Relativity & Quantum Theory: Is Locality Violated?

[RandallB]

Couldn't they come up with an experiment to show the validity of locality though?

That's exactly what John Bell was doing when he came up with his Bell's theorem.
Only thing is, so far it has shown that locality, and the action required of an ‘unknown variable’ of any kind requires impossible probabilities. (probability of an event being over 100% or less than 0%).
Indicating a "local" solution can only be non-sense until a different legitimate interpretation of those experiments can be found.

 

[LnGrrrR]

RandallB,

I've read about Bell's experiment, but it seems I can't quite get a grasp on exactly what it MEANS. Do you have perhaps a 'beginner's link' I could use? I've read the specifics of the experiment a few times now, but I can't seem to grasp why it invalidates 'hidden variables'.

Is it your belief that Bell's Inequality makes QM non-local (until a better interpretation is found)? Or am I misunderstanding you?

 

[DrChinese]

Do you have perhaps a 'beginner's link' I could use? I've read the specifics of the experiment a few times now, but I can't seem to grasp why it invalidates 'hidden variables'.

I maintain a page with a lotta links, from beginner on upward. Try:

http://drchinese.com/Bells_Theorem.htm

...and scroll down to the bottom for the links.

-DrC

 

[ttn]

If you look carefully, the standard QM makes no mention of anything being transferred from one physical location to another. In other words, no signal moved from one of the entangled particle to its partner. Furthermore, as has been discussed many times on here (see several postings by vanesch and DrChinese), superluminal communication cannot be accomplished via such "quantum teleportation".

Standard QM has, as part of its dynamics, the collapse postulate. This involves a change to the wave function at one location, due to a measurement that may have been made at a distant location. Standard QM also includes the so-called "completeness doctrine" according to which the wave function provides a complete description of the real physical state of the system. So the change in the wave function at a distant location must be interpreted as a physical change, yes? And, as the original poster suggested, this is in conflict with relativity's prohibition on superluminal causation.

This can all be made rigorous and precise by simply noting that orthodox QM violates a very plausible criterion expressing relativity's prohibition -- what I and others call "Bell Locality." (This is *not* the same thing as the Bell Inequality. Bell Locality is the "local causality" requirement that is one of the premises from which the Inequality is derived.)

You confuse things above by bringing up "signals" and "communication". Don't you think that relativity prohibits any superluminal action-at-a-distance *at all*, not merely such actions that can be harnessed by humans to build telephones (i.e., send "signals", i.e., "communicate")? What the heck does relativity care about humans? Or is your position that superluminal causation is perfectly consistent with relativity so long as the causation can't be harnessed by humans to build a telephone? Because it's just a fact that orthodox QM includes such a superluminal causal mechanism. So either you have to say that this is OK -- that this superluminal mechanism doesn't conflict with relativity because relativity only prohibits superluminal *communication*. Or you have to say, as the original poster suggested, that orthodox QM is inconsistent with relativity.

Saying "orthodox QM doesn't include any superluminal causal mechanisms" just doesn't fly. It just isn't true. (It would be true if you got rid of the collapse postulate and endorsed MWI, but this isn't orthodox QM... but that's crazy for different reasons!)

So in what way is Special Relativity violated here, and how come all those EPR-type experimental papers make zero mention of such an earth-shattering observation?

Because most experimentalists (and most, but not all, theorists) are confused about this stuff.

 

[ttn]

Of course, that does not *prove* absolutely that non-local forces/causes/signals are not possible; but it does help explain why non-local interpretations of QM are not more popular. Such interpretations are generally not Lorentz invariant.

Do you mean to imply that the collapse postulate of orthodox QM *is* Lorentz invariant? The theory's empirical predictions have a certain Lorentz invariance property, but the theory's *dynamics* (half of it, anyway -- the collapse postulate part) is badly Lorentz variant.

 

[RandallB]

Is it your belief that Bell's Inequality makes QM non-local (until a better interpretation is found)? Or am I misunderstanding you?

DrC links are excellent place to start.
QM doesn’t need to be “made” non-local it is because of HUP. That was Einstein’s big problem with it claiming it, QM, had to be incomplete (AKA wrong). Which lead to all those terse exchanges with Niels Bohr.
Being ‘non-local’ w/ HUP, there is no reason for to demand QM not break any logical probability rules.
But a classical local explanation must obey classical logic
– Ref: DrC pages on EPR-Bell.
And remember just because QM is “non-local” does not mean it requires observable FTL speed interactions, just HUP.

 

[quantumcarl]

I really like all this kind of talk:!) .

What I especially like is the fact that all the most experienced physicists have agreed that no one knows what's going on (yet).

Thanks to Dr.Chinese for the links page!

 

[UglyDuckling]

QM also includes the so-called "completeness doctrine" according to which the wave function provides a complete description of the real physical state of the system. So the change in the wave function at a distant location must be interpreted as a physical change, yes?

This assumes there was, in the first place, a real physical state to change. Although the Copenhagen Interpretation regards quantum mechanics as a complete theory “for which the fundamental physical and mathematical hypotheses are no longer susceptible of modification” it does not address the physical state of the unobserved quantum object.

It can only be complete in terms of the consistency of the regularity achieved between the state preparation process and the measurement process. And in this respect QM represents physics most successful paradigm.

But in the words of Bohr “there is no quantum world, 'There is only an abstract quantum physical description. It is wrong to think that the task of physics is to find out how nature is. Physics concerns what we can say about nature.

Still it would be nice to know what its all about?


We still seem to remain with no firm evidence for the violation of relativity, even though Bell’s inequality is exceeded in Aspect’s experiment?

 

[UglyDuckling]

Do you mean to imply that the collapse postulate of orthodox QM *is* Lorentz invariant? The theory's empirical predictions have a certain Lorentz invariance property, but the theory's *dynamics* (half of it, anyway -- the collapse postulate part) is badly Lorentz variant.

But do the laws of special relativity require an abstact mathematical contruct to be compliant?

 

[RandallB]

We still seem to remain with no firm evidence for the violation of relativity, even though Bell’s inequality is exceeded in Aspect’s experiment?

Bell’s inequality & Aspect’s experiment have nothing to do with a violation of relativity. It deals the issue of "local" vs the probablities used in QM. (ref: links above)

SR can be viewed in a "local" world, but QM does use SR in it's "non-local view.
GR expects a "non-local" world, as least for most; I don't see how it could be viewed locally.

 

[ttn]

This assumes there was, in the first place, a real physical state to change. Although the Copenhagen Interpretation regards quantum mechanics as a complete theory “for which the fundamental physical and mathematical hypotheses are no longer susceptible of modification” it does not address the physical state of the unobserved quantum object.

So, according to this, the Copenhagen interpretation provides no ontology for the world at all? It's a mistake to read it as describing literal "happenings" out in the world? I don't agree with this reading of the Copenhagen interpretation, but it is certainly a possible one. The reason I don't agree is that it seems to render the completeness doctrine (which I see as the *core* of Bohr's views, basically defined by his antagonism toward Einstein's ideas for an ensemble interpretation) meaningless. The completeness doctrine says that the wave function alone (without any additional so-called "hidden" variables) provides a complete description. Well... a complete description of what? If it's not supposed to be a description of anything, how can that description be meaningfully claimed to be complete?

But this is all beside the point. There are two possible interpretations. You can take the wave function as a complete description of physical states, or you can say that the wave function isn't a literal description of anything physical and the whole QM formalism is just a black-box recipe for calculating measurement outcomes. Let's just call those "interpretation 1" and "interpretation 2" respectively.

I think it's incontestable that "interpretation 1" is non-local. The collapse postulate describes instantaneous changes to distant parts of the wave function. "Instantaneous" means: lots faster than the speed of light. OK?

As I understand your point, it is that "interpretation 2" is *not* non-local. That's true. But it isn't local, either. It's not local or non-local. Why? Because the whole idea of "locality" is an issue of how fast causal effects (as described by some theory) propagate. And if "interpretation 2" doesn't *say* anything about any causal effects (because it doesn't provide an ontology at all), then it is just meaningless to apply the local/nonlocal terminology to it.

What the EPR argument combined with Bell's Theorem prove is this: no local theory can account for the empirical results. "Interpretation 1" is an example of this, as is (say) Bohm's theory (which is quite explicitly nonlocal, just like "interpretation 1"). "Interpretation 2" on the other hand is *irrelevant* to this claim. It's not a counterexample at all, because "interpretation 2" isn't a theory in the relevant sense. It doesn't *say* anything about what's going on physically.

So the claim is this: there's no way to tell a physical story that is local (i.e., "relativistically causal" to use Bell's terminology) by which you can understand the outcomes of the relevant experiment. Surely this means that nature (the physical world out there) is non-local -- it's just like if, somehow, you had a theorem that proved "there is no way to give a physical account of the data which doesn't involve the existence of stars" then you'd surely conclude: "so nature includes stars." Same deal with EPR/Bell, but replace "stars" with "relativity-violating, superluminal causal influences."

But in the words of Bohr “there is no quantum world, 'There is only an abstract quantum physical description. It is wrong to think that the task of physics is to find out how nature is. Physics concerns what we can say about nature.

Actually, Bohr didn't say this. It was someone else's attempt to summarize Bohr's thought. But that's neither here nor there; just a too-little-known historical tidbit.

Still it would be nice to know what its all about?

I certainly agree about that. I just think it's important to stress that, even though we don't yet know exactly "what it's all about" -- we *do* know (already) that whatever that ultimate physical theory looks like, it has to be nonlocal. We don't know everything about nature, but we do know at least one thing: it contains causal influences that are superluminal. And I think this is a serious problem for relativity. (Bell thought so too.)

We still seem to remain with no firm evidence for the violation of relativity, even though Bell’s inequality is exceeded in Aspect’s experiment?

I don't agree. I think the evidence is extremely firm.

 

[vanesch]

So, according to this, the Copenhagen interpretation provides no ontology for the world at all? It's a mistake to read it as describing literal "happenings" out in the world? I don't agree with this reading of the Copenhagen interpretation, but it is certainly a possible one.

Well, honestly, this is how I always understood the Copenhagen interpretation: there is a classical ontology for macroscopic objects, and there is an UNDESCRIBABLE ontology (whatever that may mean !) ontology for microscopic objects, but whose INFLUENCE on the classical macroscopic objects can be CALCULATED using the QM formalism (which is NOT a representation of any ontology, just a calculational algorithm).

I think that you are more thinking about the von Neumann view, where it is less clear in what way there's an ontology associated with the wavefunction (if you read von Neuman, you have the impression he DOES take it somehow for real) and where he clearly states that collapse happens *somewhere* between the microsystem and the conscious interpretation of the measurement, but that he states that, if it happens late enough in the process, that all places are good enough and are empirically indistinguishable. (in that sense, you could think of von Neuman's vision as a precursor of a many minds view!)

The reason I don't agree is that it seems to render the completeness doctrine (which I see as the *core* of Bohr's views, basically defined by his antagonism toward Einstein's ideas for an ensemble interpretation) meaningless.

I agree with you, but apparently, the idea was that quantum ontology is FUNDAMENTALLY UNDESCRIBABLE and that all you'll ever know of it is the algorithm that calculates the influences on classical objects ; an algorithm we call a quantum-mechanical calculation. No wonder Einstein went beserk over that concept This was a kind of positivist viewpoint, but only limited to the microscopic world.

But this is all beside the point. There are two possible interpretations. You can take the wave function as a complete description of physical states, or you can say that the wave function isn't a literal description of anything physical and the whole QM formalism is just a black-box recipe for calculating measurement outcomes. Let's just call those "interpretation 1" and "interpretation 2" respectively.

Right. I think Copenhagen was interpretation 2.

I think it's incontestable that "interpretation 1" is non-local. The collapse postulate describes instantaneous changes to distant parts of the wave function. "Instantaneous" means: lots faster than the speed of light. OK?

Unless, of course, the wavefunction never collapses and you experience only a part of it... I know, I know... I can't help bringing up MWI each time! Each time it is said that it doesn't exist !

What the EPR argument combined with Bell's Theorem prove is this: no local theory can account for the empirical results.

To keep this politically correct, you should add: "under the assumption that empirical results are ontologically real and unique." If you drop this assumption, MWI is in the run, and that IS a counter example.

So the claim is this: there's no way to tell a physical story that is local (i.e., "relativistically causal" to use Bell's terminology) by which you can understand the outcomes of the relevant experiment.

On the condition that these outcomes are ontologically real and unique...

but we do know at least one thing: it contains causal influences that are superluminal. And I think this is a serious problem for relativity. (Bell thought so too.)

OR it contains parallel worlds we are not aware of, in which case relativity has no problem. So the choice is: accept parallel worlds, or kick out relativity... (or, consider that a local realistic theory is still possible, and that all Bell tests have been circular ; or consider that the hypothesis of an ontology corresponding to a mathematical structure (the reductionist view) is not correct and it is all "just computing" or "emerging")

 

[ttn]

I think that you are more thinking about the von Neumann view, where it is less clear in what way there's an ontology associated with the wavefunction (if you read von Neuman, you have the impression he DOES take it somehow for real) and where he clearly states that collapse happens *somewhere* between the microsystem and the conscious interpretation of the measurement, but that he states that, if it happens late enough in the process, that all places are good enough and are empirically indistinguishable.

Yes, you're right about all this. I probably should have talked about the "orthodox" view rather than the Copenhagen view. The orthodox interpretation (which most people, including apparently sometimes myself, don't bother to distinguish from Copenhagen) is essentially the von Neumann view which, as you say, really does have two distinct dynamical processes ("Process 1" and "Process 2" or whatever), one of which happens when there's no measurement and the other of which happens when there is a measurement.

I think it's arguable whether Bohr agreed with this or not. Frankly, I don't think there's any answer -- Bohr was an obscurantist, not a clear thinker, and, simply put, he wasn't consistent on this very-much black-and-white question. You can find passages that are most naturally interpreted as implying what I called "interpretation 1" before -- and same for "interpretation 2".

So at the end of the day, my attitude is: who cares what Bohr thought. There are two possible consistent views, and neither one of them is any kind of counterexample to the deep conflict that Bell first perceived between quantum physics and relativity.

(in that sense, you could think of von Neuman's vision as a precursor of a many minds view!)

? As you said above, he makes the point that it is very difficult to empirically distinguish versions of the theory which place the "cut" (between "process 1" and "process 2") in different places. FAPP, you can put the cut pretty much anywhere you want and still predict the same things. But i don't see what this point has to do with the many minds view. Maybe you were just joking or something?

Right. I think Copenhagen was interpretation 2.

I lean the other way, but perhaps you're right. But really... who cares?

Unless, of course, the wavefunction never collapses and you experience only a part of it... I know, I know... I can't help bringing up MWI each time! Each time it is said that it doesn't exist !

Is it *really* "said that it doesn't exist" each time? Or perhaps you are just deluded about that because your mind only has access to one very narrow branch of the truth?

My point is: even to discuss anything, you've got to take some things as given. We normally take as given (at least) stuff like the real existence of the macroscopic physical objects around us (e.g., as you did when you accepted the real existence of certain letters spelled out on your computer screen just now). ...which is *all* I'm doing when I neglect to mention your beloved MWI "counterexample" to the claim that nature isn't local. Let me put it this way: if you were right to accuse me of ignoring your counterexample, you've accidently thereby conceded the argument to me -- for your belief about what I wrote is actually premised on the very principle grounding my not taking your counterexample seriously!

To keep this politically correct, you should add: "under the assumption that empirical results are ontologically real and unique." If you drop this assumption, MWI is in the run, and that IS a counter example.

Sure, something like that, though I don't like the precise way you phrased it. How about "under the assumption that our normal everyday perception of the familiar macroscopic external world (of such things as tables, books, and instrument-pointers) isn't delusional."

The point I will keep coming back to forever is this: *without* that assumption, there can be no such thing as science, period. Science can't exist without (among other things) the idea of *evidence* -- if there can be no evidence for a proposition, then there's no way to distinguish the true from the false, and no way to do science (or think generally). And if literally *seeing* something in front of your face doesn't count as evidence, nothing ever will. We *have* to accept the veracity of direct perception, or else (leaving aside crazy mystics) we have no access to reality at all, and there's no more point trying to do physics (or anything else).

OR it contains parallel worlds we are not aware of, in which case relativity has no problem. So the choice is: accept parallel worlds, or kick out relativity... (or, consider that a local realistic theory is still possible, and that all Bell tests have been circular ; or consider that the hypothesis of an ontology corresponding to a mathematical structure (the reductionist view) is not correct and it is all "just computing" or "emerging")

Fine. But I think the people (like, for example, Zapper Z) who were denying that there is any conflict between QM and SR had something other than "parallel universes" in mind. I'm guessing the idea was supposed to be that, even on the assumption that there is only one world (the one we perceive), it's possible for a local theory to account for the empirical results. Patrick, I believe you and I agree that these people would be wrong. Yes? Anyway, I hope some of these dissenters will clarify the basis for their claims so Patrick and I can explain why they're erroneous (or maybe they'll confess that what they had in mind all along was "parallel worlds").

 

[UglyDuckling]

Bell’s inequality & Aspect’s experiment have nothing to do with a violation of relativity. It deals the issue of "local" vs the probablities used in QM. (ref: links above)

QUOTE]
In order to supply a result that distinguishes between hidden variables and quantum mechanics it was necessary to spatially separate the detectors so the interval between measurements of paired photons was space-like. The reason for this is that in the case of the hidden variables there is no way to predict how the results on measured paired photons would correlate. Thus by chance the hidden variable interactions could have provided the same correlations as that predicted by quantum mechanics as long as there was some means of communicating between the detection events.

By isolating the detection events in space-time an upper limit is definitely established for the level of correlation that can be achieved by the hidden variables model.

As the results appear to correspond with those predicted by quantum mechanics a flavour of the super-luminal must be added to our view of the world.

 

[UglyDuckling]

Bell’s inequality & Aspect’s experiment have nothing to do with a violation of relativity. It deals the issue of "local" vs the probablities used in QM. (ref: links above)

QUOTE]
In order to supply a result that distinguishes between hidden variables and quantum mechanics it was necessary to spatially separate the detectors so the interval between measurements of paired photons was space-like. The reason for this is that in the case of the hidden variables there is no way to predict how the results on measured paired photons would correlate. Thus by chance the hidden variable interactions could have provided the same correlations as that predicted by quantum mechanics as long as there was some means of communicating between the detection events.

By isolating the detection events in space-time an upper limit is definitely established for the level of correlation that can be achieved by the hidden variables model.

As the results appear to correspond with those predicted by quantum mechanics a flavour of the super-luminal must be added to our view of the world.

 

[ttn]

In order to supply a result that distinguishes between hidden variables and quantum mechanics it was necessary to spatially separate the detectors...

You have misunderstood Bell's Theorem. The point is not to "distinguish between hidden variables and quantum mechanics." The point is to distinguish between local theories and non-local theories.

The argument is really quite simple. Orthodox QM is a non-local theory. Einstein pointed this out long ago, and noted that (perhaps) a local theory could be constructed by supplementing OQM with "hidden variables." Bell's Theorem then showed that *even* by adding hidden variables, you can't have a local theory that agrees with experiment. So locality cannot be saved, period. The question of the existence of hidden variables simply isn't touched here. Today, there exist both hidden variable theories and non-hidden-variable theories that are empirically viable. What's important and interesting, though, is that both kinds of theories are non-local. That's the lesson of Bell: in order to agree with experiment, a theory has to be non-local, period.

(Assuming we don't go off into parallel-universes MWI la-la-land... )

The reason for this is that in the case of the hidden variables there is no way to predict how the results on measured paired photons would correlate. Thus by chance the hidden variable interactions could have provided the same correlations as that predicted by quantum mechanics as long as there was some means of communicating between the detection events.

By isolating the detection events in space-time an upper limit is definitely established for the level of correlation that can be achieved by the hidden variables model.

If you think about it, this claim (which is a disturbingly widespread misconception) is really quite stupid and obviously wrong. The whole idea of "hidden variables" is to *supplement* the wave-function-description with some other variables, some additional structure. Bohmian Mechanics is of course the nicest example here, where the wave function is supplemented by actual particle positions.

Anyway, here's my point: you have some theory (orthodox QM) which people erroneously think is local; then you're going to *add* some structure to the theory; and suddenly (people erroneously think) the theory has to be made nonlocal in order to make the same predictions the earlier theory was able to make locally without the hidden variables? That's preposterous on its face. Just as an obvious counterexample, you could add deliberately pointless hidden variables -- variables that *did nothing* dynamically and didn't affect in any way the outcomes predicted already by orthodox QM. And yet people think that adding such variables will render a still-local theory in disagreement with experiment all of the sudden? *How* could it possibly do that?

Think of it this way: the basic issue here in all these Bell inequality tests is how strongly correlated outcomes in two places can be. Orthodox QM says they should be correlated a certain way. Now isn't it just obvious that if you *add* some additional variables to orthodox QM (i.e., write down a hidden variable theory) you'll maybe be able to get *stronger* correlations? How could adding structure to the theory somehow force the correlations to be weaker? It just makes no sense. Yet this is implicit in the common (mistaken) view that Bell's theorem is all about "QM vs hidden variables." It isn't, and what these people are missing is the crucial fact that orthodox QM is already non-local. So when you compare the correlations predicted by orthodox QM to those of a local hidden variable theory, you are comparing a non-local theory to a local theory. And then it's no surprise that the non-local theory can predict stronger correlations between distant measurements! But then it's outrageously stupid to conclude that hidden variables are thus refuted.

 

[RandallB]

TNN on you your comment:

But then it's outrageously stupid to conclude that hidden variables are thus refuted.

TNN I don’t think that’s fair.
I love going to Conferences and presentations where I have the chance to more than meet some truly important people in physics. There they often will take the time to actually discuss things with ordinary lay folk like myself. It helps to have rational questions and points. Kip S. Thorne, Wendy Freedman, Sylvester James Gates Jr., Wolfgang Ketterle and even noble-laureate Leon M. Lederman of Fermilab were all so kind. (Although, I did meet Brian Greene the same way, celebrity of TV & Books prevented him entering any real discussions).
My topic is always my same favorite, Einstein’s unknown variable, and are they satisfied it has been refuted. Which once they could see how well I understood the issue they always were careful and considered in their replies. And all the same on two points.
First it is after all near impossible to prove a negative and none claimed that Einstein has been definitely proven wrong on the unknown variable.
But, second that they were (and most felt ‘shouldn’t we all be’) satisfied that a unknown variable was not viable based on EPR-Bell etc.

Now I’ll grant you, they may feel stupid if you or someone can prove as a positive fact that unknown variables can work to resolve EPR-Bell type issues.
But even if that were to occur.
I think it very inappropriate to describe, those satisfied with concluding hidden variables as refuted at this time in physics, as being “outrageously stupid”.
I certainly will not even if they turn out to be wrong, that would just mean they were wrong not stupid.

 

[ttn]

First it is after all near impossible to prove a negative and none claimed that Einstein has been definitely proven wrong on the unknown variable.
But, second that they were (and most felt ‘shouldn’t we all be’) satisfied that a unknown variable was not viable based on EPR-Bell etc.

Yes, I'm very aware that lots of highly-regarded physicists have this view. Nevertheless, it's a wrong (and yes, stupid) view, for the reasons I explained in the previous post.

Now I’ll grant you, they may feel stupid if you or someone can prove as a positive fact that unknown variables can work to resolve EPR-Bell type issues.

I don't know exactly what you mean by "resolve". What's to resolve? What's clear is that there exists a hidden variable theory that makes all the same predictions as orthodox QM. So *clearly* hidden variable theories can't be regarded as non-viable.

But even if that were to occur.
I think it very inappropriate to describe, those satisfied with concluding hidden variables as refuted at this time in physics, as being “outrageously stupid”.
I certainly will not even if they turn out to be wrong, that would just mean they were wrong not stupid.

I don't think all those *people* are stupid. But non-stupid people can hold the occasional stupid view on some isolated issue (usually because they haven't thought that issue through as carefully as they do for other issues). And I stand by what I said before: it's stupid to think that *adding* extra structure to a theory will *reduce* the maximum strength of correlations.

 

[RandallB]

What's to resolve? What's clear is that there exists a hidden variable theory that makes all the same predictions as orthodox QM.

Sorry I didn't see that one in the headlines, Why is the evening news so slow on this.
What "hidden variable theory" does this.
And please don't say BM that would be stupid.
BM is a non-local theroy that just uses a non-local guide wave as a proxy for a hidden variable - that's not the same as a hidden variable.

 

[LnGrrrR]

Ok, after reading some of DocC's articles, I feel a bit better...but can anyone explain to me why hidden variables COULDN'T back up a local theory because of Bell's Experiment? Wasn't that the whole point of hidden variables? And it is impossible to test locality? The idea that 'standard QM' must imply non-locality...how is that?

PLEASE dumb it down a lot for me...as my brain refuses to work with me to jump over this hurdle. If you wish, feel free to message me privately.

 

[DrChinese]

Ok, after reading some of DocC's articles, I feel a bit better...but can anyone explain to me why hidden variables COULDN'T back up a local theory because of Bell's Experiment? Wasn't that the whole point of hidden variables? And it is impossible to test locality? The idea that 'standard QM' must imply non-locality...how is that?

PLEASE dumb it down a lot for me...as my brain refuses to work with me to jump over this hurdle. If you wish, feel free to message me privately.

Assuming you are familiar with the Heisenberg Uncertainty Principle (HUP): A first step is to ask the question: Does the HUP describe reality? Or is there a deeper level of reality and we just can't see it?

If you believe that the HUP is incomplete, and there is a deeper level of reality in which non-commuting variables are well-defined in violation of the HUP: You are a "realist". Einstein was a realist. If you are a realist, then Bell's Theorem has important implications for you.

 

[ttn]

If you believe that the HUP is incomplete, and there is a deeper level of reality in which non-commuting variables are well-defined in violation of the HUP: You are a "realist". Einstein was a realist. If you are a realist, then Bell's Theorem has important implications for you.

Hogwash. Bohm's theory is "realist" in this sense, yet is entirely consistent with all the Bell-related experimental facts.

The correct statement is: if you're a *local* "realist", then Bell's Theorem has important implications for you -- namely, your views are inconsistent with experimental data. But this is also true for *local* "non-realists" -- such a view is *also* inconsistent with experimental data. Bottom line: Bell's Theorem says nothing about "realism" one way or the other. What it says is: you can't have a local theory.

 

[ttn]

Sorry I didn't see that one in the headlines, Why is the evening news so slow on this.
What "hidden variable theory" does this.

Bohmian Mechanics.

And please don't say BM that would be stupid.

Oops. You'll have to explain why it's stupid.

BM is a non-local theroy that just uses a non-local guide wave as a proxy for a hidden variable - that's not the same as a hidden variable.

BM is non-local, that's true. But what is your other point here? You're denying that BM is a hidden variable theory? This makes me think you don't know what the term "hidden variable theory" means. Generally people use this phrase to mean a theory that attempts to solve the measurement problem by adding some kind of structure to orthodox QM (i.e., by denying the orthodox completeness doctrine). As I said before, what BM adds to the wave-function description of orthodox QM is definite particle positions (which then follow definite, deterministic trajectories at all times, even, e.g., when the wave-function isn't a position eigenstate).

Edit: Oh, by the way, the "non-local guide wave" that you mention is not a hidden variable nor a proxy for one. It's the wave function -- the same dynamical object that already appears in orthodox QM. It's the particle positions that are the "hidden variable" (though, as has been pointed out many times, this is a silly misnomer since, if anything is "hidden", it's the wave function, not the particle positions!).

 

[LnGrrrR]

DrChinese,

I would say that, based on instinct alone, I am a 'realist' (there should be some 'magical' way to determine both velocity and position that we have not discovered yet).

Assuming that I were to be a realist, how does Bell's Theorem show that this can not exist?

 

[Hurkyl]

I would say that, based on instinct alone, I am a 'realist' (there should be some 'magical' way to determine both velocity and position that we have not discovered yet).

Assuming that I were to be a realist, how does Bell's Theorem show that this can not exist?

It doesn't. But if you further assume some notion of statistical noninteraction of separated events (e.g. I don't need to know what's going on at Alpha Centauri to figure out the odds of a result here on Earth), then the math will yield that certain possibilities have negative probabilities.

 

[RandallB]

LnGrrR
Since this thread is well off topic from the OP
I’ll continue comments on Entanglement in a new thread And try to “Dumb it down" there

Ok, after reading some of DocC's articles, I feel a bit better...but ….
PLEASE dumb it down a lot for me...as my brain refuses to work with me to jump over this hurdle. If you wish, feel free to message me privately.

GO TO:
https://www.physicsforums.com/showthread.php?t=116036"

 

[UglyDuckling]

You have misunderstood Bell's Theorem. The point is not to "distinguish between hidden variables and quantum mechanics." The point is to distinguish between local theories and non-local theories.

The argument is really quite simple. Orthodox QM is a non-local theory. Einstein pointed this out long ago, and noted that (perhaps) a local theory could be constructed by supplementing OQM with "hidden variables." Bell's Theorem then showed that *even* by adding hidden variables, you can't have a local theory that agrees with experiment. So locality cannot be saved, period. The question of the existence of hidden variables simply isn't touched here. Today, there exist both hidden variable theories and non-hidden-variable theories that are empirically viable. What's important and interesting, though, is that both kinds of theories are non-local. That's the lesson of Bell: in order to agree with experiment, a theory has to be non-local, period.

(Assuming we don't go off into parallel-universes MWI la-la-land... )




If you think about it, this claim (which is a disturbingly widespread misconception) is really quite stupid and obviously wrong. The whole idea of "hidden variables" is to *supplement* the wave-function-description with some other variables, some additional structure. Bohmian Mechanics is of course the nicest example here, where the wave function is supplemented by actual particle positions.

Anyway, here's my point: you have some theory (orthodox QM) which people erroneously think is local; then you're going to *add* some structure to the theory; and suddenly (people erroneously think) the theory has to be made nonlocal in order to make the same predictions the earlier theory was able to make locally without the hidden variables? That's preposterous on its face. Just as an obvious counterexample, you could add deliberately pointless hidden variables -- variables that *did nothing* dynamically and didn't affect in any way the outcomes predicted already by orthodox QM. And yet people think that adding such variables will render a still-local theory in disagreement with experiment all of the sudden? *How* could it possibly do that?

Think of it this way: the basic issue here in all these Bell inequality tests is how strongly correlated outcomes in two places can be. Orthodox QM says they should be correlated a certain way. Now isn't it just obvious that if you *add* some additional variables to orthodox QM (i.e., write down a hidden variable theory) you'll maybe be able to get *stronger* correlations? How could adding structure to the theory somehow force the correlations to be weaker? It just makes no sense. Yet this is implicit in the common (mistaken) view that Bell's theorem is all about "QM vs hidden variables." It isn't, and what these people are missing is the crucial fact that orthodox QM is already non-local. So when you compare the correlations predicted by orthodox QM to those of a local hidden variable theory, you are comparing a non-local theory to a local theory. And then it's no surprise that the non-local theory can predict stronger correlations between distant measurements! But then it's outrageously stupid to conclude that hidden variables are thus refuted.

You are of course correct when you say “the point is to distinguish between local theories and non-local theories.” However for a local theory to be valid quantum mechanics necessarily needed to be supplemented with hidden variables! These hidden variables in turn needed to be local. The local nature of these variables is implicit in Bell’s theorem and the experimental design. Bell’s theory coupled with Aspect experiment showed that local hidden variables cannot explain the correlations between spatial remote sets of observations.

This result does not eliminate hidden variables it merely says local hidden variables alone cannot explain quantum behaviour; there is an apparent non-local element influencing of the quantum world.

The lack of ontology associated with quantum mechanics leaves plenty of room for conjecture about the nature of hidden variables be they local or non-local.

 

[ttn]

You are of course correct when you say “the point is to distinguish between local theories and non-local theories.” However for a local theory to be valid quantum mechanics necessarily needed to be supplemented with hidden variables! These hidden variables in turn needed to be local.

Maybe we agree, then, but I can't quite follow exactly what you're saying here. Are you referring basically to the EPR argument here? I think it's correct that what EPR showed was that the only way to "save" locality for quantum mechanics, was to introduce local hidden variables. (That is, they showed that QM without hidden variables -- regular orthodox QM -- is already a nonlocal theory, but that one could in principle get rid of the nonlocality by rejecting the completeness doctrine and adding hidden variables.) Is that also what you're saying?

The local nature of these variables is implicit in Bell’s theorem and the experimental design. Bell’s theory coupled with Aspect experiment showed that local hidden variables cannot explain the correlations between spatial remote sets of observations.

Agreed. Bell's theorem (+ experiment) shows that the kind of theory argued for by EPR (argued for, that is, as the only way to locally account for certain correlations) doesn't work. Such a theory will have to make some other predictions that don't agree with experiment.

This result does not eliminate hidden variables it merely says local hidden variables alone cannot explain quantum behaviour; there is an apparent non-local element influencing of the quantum world.

I agree with the last bit, but I don't know why you put the first part that way. It doesn't "merely" say that "local hidden variables alone cannot explain quantum behaviour." The whole crucial point here that so many people miss is that it says *more* than that -- it says that no local theory *at all* (whether it has hidden variables or not) can explain quantum behavior, i.e., the empirical facts.

The lack of ontology associated with quantum mechanics leaves plenty of room for conjecture about the nature of hidden variables be they local or non-local.

Conjecture is cheap. Here's what's actually known to be true: local hidden variable theories are ruled out; indeed, local theories of any kind are ruled out. So we need a nonlocal theory. And then we have several extant options: orthodox QM (which is nonlocal and has no hidden variables -- and btw suffers from the measurement problem) and Bohm's theory (which is nonlocal and does have hidden variables -- which means no measurement problem). And there are some others, too, such as the GRW type theories. And of course there is always MWI, which is a kind of counterexample to the claim that we need a nonlocal theory... but not a counter example to the more precise claim that we need a nonlocal theory *in order to explain the observed experimental results*. MWI doesn't explain those observations; it dismisses them as delusional.

 

[RandallB]

Bohmian Mechanics.

Oops. You'll have to explain why it's stupid.

Bohm's theory is non-local and has hidden variables – so big deal.
What’s stupid is to argue that hidden variables in a non-local theory has anything to do with the local hidden variables in Einstein’s view, EPR, or a classical view.

It’s grossly miss-leading to those trying to understand hidden variables.
Do everyone a favor, when describing BM hidden variables at least call them “non-local hidden variables” at least until you can describe them in something other than a non-local theory.

Since this thread has long ago addressed the issue of these things not affecting the “Validity of Relativity” from the OP.
I’d recommend that detailed debate on the various issues this discussion has split into move into individual threads that can focus on one topic/issue.

See you in another thread.

 

[ttn]

Bohm's theory is non-local and has hidden variables – so big deal.
What’s stupid is to argue that hidden variables in a non-local theory has anything to do with the local hidden variables in Einstein’s view, EPR, or a classical view.

It’s grossly miss-leading to those trying to understand hidden variables.
Do everyone a favor, when describing BM hidden variables at least call them “non-local hidden variables” at least until you can describe them in something other than a non-local theory.

I don't get your point. There are (in principle) local hidden variable theories, and there are also nonlocal hidden variable theories. You suggest these have nothing to do with each other, but obviously they do: they're both hidden variable theories.

Since this thread has long ago addressed the issue of these things not affecting the “Validity of Relativity” from the OP.

Do you mean that we've reached some consensus about the "validity of relativity"? It seems to me there are still lots of people around who want to say that there is no problem here for relativity, because the problematic non-locality only afflicts hidden variable theories. I have been at pains to argue that this conclusion is too narrow, that the real conclusion from EPR/Bell is that no local theory *at all* is empirically viable -- and that therefore relativity is, so to speak, broken.

But there seems to be nobody left who's interested in actually debating this point. So I guess either they've all come around to the side of the truth, or they're too scared to discuss it. Oh well.