beethoven's 5th symphony qualifies as a peanut butter sandwich!

Sorry, I couldn't resist.

By the way, if you're interested in the relation of Bell Locality to Outcome Independence and Parameter Independence, you absolutely must read chapter 4 (esp. the section called "What does factorization signify?") of Tim Maudlin's spectacular book "Quantum Non-Locality and Relativity." He shows that the parsing of Bell Locality into "OI" and "PI" isn't unique, and that all the conclusions change if you do it a different way. Let me clarify that. Bell Locality is, as you noted, the conjunction of OI and PI. But it is also the conjunction of another pair of requirements, call them OI' and PI'. OQM violates OI but not PI. But on the other hand OQM violates PI' but not OI'. And vice versa for Bohmian Mechanics -- it violates PI but not OI, and OI' but not PI'. So which one is really local and which one isn't? It simply isn't clear... which is precisely why it's better to drop the whole "deflated elephant" (as one researcher once described it to me) of Outcome- and Parameter-Independence, and simply talk about Bell Locality.

(BTW, note also that Jarrett, who discovered the whole OI/PI thing using different terminology, initially asserted that a violation of PI means a violation of signal locality. And that is just plain wrong. So you have to be very very careful believing what people -- even "experts" -- say on these topics. Except Maudlin. He's right about everything.)

To be fair to your position, we could also say that oQM does not fall within the class of theories affected by Bell's Inequality. Now that we have agreed on this point, what possible connection does any of this have to Bohmian Mechanics? Why would Bell have felt compelled to mention BM IF he already saw oQM as a good non-local theory (as you think is obvious and always has been) ?
I personally think the answer is (and this is somewhat speculative): Bell was not sure if OI made sense as a requirement of locality. After all, he saw oQM as Lorentz invariant! On the other hand, mentioning BM had the advantage of at least indicating that there existed theories other that oQM which might meet the standards of Bell's Theorem.

I'm not sure what you're getting at here. Bohm's theory doesn't really have anything to do with Bell's Theorem, except that we know, in part from Bell's Theorem, that an empirically viable theory (like Bohm's) will have to violate Bell Locality (which Bohm's theory indeed does).

Were you wondering why anyone would believe in Bohm's theory when there's already a perfectly good (but non-Bell-Local) theory out there, namely OQM? Here there's an easy answer: because Bohm's theory solves the measurement problem.

 

[ttn]

1. We know that one cannot construct a viable theory which is both local and realistic, and we agree on this point. If it is possible to construct a theory which mimics the predictions of QM but is non-local and realistic - that would be BM - then... it might also be possible to construct a a theory which mimics the predictions of QM but is local and non-realistic. That is simple logic.

Well, I don't agree. The kinds of hidden variables Bell assumes in deriving the inequality turn out to be the minimally necessary ones to build a Bell Local explanation of the perfect correlations. And that means it is *not* going to be possible to build an empirically viable Bell Local theory. That is, it is not "possible to construct a theory which mimics the predictions of QM but is local and non-realistic."

But I'm sure that if I'm wrong about this, you will be forthcoming with a counterexample to prove me wrong.

 

[Careful]

Just a few notes:

(a) Experiment does not show at all that the predictions of OQM are correct; to be precise, every experimental outcome has a local realist explanation up till now.
(b) It would be better for the discussion to specify precisely which Bell type of inequalities you are referring to: the orginal one, the CHSH inequality or the CH74 no enhancement inequality..
(c) There are factors in the experiments which are not explained by the simple singlet state description of OQM, such as an assymetry between detectors readings or detection probabilities which are dependent upon the relative detector settings.

Moreover, as I seem to remember, there are even contrived local realist models which fully agree with the outcome of OQM (the assumptions for Bell's theorem are violated here in a more subtle way - particles in the cavity are supposed to figure out what the dectors are going to do in the future and act accordingly). All processes involved are stil causal, but not Bell local...

 

[DrChinese]

And loosening the standard for what we deem "a peanut butter sandwich", Beethoven's 5th symphony qualifies as a peanut butter sandwich!

Sorry, I couldn't resist.

You must realize, as Bell did, that the $64,000 question is whether relativistic theories are compatible with Bell's Theorem. So naturally, the Bell Locality condition does have some baggage associated with it since it is a slightly different definition. Is the light cone a fundamental border for the propagation of causes and effects? And how does collapse fit into the equation... is this a cause or effect or what? So asking if OI could be dropped is not an irrelevant issue by any means. And again, I believe that generally accepted scientific opinion is more along the lines that I have outlined.

 

[NateTG]

Well, I don't agree. The kinds of hidden variables Bell assumes in deriving the inequality turn out to be the minimally necessary ones to build a Bell Local explanation of the perfect correlations. And that means it is *not* going to be possible to build an empirically viable Bell Local theory. That is, it is not "possible to construct a theory which mimics the predictions of QM but is local and non-realistic."
But I'm sure that if I'm wrong about this, you will be forthcoming with a counterexample to prove me wrong.

Bell's theorem relies on the assumption that certain physically untestable probabilities are well-defined. Without those probabilities, Bell's inequality is invalid, and the theorem does not apply. And, since these probabilties are physically untestable, they are not a necessary aspect of a hidden variable theory (or any physical theory.)
Thus, the assertion that "the kinds of hidden variables Bell assumes in deriving the inequality turn out to be the minimally necessary..." is questionable.
In fact, it should be quite easy to prove that any realistic theory which allows for 'faster than light correlation' but not for 'faster than light communication' can be mimiced by a theroy that is local but (potentially) unrealistic.

 

[Careful]

Two comments NateTG :

(a) By *Bell's theorem relies on the assumption that certain physically untestable probabilities are well-defined*, you probably refer to the fact that you need more than one dector setting on one side to get the inequality. If you check the derivation of the Bell inequalities carefully, you will see this is not a problem at all. The correlations can be (and have to be) interpreted as coming from independent measurements; otherwhise the Bell inequalities would be just trivial arithmetic inequalities (and therefore trivially satisfied by any experiment).

(b) The common statement that quantum field theory does not allow faster than light signalling is false. You can use the correlations beyond the lightcone in an experiment with three observers to send info faster than the speed of light (see Sorkin: Impossible measurements on Quantum fields). Another point of view would be that the measurement theory in QFT is incomplete.

 

[DrChinese]

Just a few notes:
(a) Experiment does not show at all that the predictions of OQM are correct; to be precise, every experimental outcome has a local realist explanation up till now.

As you are new to this forum, you may not realize that the subject of violation of Bell Inequalties is often a source of contentious discussion. I would recommend that you move to an existing thread - or start a new one - if you want to discuss that subject - or the accuracy of the predictions of QM. This thread is not related to tests of QM or Bell's Theorem.

If you would care to provide an example of any experiment which does not support the predictions of QM, please, go ahead. QM is a very successful theory.

As to the second part of your comment, local reality has been ruled out by at least a dozen different experiments. Besides the Aspect experiments and its subsequent highly improved PDC variants, there are the more recent GHZ type which violate local reality in individual tests. The only folks still adhering to the local realistic program are a relatively small band of diehards (such as Santos, Thompson, and our own nightlight).

Again, further discussion on this should be carried on elsewhere.

 

[Careful]

I do not think I have to inform you why these *diehards* have a point. Indeed, this thread is *is QM inherently non-local?*; however most people, including yourself, have turned this question into *Is nature inherently non local* with of course an affirmative answer attached. Now, I have nowhere seen any interception from your part when this deviation occured. Moreover, it is completely nonsensical to have a discussion about the *exact* issue of this thread; the answer is clearly yes (as well in the Bell as the operational sense of non-locality). You did not say anthing my comments concerning the violation of causality in the operational sense at all, and this is clearly relevant for the discussion.

Concerning your second and third paragraphs, I have already given the answers on that in my previous posting, I would appreciate any accurate comment on that.

 

[NateTG]

Two comments NateTG :
By *Bell's theorem relies on the assumption that certain physically untestable probabilities are well-defined*, you probably refer to the fact that you need more than one dector setting on one side to get the inequality. If you check the derivation of the Bell inequalities carefully, you will see this is not a problem at all. The correlations can be (and have to be) interpreted as coming from independent measurements; otherwhise the Bell inequalities would be just trivial arithmetic inequalities (and therefore trivially satisfied by any experiment).

My post was in response to the assertion that Bell's theorem cannot be circumvented by local non-realistic theories.
Let's say that I have a neutral electron source and I have a single detector that will randomly measure the electron's spin orientation along one of three axes $a,b$ or $c$ at $0,120$ and $240$ degrees off of the vertical respectively. Then there are six possible results this device can produce: $a^+,a^-,b^+,b^-,c^+,$ and $c^-$.
Now, if I take all of the electrons that gave a result of, say $a^+$ what is the probability that those electrons would have given $b^-$ if they had been measured along the $b$ axis instead, and, more importantly is it possible devise an experiment that will test the prediction?
AFAICT QM essentially dictates that we cannot know - so the 'probability' is meaningless in an experimental sense. From a theoretical perspective, one can either postulate that there is a probability -- which is AFAICT an example what the 'realistic' in QM cannot be local and realistic corresponds to, or one could say it's undefined - like, for example $\frac{0}{0}$.
In the latter case, Bell's theorem doesn't make sense anymore since it involves adding and subtracting undefined quantities.