How can one event affect another instantly over a distance

[DrChinese]

The phrase "more complete specification of the system...beyond QM" refers to hidden variable theories, right?

Well then it's just outrageous to say that Bell never stated such a thing was possible. For about 20 years he was one of the only people to take Bohmian Mechanics seriously, i.e., to recognize clearly that Bohm's theory *existed* and that it was a *counterexample* to all the stale old claims that no hidden variable theories were possible. Indeed, Bell did a lot of work on this theory and moved it forward in several important ways. And of course his famous Theorem was inspired precisely by Bohm's hidden variable theory.

In any case, it is definitely not the case that Bell "believed in the HUP all the way" if that's supposed to mean he didn't recognize the possibility of hidden variable theories.

I don't think he foresaw dropping the HUP, or re-instating EPR's conclusion that QM was incomplete. That was my point, and I am aware of his interest in BM.

 

[Hurkyl]

I don't understand what you mean by "observation independence." By "observations" do you mean the *outcomes* of the experiments, or the fact that observations are made at all, or what?

I'm talking about outcome independence from 0408105, but I misspoke. I agree entirely that Outcome Independence is violated.

The point I am trying to make is that, despite 0408105's claims, OI is not required by my reading of Bell Locality. But more importantly, even if I've completely misunderstood Bell, that my version of it is what's relevant for compatability with special relativity.

The initial entangled wave function alone is *not* sufficient -- according to orthodox QM -- to calculate the correlations. That's precisely what is shown in quant-ph/0408105.

Only if you assume OI, which 040815 does, since it wraps OI up in its criterion for locality. Specifically, OI was used in the derivation of (18). In other words, the article proves that completeness implies violation of OI.

(Well, I suppose it could mean violation of some of the other conditions instead, but I'm willing to grant those other ones)

You might calculate an expectation value that way, yes. But that's not the same as accounting for the correlation on an event-by-event basis.

Why isn't it? The correlation of two random variables is simply a number: it doesn't matter how you go about computing it.

Yes, that's true. It's parallel to the following point about QM: if you had experimental access to the wave function associated with a given particle (in your lab, say), you would be able to detect a sudden change when somebody far away makes a measurement and collapses the wf.

Only if you assume that wave function collapse is something that really happens to the system, rather than a mathematical trick.

Namely, there are no actual definite outcomes to the two experiments on the two sides -- there is only this "comparison" that happens later in the middle?

I take the idea of an "outcome" to be closely analogous with the concept of a "random variable" in statistics.

What I'm trying to emphasize is that nonlocality is an artifact of our external viewpoint on QM. Specifically, we can ask nonlocal questions.

So what I was trying to do is to replace the nonlocal question with a seemingly (to me) equivalent local question, "What is the output of the third detector?" Even better than that, I've turned it from an external question into an internal question.

If you only ask local questions, then all sorts of "problems" vanish. In particular, the question of outcome independence cannot come up.

I think this is an important point to make, since it emphasizes that things interpreted as nonlocality arise when answering nonlocal questions.

 

[Sherlock]

Bell's Theorem clearly shows that local hidden variables are incompatible with QM, and on this point there is really nothing ambiguous. You are completely off with regards to your characterization the entire EPR/Bell regime.

...

Bell has never, as far as I know, stated that a more complete specification of the system is possible beyond QM - per any actual science (theory or experiment) ...

Apparently there is something ambiguous about saying that
local hidden variables are incompatible with qm.

From page 196 of "On the Einstein Podolsky Rosen Paradox" Bell
writes "... there is no difficulty in giving a hidden variable account
of spin measurements on a single particle." Then he shows this
mathematically, concluding with ... "So in this simple case there
is no difficulty in the view that the result of every measurement
is determined by the value of an extra variable, and that the
statistical features of quantum mechanics arise because the value
of this variable is unknown in individual instances."

So what happens to these local hidden variables when
we incorporate these individual measurement events into
a correlational context involving other individual measurement
events at spacelike separations from these? Do the hidden
variables just vanish? Or is it simply that they aren't
determining the joint results?

 

[ttn]

The point I am trying to make is that, despite 0408105's claims, OI is not required by my reading of Bell Locality. But more importantly, even if I've completely misunderstood Bell, that my version of it is what's relevant for compatability with special relativity.

Well, Bell Locality is definitely the conjunction of OI and PI. There's no question about that.

However, I concede that it's at least a possible position to hold that Bell Locality is too strong -- that it requires more of a theory than relativity does, that relativity merely requires something weaker like "signal locality". Of course then one has to conede that theories such as OQM and BM (which are Bell Nonlocal but Signal Local) are consistent with SR.

Only if you assume that wave function collapse is something that really happens to the system, rather than a mathematical trick.

If (as Bohr claimed) the wave function is a complete description of the state of the system, then any change in the wf implies a change in the state of the system -- i.e., the collapse is "something that really happens to the system." Of course, one can reject this and view the collapse as merely an updating of our knowledge of the system. But to take that route is to reject the completeness claim. For us to be able to acquire additional knowledge of the state of the system (without any coincident change in the state of the system) is to discover that there were facts about the system that we didn't know about before.

In other words, to regard the collapse postulate as epistemological rather than physical is to advocate a hidden variable theory.

And then we're not really talking about "orthodox QM" anymore, are we?

 

[Hurkyl]

I assert that incompleteness is not a necessary conclusion, in the following sense:

The wave function allows us to compute the probability distribution on any measurement (and thus the conditional probabilities as well). One is not forced to assume that there is a reality beyond these distributions, thus one need not conclude that the QM description is incomplete.

(In other words, it's just like the fact a "random variable" never actually takes on any values: it's just a convenient fiction to assist the intuition. The probability distribution is all there is)

 

[ttn]

I assert that incompleteness is not a necessary conclusion, in the following sense:

The wave function allows us to compute the probability distribution on any measurement (and thus the conditional probabilities as well). One is not forced to assume that there is a reality beyond these distributions, thus one need not conclude that the QM description is incomplete.

This is wrong in an interesting way. In fact, it's wrong in precisely the same way that it's wrong to think that you can escape the apparent conflict between QM and SR by denying realism (which was suggested as a possibility on another thread).

The problem is this: "completeness" has a certain *meaning*. It means that some theory (or theoretical entity, like the wave function) captures *all* of the facts that pertain to a given system. It means our description doesn't leave anything out, doesn't miss anything that's really out there. So the very *claim* that the wf alone provides a complete description of the system, *presupposes* the external objective reality of the system. If there really is no system, then there's no possibility for the wave function (or anything else) to provide a complete description of *it*. ("It, brother?")

So, I guess, in a sense you are correct: if you deny that there's an external reality, it's not quite correct to say that the wf is *incomplete*. (That would commit the same error i just noted.) If there's no external reality, then there's simply no such issue as "completeness", so both terms ("complete" and "incomplete") become literally meaningless.

"Incompleteness" means that the wf fails to capture some relevant fact about the real system out there. (A complete description, if the wf is incomplete, would have to supplement the wf with some additional variables.) And that presupposes realism too, just as much as the concept of "completeness" does.

So where does this leave us? Well suppose we hold on to realism. Then, if you regard the collapse postulate as merely epistemological, as merely an updating of our knowledge (which doesn't coincide with any physical change to the system), then it would be correct to say that the wf is incomplete. Right?

But on the other hand, if you do drop the realism assumption (and retreat to solipsism or whatever) then *both* "completeness" and "incompleteness" are false. It's not true that the wf provides a complete description of the facts, nor is it true that the wf provides an incomplete description of the facts. There simply are no facts. Note finally how this is parallel to the locality issue. If someone objects that QM contradicts relativity's prohibition on superluminal causation, it is not a successful response to deny realism -- i.e., to deny that light exists, that the speed of light exists, that there is an external world with causal interactions in it, etc...

This is an important point, because Bohr is often interpreted as responding to EPR's claim that the wf was incomplete, by retreating to a kind of anti-realism. And it's important to grasp that this is not a successful response to that charge. In fact, the EPR argument is *valid*, so there is no successful response to it. Either you have to admit that OQM is incomplete, or that nonlocality is real, or you can deny realism and hence wipe both issues (completeness and locality) out. But then one can't come back and say "I refuted the charge that QM was incomplete or nonlocal; it's both complete and local." No, under the assumption of anti-realism, QM is *not* both complete and local. That's *false* because there's no world for QM to provide a complete description of, and no causal interactions in the world for QM to provide a local explanation of.

I hope that's somewhat clarifying...??

 

[DrChinese]

Apparently there is something ambiguous about saying that
local hidden variables are incompatible with qm.

From page 196 of "On the Einstein Podolsky Rosen Paradox" Bell
writes "... there is no difficulty in giving a hidden variable account
of spin measurements on a single particle." Then he shows this
mathematically, concluding with ... "So in this simple case there
is no difficulty in the view that the result of every measurement
is determined by the value of an extra variable, and that the
statistical features of quantum mechanics arise because the value
of this variable is unknown in individual instances."

I see the point you are making better now; however, it is out of context of the EPR and Bell progression.

1. Well before EPR, it was suspected that the observer "shaped" reality by what the observer chose to measure - but no one was sure. It was possible to see the HUP as due to our ignorance, and that future technogical improvements would cross the threshold of the HUP. So local reality was still a reasonable assumption. Local realistic interpretations of QM incorporating the HUP could be applied to single particles and would yield limits to our knowledge. That is what Bell is referring to in your quote above.

2. With EPR, it was shown that in the case of entangled particles, either QM is incomplete or there is not simultaneous reality to non-commuting variables. EPR could not say which, but they guessed that QM was incomplete. Either way, the conclusions applied to single particle interpretations - they simply used the entangled scenario as an example to demonstrate their ideas.

3. Bell came along and burst the bubble on EPR's guess as to local reality, showing it was not compatible with QM. The fact is: it is untenable to assert LR is compatible with QM - it is disproved by counterexample. The counterexample uses entangled particles, but the assumption it overturns applies generally.

QM's HUP requires limits to our knowledge about individual particles. Because of Bell, we know that it is not due to our ignorance - it is because those particles do not have local hidden variables present simultaneously. It is wrong to say that any LR theories are consistent with QM.

 

[vanesch]

The problem is this: "completeness" has a certain *meaning*. It means that some theory (or theoretical entity, like the wave function) captures *all* of the facts that pertain to a given system. It means our description doesn't leave anything out, doesn't miss anything that's really out there. So the very *claim* that the wf alone provides a complete description of the system, *presupposes* the external objective reality of the system.

Yup.

"Incompleteness" means that the wf fails to capture some relevant fact about the real system out there. (A complete description, if the wf is incomplete, would have to supplement the wf with some additional variables.) And that presupposes realism too, just as much as the concept of "completeness" does.

Ok.

So where does this leave us? Well suppose we hold on to realism. Then, if you regard the collapse postulate as merely epistemological, as merely an updating of our knowledge (which doesn't coincide with any physical change to the system), then it would be correct to say that the wf is incomplete. Right?

That's where I don't agree. After all, maybe you only consciously observe a part of the wavefunction (one term). The wavefunction is still real, and unprojected. Your relationship to the wavefunction is what makes you think it collapsed, because (by postulate) now you only consciously observe part of it. That's still some form of realism (less tangible, granted, because now largely unobservable: only one term will remain observable for your conscious observation). This doesn't mean that the wf description is incomplete, does it ? And it allows for a completely locally formulated interaction.

But on the other hand, if you do drop the realism assumption (and retreat to solipsism or whatever) then *both* "completeness" and "incompleteness" are false.

The "relative solipsism" that is needed in no way drops realism, does it ? It tells you what effects a realistic object has on your conscious observation. What could be more real ?

It's not true that the wf provides a complete description of the facts, nor is it true that the wf provides an incomplete description of the facts. There simply are no facts. Note finally how this is parallel to the locality issue. If someone objects that QM contradicts relativity's prohibition on superluminal causation, it is not a successful response to deny realism -- i.e., to deny that light exists, that the speed of light exists, that there is an external world with causal interactions in it, etc...

I think you jumped to the conclusion that because we only observe ONE TERM, that the rest isn't real for some reason. I don't see the reason for that. And if this is correct, then the WF DOES describe ENTIRELY reality (of which we only observe a part).
Also, if this is correct, there is no non-locality issue with Bell, because the remote measurement happened BOTH WAYS at once. Bob saw and Bob didn't see the detector click, at the same time. Only, when this Bob in a superposition gets to you, interference happens when he interacts with you, and out of it come the strange correlations of entangled pairs. So there is no objective probability to be assigned to whatever Bob is doing. There are only objective probabilities to be assigned to what YOU observe, on your worldline, because that's how you hop from interaction to interaction, and each time you only see part of what really happens - hence the probabilistic aspect in your observations, which comes from the hopping, and not from what happens out there (because EVERYTHING happens out there).

This is an important point, because Bohr is often interpreted as responding to EPR's claim that the wf was incomplete, by retreating to a kind of anti-realism. And it's important to grasp that this is not a successful response to that charge. In fact, the EPR argument is *valid*, so there is no successful response to it. Either you have to admit that OQM is incomplete, or that nonlocality is real, or you can deny realism and hence wipe both issues (completeness and locality) out.

I think you're missing the possibility that QM is complete, that locality holds, but that we only observe part of what is really out there, and that this partial observation is responsible for the probabilistic impression we have.

However, all you say is valid if you insist upon that what is observed is real and that the alternatives really didn't take place. Then, indeed, there's no way out. But if what's observed is part of what's real, and what you observe can be different from what I observe - different aspects of the same reality - then I don't see how you come to your conclusion. Except that you "find this silly" ...

 

[Sherlock]

I see the point you are making better now; however, it is out of context of
the EPR and Bell progression.

The point is this: we're talking about two different experimental
setups. One is detecting single particles, the other is correlating
detections of two particles. In the former, an lhv description is
not incompatible with qm. In the latter, an lhv description is
incompatible with qm (and experiment).

I'm asserting that the reason for this is because in the
individual setup an lhv is a factor in determining the results
(per Bell 1964), and in the correlational setup an lhc
(local hidden constant) is a factor in determining the results.

Showing that an lhv is not a factor in determining the
results in the correlational setup ( per Bell, 1964) does not
then mean that an lhv is not a factor in determining individual
results, or that lhv's don't exist.

The counterexample to local realism has only to do with the
correlational setup, but this is not a counterexample to
local realism for the simple reason that lhv's are just not
relevant to the joint results in the correlational setup.

So, I'll repeat my question that you didn't answer. :-)
What happens to these local hidden variables
when we incorporate these individual measurement
events into a correlational context involving other individual
measurement events at spacelike separations from these?
Do the hidden variables just vanish (along with local
reality)? Or is it simply that they aren't determining the
joint results?

If the lhv's simply aren't a factor in determining the joint
results, then isn't it incorrect to say that these setups
show that lhv's don't exist, or that there is no locally
realistic behavior occurring in these setups, or that lhv
descriptions of any setup are therefore ruled out?

 

[ttn]

1. Well before EPR, it was suspected that the observer "shaped" reality by what the observer chose to measure - but no one was sure. It was possible to see the HUP as due to our ignorance,

That's still possible, as shown by the extant hidden variable theories like Bohmian Mechanics.

So local reality was still a reasonable assumption. Local realistic interpretations of QM incorporating the HUP could be applied to single particles and would yield limits to our knowledge. That is what Bell is referring to in your quote above.

What is the source of this seemingly irresistable desire people have to associate locality and realism, as if there were only one issue: "local realism" vs everything else? Whether certain facts exist or not prior to observation, and whether a theory's dynamics respects relativity's prohibition on superluminal causation aren't the same question.

"Reality" is still a damn reasonable assumption. (Really, it's an axiom -- it is necessarily presupposed by any physics at all, and any attempt to deny it refutes itself.) If you mean something narrower, like whether spin-components are real properties (as opposed to "contextual" or "emergent" properties) of particles, well then you should be specific and not imply that somehow anything in QM refutes realism *generally*.

In regard to locality, it depends on what you mean. Signal Locality remains a reasonable assumption. Bell Locality is definitely violated.

Why lump all these together into one vague issue when really there are several distinct issues?

2. With EPR, it was shown that in the case of entangled particles, either QM is incomplete or there is not simultaneous reality to non-commuting variables. EPR could not say which, but they guessed that QM was incomplete.

The alternative you pose is simply the question of whether or not OQM is complete. (The EPR argument attempts to show that non-commuting properties like different spin components *do*, despite the orthodox eigenstate-eigenvalue link, possesses simultaneous definite values. Either they do and OQM is incomplete; or they don't and QM is complete. That's all that issue means.)

It's ridiculous to say that EPR *guessed* that QM was incomplete. This makes it sound like the entire content of the EPR argument is to pose a trivial dilemma (either X or not-X) and then to take a wild stab at answering. ("Ummm, I dunno, how about... not-X!??") The fact is, EPR actually had an *argument* for their conclusion. And the *premise* of this argument was: locality. So EPR didn't simply *guess* that maybe OQM was incomplete. They *proved* that OQM *has* to be incomplete if one insists on respecting locality. Or just saying the same thing differently, they proved that anyone who insists on treating OQM as complete has to contend with the fact that the theory is nonlocal.

3. Bell came along and burst the bubble on EPR's guess as to local reality, showing it was not compatible with QM. The fact is: it is untenable to assert LR is compatible with QM - it is disproved by counterexample. The counterexample uses entangled particles, but the assumption it overturns applies generally.

Now that is doubly ridiculous. Bell "burst the bubble on EPR's guess"?!??! What does this mean? Bell somehow refuted the EPR argument? He absolutely did not. What he showed is that the particular kind of theory lobbied for by EPR as a way of saving the locality principle in the face of the QM predictions (namely, a local hidden variable theory) couldn't work. Bell showed that the kind of theory Einstein probably hoped for couldn't exist, yes. But he didn't refute the EPR argument. It's still true that the completeness claim for QM entails nonlocality. (See quant-ph/0408105.) And Bell proved that the opposite claim -- that QM is *not* complete, that there are hidden variables -- also requires nonlocality. So locality fails. (BTW, by "locality" in this paragraph I mean "Bell Locality".)

QM's HUP requires limits to our knowledge about individual particles. Because of Bell, we know that it is not due to our ignorance - it is because those particles do not have local hidden variables present simultaneously. It is wrong to say that any LR theories are consistent with QM.

There you go again, conflating the issue of hidden variables with the issue of locality. Bell did *not* show that the HUP "is not due to our ignorance." He showed that if you want to have a theory in which the HUP is epistemological rather than fundamental, that theory will have to be nonlocal. EPR had already shown that if you regard the HUP as fundamental (i.e., as "not due to our ignorance") then the resulting theory (OQM) is nonlocal.

So it is totally misleading to say: "It is wrong to say that any L[ocal] R[ealistic] theories are consistent with QM." The correct statement is: "It is wrong to say that any [Local] theory is consistent with QM."

Neither Bell nor EPR nor their combination proves one way or the other whether or not there are hidden variables. The argument for these lies elsewhere (e.g., in the fact that a hv theory can solve the measurement problem).

 

[DrChinese]

So, I'll repeat my question that you didn't answer. :-)
What happens to these local hidden variables
when we incorporate these individual measurement
events into a correlational context involving other individual
measurement events at spacelike separations from these?
Do the hidden variables just vanish (along with local
reality)? Or is it simply that they aren't determining the
joint results?

If the lhv's simply aren't a factor in determining the joint
results, then isn't it incorrect to say that these setups
show that lhv's don't exist, or that there is no locally
realistic behavior occurring in these setups, or that lhv
descriptions of any setup are therefore ruled out?

And I'll repeat my answer: There are no local hidden variables in QM.

All LHV theories are incompatible with all of the predictions of QM. This has been known for 40 years (per Bell). If you want to postulate a LHV which mimics SOME of the predictions of QM, no one is disputing your ability to do that. But since such a theory makes erroneous predictions about some experiments (such as Aspect), it is not likely to find much acceptance among scientists.

Entangled systems are merely a tool that enables us to realize Bell's Theorem (i.e. that LR is incompatible with QM). It is not a boundary condition, i.e. that the world is local realistic everywhere EXCEPT entangled systems. It is a misreading of the literature to assert otherwise.

 

[DrChinese]

What is the source of this seemingly irresistable desire people have to associate locality and realism, as if there were only one issue: "local realism" vs everything else? ...

"Reality" is still a damn reasonable assumption. (Really, it's an axiom -- it is necessarily presupposed by any physics at all, and any attempt to deny it refutes itself.) If you mean something narrower, like whether spin-components are real properties (as opposed to "contextual" or "emergent" properties) of particles, well then you should be specific and not imply that somehow anything in QM refutes realism *generally*.

So it is totally misleading to say: "It is wrong to say that any L[ocal] R[ealistic] theories are consistent with QM." The correct statement is: "It is wrong to say that any [Local] theory is consistent with QM."

This is factually incorrect. The reason L and R MUST be mentioned together is because Non-local HV theories are not excluded by Bell's Theorem. But that does not mean that Non-locality is the only solution. You assume by your statement ("Reality is still a damn reasonable assumption"), exactly as EPR did, that there is simultaneous existence of non-commuting observables. Maybe, maybe not!

There is an explicit assumption in Bell's argument: that of reality ("It follows that C is another unit vector [in addition to A and B] ..."). This is the specific narrower context you are asking about. Sure, it is reasonable, but that does not make it true! Please note that Locality is implicitly added into Bell's argument - he mentions it, but basically takes if for granted that if there is some FTL communication between A and B (a la Bohmian Mechanics or similar) then there is no problem reproducing the results of QM.

So the fact is: the Reality and Locality assumptions are both part of Bell. So if you wonder why they are mentioned together so strongly... well, there you are! :)

 

[vanesch]

You assume by your statement ("Reality is still a damn reasonable assumption"), exactly as EPR did, that there is simultaneous existence of non-commuting observables. Maybe, maybe not!

This is exactly why MWI can "weasel out": it takes it (from Alice's point of view) that Bob both did and did not see his detector click. So Alice cannot talk about the "probability that Bob's detector clicked". It did both, each in a separate branch. However, when Alice MEETS Bob, she has to make a choice between the two Bobs and NOW, locally, she assigns a probability to Bob's result. But as this is local, no parameter independence is required anymore (the probability can locally depend as well on Alice's choices of the polarizer as on Bob's, because this information is present locally now).

Again, one can dislike MWI for many reasons, but the very existence of this view means that one cannot say that the observed outcomes of QM mean that the theory is non-local ; in the same way as Bohm's theory means that one cannot say that no hidden variable deterministic theory can make identical predictions as QM. Whether one thinks that Bohm was right or not.

cheers,
Patrick.

 

[ttn]

This is factually incorrect. The reason L and R MUST be mentioned together is because Non-local HV theories are not excluded by Bell's Theorem. But that does not mean that Non-locality is the only solution.

If you only consider Bell's Theorem, you might fool yourself into thinking that a local theory which dispensed with hv's (e.g., the simultaneously real spin components you were talking about before) could work. But this would be to ignore something that we know thanks to EPR: if you *don't* have those simultaneously real spin components (i.e., if you don't have exactly the kind of hv's Bell assumes in his derivation) you also cannot get the empirically correct predictions without violating locality. Summary: whether you have those extra elements of reality or not, a local theory will conflict with experiment. So... putting *all* the relevant arguments and evidence on the table... non-locality *is* the only solution.

It's interesting that the logic here is the same as the point you made so eloquently to Sherlock. Yes, a LHV theory can explain certain facts. But it can't explain other facts. So LHV theories are excluded. When you put *all* the evidence on the table, it is clear that LHV theories can't account for it. Likewise, when you put all the evidence on the table, it is clear that Bell Locality fails (regardless of what position you want to take on "realism", i.e., hidden variables).

You assume by your statement ("Reality is still a damn reasonable assumption"), exactly as EPR did, that there is simultaneous existence of non-commuting observables. Maybe, maybe not!

No, you're quoting me out of context. There I was using the word "reality" to refer to scientific realism *generally* -- the belief that there is an external physical world independent of my consciousness. (Not *experiments*, mind you, because experiments are part of that physical world -- when I say consciousness I mean it literally.)

If you meant above that, like me, Einstein believed in scientific realism generally, you are absolutely correct. But if you mean by "realism" specifically belief in some particular elements of reality like spin components, then it is absurd to say that EPR *assumed* their existence. They *proved* that they must exist, subject to the assumption of locality. Of course now we know that that assumption isn't true (and Einstein knew all along that it was at least logically *possible* that nature would turn out to violate locality). But that doesn't mean the argument is wrong! Orthodox QM (with the completeness assumption) violates locality, and EPR pointed out that you could perhaps construct an empirically adequate local theory to replace it if you dropped the completeness assumption -- that is, they showed that a LHV theory was the only hope for locality.

But forget all this. Which is more likely? That the EPR paper really is nothing but an emotional ejaculation ("we'd sure would like a hidden variable theory")? Or that you have failed to grasp the *argument* presented in that paper?

There is an explicit assumption in Bell's argument: that of reality ("It follows that C is another unit vector [in addition to A and B] ..."). This is the specific narrower context you are asking about.

Yes, and to avoid any future misunderstanding, we should both refer to Bell's assumption by its standard name ("hidden variables") and not "reality".

Sure, it is reasonable, but that does not make it true!

Bell jumped off from what EPR had proved. They proved that, under the assumption of locality, certain hidden variables had to exist. Bell assumed that those hidden variables existed, imposed the locality condition again, and (by considering more general correlations than EPR had considered) showed that a certain statistical constraint could be derived, the inequality.

Again, your interpretation makes it sound as if Bell just arbitrarily assumed these hidden variables existed. He just woke up one morning and happened to share the emotion that had been previously ejaculated by EPR, so he messed around and found that this contradicted some experiments. So too bad for reality.

But that reading is inexcusably sloppy (not to mention disrespectful to Bell). If you are at all skeptical of my view here, you simply need to read Bell again. He makes it abundantly clear, e.g., here:

"Let me summarize once again the logic that leads to the impasse. The EPRB correlations are such that the result of the experiment on one side immediately foretells that on the other, whenever the analyzers happen to be parallel. If we do not accept the intervention on one side as a causal influence on the other, we seem obliged to admit that the results on both sides are determined in advance anyway, independently of the intervention on the other side, by signals from the source and by the local magnet setting. [*] But this has implications for non-parallel settings which conflict with those of quantum mechanics. So we cannot dismiss intervention on one side as a causal influence on the other."

Everything before the "[*]" is a summary of the EPR argument. The subsequent sentence refers to Bell's theorem: the thing that EPR showed to be required by locality has further implications which turn out to conflict with the QM predictions. And the final sentence is admirably (and characteristically) precise. Note in particular that no mention of "realism" or "hidden variables" (or any relevant synonyms) appear in this final conclusion.

Please note that Locality is implicitly added into Bell's argument - he mentions it, but basically takes if for granted that if there is some FTL communication between A and B (a la Bohmian Mechanics or similar) then there is no problem reproducing the results of QM.

Locality is one of the crucial premises of Bell's derivation of the inequality. Are you suggesting this assumption isn't important, or that Bell didn't think it was important? I think the quote above should dissuade you of that. Or see practically anyone of Bell's later papers, where the locality assumption is highlighted more, e.g., "la nouvelle cuisine."

 

[DrChinese]

It's ridiculous to say that EPR *guessed* that QM was incomplete. This makes it sound like the entire content of the EPR argument is to pose a trivial dilemma (either X or not-X) and then to take a wild stab at answering. ("Ummm, I dunno, how about... not-X!??") The fact is, EPR actually had an *argument* for their conclusion. And the *premise* of this argument was: locality. So EPR didn't simply *guess* that maybe OQM was incomplete. They *proved* that OQM *has* to be incomplete if one insists on respecting locality. Or just saying the same thing differently, they proved that anyone who insists on treating OQM as complete has to contend with the fact that the theory is nonlocal.

Not ridiculous. EPR said in its closing sentences [my comments in brackets]:

"This makes the reality of P and Q depend on the process of measurement carried out on the first system, which does not disturb the second in any way." [This is an accurate statement, one which is demonstrated by EPR.]

"No reasonable definition of reality could be expected to permit this." [They just threw out a perfectly logical argument because they deemed it unreasonable.]

"While we have thus shown that the wave function does not provide a complete description of the physical reality, we left open the question of whether or not such a description exists." [The incompleteness conclusion is unwarranted, because they rejected a feasible alternative without rigorous reasoning.]

We believe, however, that such a theory is possible." [This is the guess. A good guess, but wrong. If EPR had known about Bell, they undoubtedly would never have gone out on a limb on this.]

 

[DrChinese]

Bell jumped off from what EPR had proved. They proved that, under the assumption of locality, certain hidden variables had to exist. Bell assumed that those hidden variables existed, imposed the locality condition again, and (by considering more general correlations than EPR had considered) showed that a certain statistical constraint could be derived, the inequality.

Again, your interpretation makes it sound as if Bell just arbitrarily assumed these hidden variables existed. He just woke up one morning and happened to share the emotion that had been previously ejaculated by EPR, so he messed around and found that this contradicted some experiments. So too bad for reality.

But that reading is inexcusably sloppy (not to mention disrespectful to Bell). If you are at all skeptical of my view here, you simply need to read Bell again. He makes it abundantly clear, e.g., here:

"Let me summarize once again the logic that leads to the impasse. The EPRB correlations are such that the result of the experiment on one side immediately foretells that on the other, whenever the analyzers happen to be parallel. If we do not accept the intervention on one side as a causal influence on the other, we seem obliged to admit that the results on both sides are determined in advance anyway, independently of the intervention on the other side, by signals from the source and by the local magnet setting. [*] But this has implications for non-parallel settings which conflict with those of quantum mechanics. So we cannot dismiss intervention on one side as a causal influence on the other."

Bell's paper showed where EPR went wrong. There is really no way to read either and conclude that non-locality is a actual deduction - more like a possibility considered. The stated conclusions in both papers speak for themselves. I already quoted EPR's conclusions in a separate post. Here is Bell's conclusion:

"In a theory in which parameters are added to QM to determine the results of individual measurements, without changing the statistical predictions, there must be a mechanism whereby the setting of one measuring device can influence the reading of another instrument." [I.e. Any hidden variables must be non-local, just as you argue... but he is not saying that hidden variables are a requirement of QM or even that hidden variables exist.]

"Moreover, the signal involved must propagate instantaneously, so that such a theory could not be Lorentz invariant." [He is saying clearly: If you think there are hidden variables, you must throw out Einstein's special relativity. Bell knows this will be difficult for many, making the price too high for retaining hidden variables.]

Regardless of how you read the above, Bell's paper does not prove that QM is non-local. If you are unsure of that, simply re-read the proof in which the hidden variable assumption is included: ("It follows that C is another unit vector [in addition to A and B] ..."). It is certainly a logical possibility that this assumption is invalid, how can you deny this? Everything Bell does after depends on this crucial assumption, which he makes knowing fully where it leads.

P.S. I would appreciate it if you would not accuse me of disrespect to Bell. Anyone who is familiar with my work knows that is far off the mark (you can google EPR Bell and see where I am). I suspect we agree far more than we disagree.

 

[NateTG]

This discussion is, more or less, repeated here monthly, so, as usual, here's my take on the issue:

Really, what EPR, Bell, Aspect, and a whole host of other brilliant people have demonstrated is that there is no 'nice' QM. It is possible to construct particle models that correspond to the experimental results, but all have some strange qualities. Choosing one over the other is currently more of a choice of interpretation or taste than one of science.

By not nice, I mean to say that the model must be non-local (e.g. bohmian mechanics), non-realistic (e.g. plug and chug), mathematically monstrous (involving non-measurable sets), or some other similarly strange notion (such as many worlds, or mini-wormholes).

As a fan of locality and realism, I personally like the notion of 'monstrous' particles, but recognize that such an interpretation has some philosophical issues associated with it (google Banach Tarski paradox for more information.)

 

[DrChinese]

This discussion is, more or less, repeated here monthly, so, as usual, here's my take on the issue:

Really, what EPR, Bell, Aspect, and a whole host of other brilliant people have demonstrated is that there is no 'nice' QM. It is possible to construct particle models that correspond to the experimental results, but all have some strange qualities. Choosing one over the other is currently more of a choice of interpretation or taste than one of science.

By not nice, I mean to say that the model must be non-local (e.g. bohmian mechanics), non-realistic (e.g. plug and chug), mathematically monstrous (involving non-measurable sets), or some other similarly strange notion (such as many worlds, or mini-wormholes).

LOL, You nailed it in a lot fewer words...

 

[ttn]

P.S. I would appreciate it if you would not accuse me of disrespect to Bell. Anyone who is familiar with my work knows that is far off the mark (you can google EPR Bell and see where I am). I suspect we agree far more than we disagree.

I don't know how much we agree, really. But I've made my views (including my disagreement with at least some of your views) as clear as I can make them. I think it's a waste of everybody's time to continue this back and forth about what Bell did and didn't prove. I've provided a quote from one of his later papers that, I think, completely undermines your position. If you don't agree, we'll have to just agree to disagree because nothing I say will convince you if Bell can't. And it's the same with Einstein/EPR. I've had my say elsewhere (e.g., 0404016) and if I haven't convinced you yet that you don't understand their argument, I don't think I ever will.

 

[Sherlock]

Yes, a LHV theory can explain certain facts. But it can't explain other facts.
So LHV theories are excluded.

I'd put it this way. Lhv theories apply to some setups but not to
others. What class of setups are lhv descriptions compatible with?
According to Bell, individual measurements where, eg., you're recording
random/spontaneous output of a single detector.

What class of setups are lhv descriptions incompatible with?
Composite (A,B) measurements of the sort that characterize
typical Bell tests.

Now I'll ask you the question that I asked DrChinese.
What happens to the lhv's in the composite systems?
Do we conclude that they don't exist in either individual
or composite systems? Or that they exist in one but not
the other?

My thinking on this is that they exist in both sorts
of setups. However, while they're factors in determining
the outcomes of individual measurements, they're not
factors (at least their variability isn't) in determining the
outcomes of composite setups.

Nobody has yet addressed this: what if the hidden
property in the (A,B) setup isn't varying from pair to pair?

 

[Sherlock]

So, I'll repeat my question that you didn't answer. :-)
What happens to these local hidden variables
when we incorporate these individual measurement
events into a correlational context involving other individual
measurement events at spacelike separations from these?
Do the hidden variables just vanish (along with local
reality)? Or is it simply that they aren't determining the
joint results?

If the lhv's simply aren't a factor in determining the joint
results, then isn't it incorrect to say that these setups
show that lhv's don't exist, or that there is no locally
realistic behavior occurring in these setups, or that lhv
descriptions of any setup are therefore ruled out?

And I'll repeat my answer: There are no local hidden variables in QM.

This isn't what I asked.

All LHV theories are incompatible with all of the predictions of QM.

No, some qm formulations can be supplemented with
lhv info, and some can't. What's the difference between
those that can and those that can't?

If you want to postulate a LHV which mimics SOME of the predictions of QM, no one is disputing your ability to do that. But since such a theory makes erroneous predictions about some experiments (such as Aspect), it is not likely to find much acceptance among scientists.

It's a matter of supplementing qm formulations with lhv
info. In some cases this would improve qm predictions
(eg. individual results), and in some cases (composite
setups) including lhv's as determining parameters
reduces the accuracy of predictions. Why?

Entangled systems are merely a tool that enables us to realize Bell's Theorem (i.e. that LR is incompatible with QM). It is not a boundary condition, i.e. that the world is local realistic everywhere EXCEPT
entangled systems.

We see that lhv's apply (determine outcomes) in some setups
and not in others. Am I to suppose that there are no
lhv's existing in the composite setups simply because they
don't determine the outcomes, or is there more to it than
that?

 

[DrChinese]

...We see that lhv's apply (determine outcomes) in some setups and not in others.

There are no known such situations within the realm of QM - there couldn't be, because such would violate the HUP.

You are free to contradict that with an actual example. An example would be something which displays the actual local hidden variables for us to see, not something which is a hypothetical abstraction.

 

[Sherlock]

There are no known such situations within the realm of QM - there couldn't be, because such would violate the HUP.

You are free to contradict that with an actual example. An example would be something which displays the actual local hidden variables for us to see, not something which is a hypothetical abstraction.

If we could see them they wouldn't be hidden variables,
would they? :-)

So, the whole discussion is about hypothetical abstractions ...
ie., what would happen if we supplemented some formulation
or other with hidden variable information?

And, we see that wrt some formulations it would help, and
wrt other formulations it wouldn't.

The HUP has nothing to do with more accurately predicting
detection patterns given some inferred additional information
about submicroscopic processes that's otherwise hidden from us.

This is what's happening when random individual
detections are combined to produce predictable joint
results.

 

[DrChinese]

If we could see them they wouldn't be hidden variables,
would they? :-)

So, the whole discussion is about hypothetical abstractions ...
ie., what would happen if we supplemented some formulation
or other with hidden variable information?

And, we see that wrt some formulations it would help, and
wrt other formulations it wouldn't.

The HUP has nothing to do with more accurately predicting
detection patterns given some inferred additional information
about submicroscopic processes that's otherwise hidden from us.

This is what's happening when random individual
detections are combined to produce predictable joint
results.

Well, I can't allot weight much to a theory that explains nothing new, predicts nothing new, is not falsifiable (even when experiments such as Aspect DO falsify it), applies only in occasional spots and appears to do nothing other than satisfy perceived dissatisfactions with QM. This is why Bell's Theorem is so useful. I don't even need to consider the idea of this theory further because the entire class of LHV theories are ruled out.

If I was really smart, I'd get you and ttn talking to each other... You advocating Local HV theories as being "proven", and ttn advocating Non-local HV theories as being "proven". Then I would just side-step outta here.

 

[Sherlock]

Well, I can't allot weight much to a theory that explains nothing new, predicts nothing new, is not falsifiable (even when experiments such as Aspect DO falsify it), applies only in occasional spots and appears to do nothing other than satisfy perceived dissatisfactions with QM. This is why Bell's Theorem is so useful. I don't even need to consider the idea of this theory further because the entire class of LHV theories are ruled out.

If I was really smart, I'd get you and ttn talking to each other... You advocating Local HV theories as being "proven", and ttn advocating Non-local HV theories as being "proven". Then I would just side-step outta here.

I'm not advocating lhv theories as being proven. I just think that
some important points are being overlooked. Lhv's (not lhv theories,
just lhv's) can exist and still not be relevant in some setups. So,
where they're not relevant you just don't use them. That's all.
This doesn't rule out lhv theories in general. It doesn't mean
that lhv's don't exist. There's still some real stuff happening
between emitters and detectors and we use what we can
infer about it to develop better, more complete, descriptions
of physical reality.

 

[NateTG]

Now I'll ask you the question that I asked DrChinese.
What happens to the lhv's in the composite systems?
Do we conclude that they don't exist in either individual
or composite systems? Or that they exist in one but not
the other?

The problem with 'nice' LHV theories for explaining composite systems is that they don't provide a mechanism for the HUP. Since I'm not particularly interested in the QM nuts and bolts I can't be certain of this, but Bells theorem looks like it rules out any 'hidden local realistic' theory that assigns a value to the chance of correlating non-commuting measurements, for example, measuring spin direction along a couple of different axes.

Before discussing them, I will warn you that this type of model is not AFAIK well received in mainstream physics. However, there are hidden variable theories that do not assign values to the correlations of non-commuting measurements, and hence are not invalidated by Bell's theorem + Aspect et al., but that involves unmeasurable sets. Moreover, it's clear that models of this type that make identical predictions to the 'wave equation' model can be constructed.

 

[NateTG]

You are free to contradict that with an actual example. An example would be something which displays the actual local hidden variables for us to see, not something which is a hypothetical abstraction.

Ah, but Science works by falsification, not by demonstration, and I can suggest an experiment that could falsify the notion that spin state can be completely explained by LHV theories:

This is only a thought experiment, but consider the following:
Let's say we have an entangled positron source, and an entangled electron source, separated by two light seconds, covered so that only pairs that send one member towards the other source are emitted. So the set up might look something like

   ______         ______
     E+             E-
   ______         ______

So, we have the source on the left emitting positrons, and the source on the right emitting electrons, and sending them into the middle where they anihillate pairwise.

We can time the anihillation, and measure the spin orientations (along the up-down axis only) of the particles that are sent out the outside ends of the apparatus.

If the anihillation occurs readily for all of the particle-antiparticle pairings, and the spins correlate then we have correlating measurements that cannot be drawn back to a single (non-hidden) event since they are (or at least could be) separated from the creation of the other particle by more than ct. This cannot be explained by any local hidden variable theory.

 

[ZapperZ]

I suppose at this point I should add that there is an obscenly comprehensive review of hidden variable theories written by Marco Genovese[1]. It is a 78-page review of the theories and experiments on the EPR-type issues, and contains 504 references! It took me a week to actually finish reading the damn thing, and I need to go over it again. It covers the historical development of the field along with practically all the important theoretical and experimental results in this field, except for the 3 that I have mentioned that was recently published (see PF Blog).

There are some strange sentences in the article (a more accurate proof-reading might have made it better), but I still highly recommend this for anyone wishing to understand this area of physics. I think other than Special Relativity, this is one area of physics that has a lot of misunderstanding.

Zz.

[1] M. Genovese, Phys. Rep. v.413, p.319 (2005).

 

[Locrian]

It may be true that this argument is repeated once a month, as NateTG noted, but I learn something new each time.

 

[Sherlock]

Ah, but Science works by falsification, not by demonstration, and I can suggest an experiment that could falsify the notion that spin state can be completely explained by LHV theories:

This is only a thought experiment, but consider the following:
Let's say we have an entangled positron source, and an entangled electron source, separated by two light seconds, covered so that only pairs that send one member towards the other source are emitted. So the set up might look something like

   ______         ______
     E+             E-
   ______         ______

So, we have the source on the left emitting positrons, and the source on the right emitting electrons, and sending them into the middle where they anihillate pairwise.

We can time the anihillation, and measure the spin orientations (along the up-down axis only) of the particles that are sent out the outside ends of the apparatus.

If the anihillation occurs readily for all of the particle-antiparticle pairings, and the spins correlate then we have correlating measurements that cannot be drawn back to a single (non-hidden) event since they are (or at least could be) separated from the creation of the other particle by more than ct. This cannot be explained by any local hidden variable theory.

The simpler, optical Bell tests can't be explained by lhv
theory either -- unless you don't assign a value to the
hidden parameter. Then it's not an lhv theory, but a
local hidden whatever (some mysterious constant
relationship between members of each pair?) theory. :-)

Anyway, I'm curious. Regarding your thought experiment,
how would you correlate anihilations to coincidences? Or,
would you just compare the counts per unit of time, or what?

 

[Sherlock]

The problem with 'nice' LHV theories for explaining composite systems is that they don't provide a mechanism for the HUP. Since I'm not particularly interested in the QM nuts and bolts I can't be certain of this, but Bells theorem looks like it rules out any 'hidden local realistic' theory that assigns a value to the chance of correlating non-commuting measurements, for example, measuring spin direction along a couple of different axes.

Before discussing them, I will warn you that this type of model is not AFAIK well received in mainstream physics. However, there are hidden variable theories that do not assign values to the correlations of non-commuting measurements, and hence are not invalidated by Bell's theorem + Aspect et al., but that involves unmeasurable sets. Moreover, it's clear that models of this type that make identical predictions to the 'wave equation' model can be constructed.

This sounds like the way I've been thinking about it. Do you
happen to have any references handy?

 

[NateTG]

The simpler, optical Bell tests can't be explained by lhv
theory either -- unless you don't assign a value to the
hidden parameter. Then it's not an lhv theory, but a
local hidden whatever (some mysterious constant
relationship between members of each pair?) theory. :-)

Actually, Bell's theorem only eliminates 'nice' lhv theories. To avoid confusion, I will refer to the local hidden theories that it does not eliminate as local hiden monster theories (lhm). The experiment I described may be able to falsify these lhm theories because it has a larger separation between the measurements than a traditional EPR experiment.

Anyway, I'm curious. Regarding your thought experiment,
how would you correlate anihilations to coincidences? Or,
would you just compare the counts per unit of time, or what?

I was thinking that you control the emitters so that the events are sparse. Then count per unit time should work. I expect that a macroscopic count per unit time would be theoretically nice. However, the net spin of a bunch of particles is going to be about the square root of the number of particles or less, which means that if you deal with large numbers of particles and anihillations there should be more 'noise'.

From my perspective the experiment has the bigger problem that I have absolutely no idea what sort of prediction conventional models make for the experimental results - so it may be a non-experiment in that the predictions made by the theories do not differ.

 

[Sherlock]

From my perspective the experiment has the bigger problem that I have absolutely no idea what sort of prediction conventional models make for the experimental results - so it may be a non-experiment in that the predictions made by the theories do not differ.

A more detailed diagram of what you have in mind
is necessary (at least for me).

My intuitive prediction is that local hidden monster
(I prefer local hidden constant) formulations won't be
falsified. :-)

If I get time, I'll do some homework about how your
proposed setup might work.

 

[ohwilleke]

Let me look at QM v. Special Relativity from a slightly different perspective.

The basic deal in entanglement is that when you locally learn facts about half of a set of entangled particles, you know something about the other half of the set of engtangled particles, regardless of their distance.

In advance, you can't know what data will be found when you collapse the wave function on either set of particles. And, viewed alone, each set of particles will comply with QM predictions.

To use the Copenhagen interpretation (or as it has been called in this thread) OQM, the reason that you can't know in advance what data will be found when you collapse the wave function on either set of particles is that this data doesn't exist yet. Until there collapse happens, any result is possible. A set of particles does not have a deterministic pre-ordained state upon collapse.

OK, so enough of OQM for a moment. We'll come back to that.

Now, one of the most basic elements of SR is that light in a vacuum travels at speed c. It has been assumed that one can infer from SR the stricter condition that information also does not travel in excess of speed c.

It seems to me that one way to reconcile SR and OCM is to violate neither of these propositions, but instead to violate the implicit, but mathematically unnecessary assuption of SR that information and light always go forward in time.

The way that you would do this is to stick to OCM. The state of one half of a set of entangled particles does not exist until you collapse the wave function. So, how does the other part of the set of entangled particles end up corollated?

Maybe, at the moment that one collapse happens, that information goes backwards in time (at a speed not greater than c) to the point of entanglement, and then goes forward in time from there to the other entangled particles, communicating the information to the second set without the message ever having traveled faster than c.

This looks like FTL, but it isn't. One of the keystones of entanglement is that it only happens to particles that have been physically local at some point in time, making this backward to forward in time communication possible.

Unlike a traditional hidden variable theory, nothing that has yet happened makes it possible to determine the final set of either part of the set of entangled particles.

But, only events within the light cone of the initial entanglement (the light cone of the global system if you will) can influence either result.

Does that make sense? Where have I gone wrong?

 

[NateTG]

This sounds like the way I've been thinking about it. Do you
happen to have any references handy?

I'll warn you again, that this is non-standard stuff. I'm also going to warn you that it involves non-measurable sets (if you don't know what this means, you might want to look into it a bit google unmeasurable sets, and banach-tarski for a taste).

That said, you might check out this thread:
https://www.physicsforums.com/showthread.php?t=30947&page=1&pp=15

 

[DrChinese]

It seems to me that one way to reconcile SR and OCM is to violate neither of these propositions, but instead to violate the implicit, but mathematically unnecessary assuption of SR that information and light always go forward in time.

Maybe, at the moment that one collapse happens, that information goes backwards in time (at a speed not greater than c) to the point of entanglement, and then goes forward in time from there to the other entangled particles, communicating the information to the second set without the message ever having traveled faster than c.

This looks like FTL, but it isn't. One of the keystones of entanglement is that it only happens to particles that have been physically local at some point in time, making this backward to forward in time communication possible.

This explanation makes AT LEAST as much sense as the other interpretations. Probably a lot more, since all physical laws are otherwise time symmetric. If the future were to exhibit SOME influence on the past, then it would appear to those in the past as being random (uncaused).

There are some articles out there which hint at this. See http://www.arxiv.org/abs/quant-ph/0507269 for example.