A Paradox: Do LHV Theories Need the HUP?

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DrChinese

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ttn said:
The whole point of the EPR argument is to show that ***if*** you assume that QM is complete, the theory violates locality. Completeness entails non-locality.
Perhaps you would care to provide a quote from EPR that backs this up. FYI, here are the actual last 4 sentences of EPR. Note that the last 3 are now known to be WRONG and this is what got us debating in the first place.

"This makes the reality of P and Q depend on the process of measurement carried out on the first system, that does not affect the second system in any way."

(This is standard Copenhagen interpretation. It is also a way of saying that reality is observer dependent. Please note that this applies equally in any test of QED in which the HUP is used. The HUP tells us that reality IS observer dependent.)

"No reasonable definition of reality could be expected to permit this."

(This is an ad hoc assumption and is not warranted from the argument presented in EPR.)

"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."

(This deduction is invalid because the previous sentence is unwarranted. The correct conclusion is that EITHER QM is incomplete, OR there is not simultaneous reality to non-commuting observables. This correct conclusion was stated earlier in EPR.)

"We believe, however, that such a theory is possible."

(Because of Bell, we now know that NO such theory is possible, regardless of Einstein's faith in the matter. R.I.P. Local reality.)
 

ttn

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DrChinese said:
Perhaps you would care to provide a quote from EPR that backs this up.
See quant-ph/0404016 for a detailed discussion, including lots of juicy quotes.
FYI, here are the actual last 4 sentences of EPR. Note that the last 3 are now known to be WRONG and this is what got us debating in the first place.
"This makes the reality of P and Q depend on the process of measurement carried out on the first system, that does not affect the second system in any way."
(This is standard Copenhagen interpretation. It is also a way of saying that reality is observer dependent. Please note that this applies equally in any test of QED in which the HUP is used. The HUP tells us that reality IS observer dependent.)
Huh? The point of this sentence you quote is to stress the locality assumption. EPR (i.e., *Podolsky*!!) are here pointing out the non-locality implied by the standard "disturbance" view which says: the distant particle doesn't have a definite X or a definite P until the nearby measurement is made, the distant particle then "collapsing" into a state with a definite value for the appropriate operator. Their point is that this non-local collapsing violates locality.
"No reasonable definition of reality could be expected to permit this."
(This is an ad hoc assumption and is not warranted from the argument presented in EPR.)
Substitute "local" for "reasonable" and then it makes perfect sense.
"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."
(This deduction is invalid because the previous sentence is unwarranted. The correct conclusion is that EITHER QM is incomplete, OR there is not simultaneous reality to non-commuting observables. This correct conclusion was stated earlier in EPR.)
Blah.
"We believe, however, that such a theory is possible."
(Because of Bell, we now know that NO such theory is possible, regardless of Einstein's faith in the matter. R.I.P. Local reality.)
Agreed. But you still seem to be missing the main point here: EPR showed that orthodox QM violates locality. And that is simply a different point than their belief that a local theory might be possible. They hoped a local theory would be possible, yes. And now we know it isn't, yes. But none of that undermines their argument that orthodox QM (i.e., QM with the completeness doctrine) violates locality. That is, was and always will be true. So it's a mistake to say "RIP Local Reality" as if there was some "non-reality" *alternative* that *was* local. There isn't. Orthodox QM is nonlocal. It violates Bell Locality, which you can just test for yourself if you know how QM works and what Bell Locality means. EPR showed that the only way to save Locality was to reject the completeness doctrine, and supplement QM with some local hidden variables. But then Bell showed that such a LHV theory can't work. So the only way to save locality doesn't work. Locality can't be saved. Locality is false.
That is the real conclusion of this whole EPR + Bell issue, and you'll never see it so long as you bury your head in the sand w.r.t. EPR.
 

DrChinese

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ttn said:
Maybe you should clarify your notation, but if it means what I think it means, then you are confused: the unit vector c isn't a hidden variable, it's the direction a certain person measures the spin of a certain particle along. The hidden variable is the "lambda" (Bell's notation) which determines what the *outcome* of that measurement will be.

Your claim was that the "unit vector c" was a *non-local* hidden variable.
I don't need to clarify my notation - it is straight from Bell. Please see just after equation (14) where this is introduced. It is axiomatic that vector c exists IF there is SIMULTANEOUS REALITY TO NON-COMMUTING OBSERVABLES. You can call it a hidden variable, hidden observable or anything you want to really. The point is that it maps to what EPR was assuming existed independently of the act of observation.

As presented by Bell, it is neither local nor non-local. This *despite* his statement that particle 1's result does not depend on particle 2's setting, and vice versa. Sure, this matters to the final conclusion (please don't misquote me on this point) but it is not part of the formal proof. Please note that a reasonable person could read Bell's words and conclude that he believes that Bohmian mechanics is the only possible solution to the conclusion he arrives at. Clearly, BM is not the same as QM! Yet, the fact is, today Bohmian mechanics is not really pursued too seriously. Why is that? Because there is absolutely no need to add anything to QM to fit with experiment.
 

DrChinese

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ttn said:
That is the real conclusion of this whole EPR + Bell issue...
I'm waiting for the quotes. Meanwhile, please continue to give yourself pats on the back.

Meanwhile, on the original subject of this thread: can a local realistic theory (such as S.E.D. - see Vanesch's earlier references) operate without incorporating non-classical ideas such as the HUP or the projection postulate?

As a result of Bell, we now know that LHV theories cannot replicate all of the results of QM. With this knowledge in hand, my curiously was piqued. Presumably, there must be other areas in which the ideas of QM conflict with various fundamental elements of LHV theories. And sure enough, there is work being done to further distance QM from such theories. It turns out that attempts to present LHV theories consistent with Bell tests have not been going very well.
 
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ttn

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DrChinese said:
I don't need to clarify my notation - it is straight from Bell. Please see just after equation (14) where this is introduced. It is axiomatic that vector c exists IF there is SIMULTANEOUS REALITY TO NON-COMMUTING OBSERVABLES. You can call it a hidden variable, hidden observable or anything you want to really. The point is that it maps to what EPR was assuming existed independently of the act of observation.
I'm sorry, but none of this makes any sense. The lowercase a, b, and c refer simply to directions in space. They are the directions along which some hypothetical S-G apparatus is oriented to measure the spin component (along that direction) of an electron.

But if you think "c" is a hidden variable that has something to do with there being "SIMULTANEOUS REALITY TO NON-COMMUTING OBSERVABLES", there's really no point arguing further with you about this. You obviously just haven't seriously tried to understand Bell's paper(s).



Clearly, BM is not the same as QM! Yet, the fact is, today Bohmian mechanics is not really pursued too seriously. Why is that? Because there is absolutely no need to add anything to QM to fit with experiment.
Apparently the reason Bohmian Mechanics isn't pursued more seriously is that, like you, there are a lot of physicists out there who are seriously confused about these issues. Bohm's theory *shouldn't* be pursued because it conflicts with relativity, right? Oh, but we don't need to worry about the fact that orthodox QM also conflicts with relativity -- that's just a subjective interpretation. Yeah, right. Good physics.
 

ttn

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DrChinese said:
I'm waiting for the quotes.
OK, fine, here's one:

"...the paradox [EPR] forces us to relinquish one of the following two assertions:

1. the description by means of the psi-function is complete

2. the real states of spatially separated objects are independent of each other

...it is possible to adhere to (2) if one regards the psi-function as the description of a (statistical) ensemble of systems (and therefore relinquishes (1) ). However, this blasts the framework of the 'orthodox quantum theory.'"

-Albert Einstein, "Reply to Criticisms", from Schilpp (Albert Einstein: Philosopher Scientist), pg 681.


As I said, see quant-ph/0404016 for a more detailed discussion.



Meanwhile, on the original subject of this thread: can a local realistic theory (such as S.E.D. - see Vanesch's earlier references) operate without incorporating non-classical ideas such as the HUP or the projection postulate?
I don't really have any interest in that question, since I don't think local realistic theories are viable. They're refuted by Bell's theorem, so who really cares what weird ideas they can or can't incorporate? Well, not me.


As a result of Bell, we now know that LHV theories cannot replicate all of the results of QM.
True. But an equally important point is: As a result of Einstein, we now know that orthodox quantum mechanics is not local.
 

DrChinese

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ttn said:
... As a result of Einstein, we now know that orthodox quantum mechanics is not local.
No quote provided from EPR, as I said you won't be able to provide one which is relevant. EPR is about the reality of observables, and so is Bell. You know, is the moon there when no one is watching? If it was, then you could use information from an observation on one entangled particle to augment your knowledge of the other. But that doesn't happen, because there are limits to what we can know about any particle (entangled or not). So I guess I would ask you: does a single particle have a well defined position AND momentum simultaneously? If the answer is NO, then the results of Bell tests shouldn't seem surprising. Entangling them does not change this basic fact. Separating them also does not change this basic fact.

I cannot imagine too many scientists agreeing with your statement above. As a result of EPR, there was no significant change in the view of QM by the founders/followers of QM. You need to re-read the source papers and drop your bias. You can find them on my site if you don't have them: http://www.drchinese.com/David/EPR_Bell_Aspect.htm

And I'm not sure why you are hanging out in this thread if you are not interested in the subject matter.
 

ttn

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DrChinese said:
You need to re-read the source papers and drop your bias.
Oh please. This discussion proves that you don't know the literature. I'm perfectly familiar with the "source papers" -- including the EPR paper itself and Bell's first proof of Bell's Theorem. But unlike you, I'm also quite familiar with the rest of the literature on this topic, both "source" and "secondary".



And I'm not sure why you are hanging out in this thread if you are not interested in the subject matter.
You made some false and misleading statements about EPR. Since you seem to comment on this topic a lot, I thought you might be interested to get straight on a few things. But that is apparently not the case... which leaves me with very little reason for continuing to hang out on this thread.
 

DrChinese

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Moving on to a hopefully more productive discussion...

I have been reviewing Zz's reference on the state of the art in EPR and Bell written by Genovese. This monster has 504 references and is 78 pages long. It covers the history of EPR/Bell plus the state of the art in experiments; plus a discussion of the hypothetical loopholes.

Amazingly, virtually every LHV theory designed to be compatible with Bell test results has been eliminated. As detection efficiency has increased, there has been no degradation in the violations of Bell inequalities. It had previously been postulated that experiments ruling out of LHV theories might be flawed in some respects. While simultaneous elimination of all "loopholes" has not yet occurred, it is getting closer.

No amount of testing has yet indicated that locality vs. non-locality is an issue. I.e. changing the settings of polarizers mid-flight has no influence whatsoever on the results. In addition, no amount of testing has indicated that the sample size has any effect on the outcome. I.e. the detection efficiency is not a variable in the results in any experiment performed so far. However, the goal is to get a detection efficiency in excess of about 81% while simultaneously making random changes in the polarizer settings mid-flight. This is proving to be a difficult goal to achieve.
 

DrChinese

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Genovese has also acknowledged the difficulty in getting all controversy removed from discussion of experimental results. In his opinion, the ultimate experiment might be performed within the next generation. Yet, even this would be unlikely to silence all voices on the subject.

Regardless, the door would still be open for non-local HV theories which would be possible alternatives to QM. So he expects there will be plenty to investigate for many years to come.

The shear amount of work that has been done in this area in recent years is absolutely astonishing to me. When Bell wrote his paper, he had trouble getting anyone to give it serious attention. Today, interest in the EPR Paradox and Bell's Theorem has never been greater. The advent of PDC technology has even made table-top tests in undergraduate settings possible.
 

DrChinese

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One might wonder what I mean when I say that there are LHV theories compatible with Bell test results. Obviously, LHV theories are NOT compatible with the predictions of QM. And all tests to date soundly support QM. However...

There are those who hold out hope that QM is simply wrong. This is a difficult position to maintain given current experiments, but that does not stop diehard supporters of LHV theories.

Bell has already pointed out a substantial burden on LHV theories - the Bell Inequality. So if you assume that there is local reality, you are saying in essence that there is something that causes the experimental results to appear to support QM. Thus:

f(experiment) - f(LR) = f(experimental error/loopholes/etc.)

and yet:

f(experiment) - f(QM) = 0 as efficiency increases (frequently at the hundreds of Std Deviation level).

These equations are becoming increasing more difficult to explain for the local realist. Why - if QM is wrong - is it the exact result you get in every experiment? Why - if there are loopholes - is there no variation in results when any single loophole is closed?

It is almost as if someone is arguing that the earth is 70,000,000 miles from the sun while experiments all say it is 93,000,000 miles away.

In addition, there are other burdens on LHV theories. After all, they need to explain all of the things that QM does too. Double slit, uncertainty, quantum model of the atom, etc. Is it possible that another competing theory could ever jump through all of these hoops?
 

vanesch

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ttn said:
Yes, I don't think you understand Bell's derivation. Look at his paper, "On the EPR Paradox." Right after equation (1) he states openly: "The vital assumption is that the result B ... does not depend on the setting a ... nor A on b." (Check out the footnote, too.) This is the locality assumption. It's only because of the locality assumption that we *forbid* A to depend on b, and vice versa.
Well, a way of sneaking out is to say simply that there WAS no result at A, until it got transported at b. That A still contained both possible results, and that it was only decided upon when the signal arrived at b.
And without that assumption, the derivation (obviously) does not go through. Bell's inequality is a bound that applies to *local* theories only. So... how you can say "his primary argument...has nothing to do with locality", I have no idea. Again, all I can conclude is that you simply don't understand either EPR or Bell's Theorem. *Both* of these *crucially* involve locality.
This is correct, but they also assume that a measurement at A had a definite outcome.
You will have to flesh this out. What, exactly, is "observer dependent"? And please note: if your "observer dependence" includes a dependence of the A-side outcome on the B-side observation, your theory is *nonlocal*. Which means it isn't "both local and observer dependent."
What is observer-dependent is what the observer at b *learns* from what was supposed to be measured at a. He learns something about b and thinks that this was the "definite" result at A. But the observer at a might just "learn" something quite different about exactly that same outcome (in that case he'll be living in a different branch).
Or was your point that regular old Copenhagen-ish QM is "both local and observer dependent"? That is just patently false.
Orthodox Copenhagenish QM is both observer dependent and non-local in its mechanism.
In fact, I challenge you to provide any example of a theory that is local, predicts that experiments have definite outcomes, and consistent with the QM predictions.
*that* is impossible of course. So you shoot on the one you like least. As I said, it's a hard choice to have to let go either one of the 3 ; I take out the one in the middle, you take out the first one, and the LR crowd takes out the third one.
 

DrChinese

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vanesch said:
What is observer-dependent is what the observer at b *learns* from what was supposed to be measured at a. He learns something about b and thinks that this was the "definite" result at A. But the observer at a might just "learn" something quite different about exactly that same outcome (in that case he'll be living in a different branch).
Looking at it from the point of view of MW:

Bell's derivation is fully consistent with a MW interpretation in the sense that in MW, each branch contains only the actual outcomes. There is no c outcome in a branch in which a and b are measured. This exactly corresponds to Bell's proof.

It is true that Bell incorporates the idea that the setting at A does not affect the outcome at B. (For this to matter, there has to be a mechanism for this to take place.) BUT... In MW, this is not an issue because the main premise - that an observation could have been made at setting c - is explicitly false. A setting at c would be a different branch, and you cannot have settings at a, b and c simultaneously.
 
vanesch said:
Yes, of course entanglement is locally produced at emission, that is entirely correct. I'm sorry, and no offense, but I think you never got the "click" of what Bell is all about.
Entanglement is a special kind of state description entirely proper to the formalism of quantum theory, and simply means that the way states of systems are described are, eh, well, entangled, meaning, you cannot disentangle the state description as "the state of A" and "the state of B". I don't know if you realise how revolutionary a concept that is. Never this occured before in physics. In classical physics, if you have two systems which are in DIFFERENT LOCATIONS, it is always possible to describe the state of the TOTAL SYSTEM as "the state of A" and "the state of B". Of course there can be interactions between these states, and of course these states can have a "common origin" even if they are not interacting, because the systems interacted before.
But "the state of the sun-Betelgeuse system" can always be written as "the state of the sun" and "the state of Betelgeuse". All you can measure about the sun will depend ONLY on "the state of the sun" and all you can measure about Betelgeuse will depend only on "the state of Betelgeuse".
THAT DOESN'T MEAN that there cannot be correlations between both. Indeed, it could be that the sun and Betelgeuse had some interaction long ago, and the correlations of our measurements only measure that "common part" induced by that interaction long ago.
But, as Bell showed, when you have such a separate "this is the state of A" and "this is the state of B" state description EVEN IF THERE WAS A COMMON PART, the correlations you can obtain in such a way have to satisfy certain properties.
This is what is violated in QM. And it is (after the fact) not surprising because the states in QM are NOT of the form "the state of A" and "the state of B". There is only the state of AB.
Again, don't think that this is "simply because they have common origin". Classical systems can have "common origin", but that doesn't deny a reality both to the "state of A" and "the state of B". Measurements on A and B will show statistical correlations, but these correlations WILL SATISFY CERTAIN RULES (like Bell's inequalities). Quantum entanglement goes further and DENIES the existence of a state of A and a state of B.
But people feel uneasy about the fact that there is "no state of A" and "no state of B", and only a "state of AB", because we're used to thinking that what is right here, should have its own 'reality' (state). That's what local reality is all about.
Now it could be that you find it very normal for something NOT to have a state of its own (a reality of its own) just because it is confined to some space. You might have the intuition that "reality is holistic". But it is a very special way of thinking in physics, because VERY MANY USEFUL laws are based upon the locality principle. This is why people like Einstein thought that QM had an UNDERLYING theory which had identical predictions, but which had a (totally different) state description for A and for B. He called those state descriptions "hidden variables".
But Bell's theorem shows that this is not possible.
What is possible is that A and B DO have an individual state description ON THE CONDITION THAT they keep constantly in immediate interaction over long distances. Then, of course, the measurement of one can immediately change the STATE of the other, thus mimicking the quantum result. This is Bohmian mechanics.
Something did 'click' for me after I read this. It suddenly became clear to me why Bell *had* to use the form that he did. I don't really know if it's entirely due to the way you laid it out here, but this certainly had the effect of making me realize that at least one aspect of how I had been thinking about Bell-EPR consideration was wrong. So, I appreciate your efforts, as well as other mentors and advisors, here and in other threads. And, don't worry about offending -- some humiliation is part of learning.
 

vanesch

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DrChinese said:
Looking at it from the point of view of MW:
Bell's derivation is fully consistent with a MW interpretation in the sense that in MW, each branch contains only the actual outcomes. There is no c outcome in a branch in which a and b are measured. This exactly corresponds to Bell's proof.
Yes, but that could be considered not sufficient, because you can consider the branch of that single observer that has done a lot of measurements, in different directions, and who would then have to conclude that in HIS branch, there seems to be something statistically fishy (as long as we suppose that there is some "free will" to set the polarizers - but let's for the moment not go into that mine field!) with his data.
I think the real point in MW is that when A has her results, but didn't learn yet from B what were the results at B, these results (with Bob and everything included) are still in a superposition. It is only when Bob gets to A, and tells Alice the result, that Alice "joins" one of the two possible branches of B (the two terms of the superposition). But this happened LOCALLY (at Alice's place). Now Alice will conclude from that that Bob DID have a result when he was far away, and that that result seemed fishy, but in fact, Bob DIDN'T have a result: he was in a superposition. So there is influence all right of Alice's settings on what she THINKS Bob measured (when she extrapolates back in time), but that influence was transmitted LOCALLY when Bob in superposition got to Alice and told her (in fact, when the TWO Bobs came to Alice, and Alice only saw one! The one that fitted her settings and result).
So YES, there is an action of Alice's settings on "Bob's" results, but we think that this happens when Bob does his measurement, and in fact it only happens when Bob tells Alice (when Alice chooses which Bob she will see!).
This is what circumvents Bell's theorem: there IS an action of the settings of Alice on "Bob's results" (but it is not at a distance! It happens when Bob tells Alice).
 
vanesch said:
Yes, of course entanglement is locally produced at emission, that is entirely correct.
...
... as Bell showed, when you have such a separate "this is the state of A" and "this is the state of B" state description EVEN IF THERE WAS A COMMON PART, the correlations you can obtain in such a way have to satisfy certain properties.
This is what is violated in QM. And it is (after the fact) not surprising because the states in QM are NOT of the form "the state of A" and "the state of B". There is only the state of AB.
Again, don't think that this is "simply because they have common origin".
Please critique these statements and line of reasoning:
1. Bell tests are testing the viability of a certain general formulation wrt correlations of spatially separated quantum measurements involving entangled particles.
2. This general formulation involves the assumption (local realism) that the correlations can be adequately described in terms of the juxtaposition of the physical evolutions of the individual particles, A and B, of any and all entangled pairs.
3. In qm there is no description of the physical evolution of the individual particles, and due to HUP such a description is impossible in principle.
4. In qm, because there is no description of the physical evolution of the individual particles, but because the average joint results can be quantitatively reproduced, the state of the system is described holistically. That is, as the nonseparable, AB, state.
5. The entangled, qm nonseparable, state is locally produced at emission.
6. The correlations are produced via common settfings of spatially separated analyzers.
7. The observed predictable variations in the correlations occur because, i) the paired -- entangled -- particles have a common source, and ii) the paired particles are being analyzed by a common instrumental variable.
8. To date, the qm description of nonseparable states is quantitatively accurate, and the qm canon that a description of the physical evolution of individual particles is limited by HUP, and therefore that a complete description of such evolution is in principle impossible, is confirmed.
9. Since local realist theories are, to date, not quantitatively accurate, and since in order to match the quantitative accuracy of the qm formulation they would seem to require a more complete specification of the physical evolution of entangled particles than is physically possible according to qm, then *assuming* that the fundamental quantum of action is a universal limiting factor, then it can be concluded that the local realist form is, in principle, disallowed.

And, this tells us nothing about whether or not there are superluminally propagating disturbances in nature, or whether or not there is a reality independent of our observations.
 

vanesch

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Sherlock said:
Please critique these statements and line of reasoning:
1. Bell tests are testing the viability of a certain general formulation wrt correlations of spatially separated quantum measurements involving entangled particles.
Bell tests verify whether the correlations found in spatially separated systems respect of violate Bell locality conditions, where Bell locality conditions are the conditions that come from NON-QUANTUM local realist theories. Local realist theories are a class of theories that assign a reality (a definite state) to each local system individually, and the outcome of ANY experiment on that local system is purely determined by that local state. So a Bell test has a priori NOTHING to do with quantum theory. It tests whether certain correlations could eventually be produced by a theory within the class of Local Realist theories.
We only use quantum theory (which is NOT a LR theory) to hint us where to look for VIOLATIONS of these conditions, and then do the experiments accordingly.
2. This general formulation involves the assumption (local realism) that the correlations can be adequately described in terms of the juxtaposition of the physical evolutions of the individual particles, A and B, of any and all entangled pairs.
Entangled or not. Because that's not the LR business. But the Bell tests are of course only INTERESTING with entangled quantum systems, as in this case, QM predicts violation of the Bell conditions. In non-entangled quantum systems, QM does NOT violate the Bell conditions, so we expect the Bell test to be satisfied (and hence allow for a LR theory). This is not interesting.
3. In qm there is no description of the physical evolution of the individual particles, and due to HUP such a description is impossible in principle.
I don't know if it is the HUP which does this. I would say that it is the core principle of quantum theory which does it: the superposition principle.
4. In qm, because there is no description of the physical evolution of the individual particles, but because the average joint results can be quantitatively reproduced, the state of the system is described holistically. That is, as the nonseparable, AB, state.
Yes. I could make a comment, but I'm affraid it would lead us away from the main topic.
5. The entangled, qm nonseparable, state is locally produced at emission.
Yes.
6. The correlations are produced via common settfings of spatially separated analyzers.
"produced" is maybe a bad choice of words. I'd say, that you can calculate the expected correlations of the measurements in QM, by considering a "common measurement" operator which describes the two settings of the two analysers, applied to the "holistic state" AB.
7. The observed predictable variations in the correlations occur because, i) the paired -- entangled -- particles have a common source, and ii) the paired particles are being analyzed by a common instrumental variable.
In the quantum formalism, the correlations are indeed, as I said, the result of applying a "holistic" observable (containing the two settings of the two analysers) to the "holistic state" (the entangled pair). I don't know if that is what you are saying.
8. To date, the qm description of nonseparable states is quantitatively accurate, and the qm canon that a description of the physical evolution of individual particles is limited by HUP, and therefore that a complete description of such evolution is in principle impossible, is confirmed.
Let's simply say that there are 2 results:
1) purely theoretically: Bell's theorem tells us that the ("holistically calculated") correlations we can CALCULATE in QM violate the Bell conditions for certain, entangled, states. As such, we already know that there can be no LR theory which makes, in all cases, identical predictions as QM. That's without any discussion (although some try to do so).
2) experimentally: it could of course simply be that QM is experimentally wrong in these cases. However, there are many experimental results which suggest very strongly that the quantum predictions are correct, even in those cases where QM predicts violation of the Bell locality conditions. The reason why I'm using a cautious tone is just for the sake of not being rebutted by "loophole finders", because indeed, for (to my knowledge) every experiment, there are "loopholes" in the setup, which allow LR proponents to 'save their ass'. These loopholes result from the fact that the setup is complicated and that you need to apply correction techniques (such as subtraction of background, and taking into account efficiencies - things that are usually accepted as standard experimental techniques).
So from the LR-proponent side, yes, we don't have ultimate experiments without any correction that show in the raw data clear Bell locality violations. We only obtain that with standard corrections of experimental techniques.
From the QM-proponent side, these measurements are awfully close to what standard QM predicts, including the prediction of the experimental corrections. So at least, QM is not falsified, even when it is used in the domain where its states predict ideally violations of Bell locality conditions.
9. Since local realist theories are, to date, not quantitatively accurate, and since in order to match the quantitative accuracy of the qm formulation they would seem to require a more complete specification of the physical evolution of entangled particles than is physically possible according to qm, then *assuming* that the fundamental quantum of action is a universal limiting factor, then it can be concluded that the local realist form is, in principle, disallowed.
I think this is badly formulated. Local realist theories are not "quantitatively inaccurate". We don't consider specific examples of LR theories. We know that ALL of them need to respect the Bell locality conditions, so we already know that they can never give identical results in all cases as QM.
But the reason is not that QM has a "less complete specification", or that this would "violate the HUP" or something. In fact, a priori, there would be nothing against a totally different theory, that allowed for "more complete state specifications" and would overthrow the HUP, as long as it made the same statistical predictions (statistical because of our ignorance of this "more complete part" - the hidden variables) as QM. This was in fact what Einstein was hoping for. What kills such a possibility is not QM's axioms of course (because we're talking about a DIFFERENT theory, with identical outcomes). What kills it is the fact that such a theory can never make identical predictions, as it has to respect Bell conditions, which are violated by QM.
However, if you drop the Locality condition, then you CAN construct a theory with a more complete specification of the state, which DOES assign individual reality to the systems A and B, and which makes identical predictions as QM. That theory exists, is known, and is called Bohmian mechanics.
Bohmian mechanics looks a lot like classical mechanics, except that there are forces at a distance (non-local dynamics). It makes identical predictions with QM. But it is of course NOT relativistically invariant in its mechanism.
And, this tells us nothing about whether or not there are superluminally propagating disturbances in nature, or whether or not there is a reality independent of our observations.
No, but at least it tells us that you cannot have both, in the way they were assumed in a LR theory. One thing has to give.
 
Sherlock said:
And, this tells us nothing about whether or not there are superluminally propagating disturbances in nature, or whether or not there is a reality independent of our observations.
vanesch said:
No, but at least it tells us that you cannot have both, in the way they were assumed in a LR theory. One thing has to give.
I'm just saying that the Bell issue isn't telling us anything about nature, which you seem to agree with. Is that correct?

Thanks for the extended comments. I'm redoing my list of statements, and will resubmit (until I get it right :-) ). Is this thread ok for that or should I start a new one?
 

vanesch

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Sherlock said:
I'm just saying that the Bell issue isn't telling us anything about nature, which you seem to agree with. Is that correct?
It tells us something about quantum mechanics. Whether quantum mechanics describes nature is another issue, but it tells us that, concerning a combined set of properties one would like to have, and which quantum mechanics doesn't possess, namely local reality, you will never find ANOTHER theory which does have these property (local reality) and which has identical predictions in all circumstances as quantum theory.
In as far as experiments seem to confirm the QM predictions for exactly these cases, I would say that it DOES tell us something about nature, namely that local reality, in the way it is imagined in the kind of theories considered by Bell, is not valid in nature.

However, I do not agree with the shortcut that people take, and that say that, for instance, Bell disproves *locality*. This is not correct, as there exists a version of QM that is local, namely MWI (but MWI does not assign any reality to the outcomes of remote measurements). Bell does not disprove either what one could call "reality" (or even determinism). Indeed, Bohmian mechanics is such a theory (but it contains non-local dynamics).
 

ttn

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vanesch said:
However, I do not agree with the shortcut that people take, and that say that, for instance, Bell disproves *locality*. This is not correct, as there exists a version of QM that is local, namely MWI (but MWI does not assign any reality to the outcomes of remote measurements). Bell does not disprove either what one could call "reality" (or even determinism). Indeed, Bohmian mechanics is such a theory (but it contains non-local dynamics).
Well, I'm one of the people who take the "shortcut" of saying that locality has been disproven (by some combination of EPR and Bell and experiment). So we might as well be clear on exactly what it is that is cut short -- i.e., exactly what other tacit premises are really needed to justify the claim that locality as such is refuted.
It's pretty simple and, Patrick, I'm pretty sure you and I agree about this: the other premise that you need is the idea that for each photon pair in the experiments, the measurements on both sides *have definite outcomes*. That is, for any incoming photon pair, both Alice and Bob see definitely either spin up or spin down (along whatever direction is being measured at that moment) for their photon. In particular, Alice has to say: I know Bob just made a measurement and I don't know yet what the outcome of that measurement was, but I know it had some one particular definite outcome.
If one accepts this, then there is no way around the conclusion that locality is false. Anyone disagree with that?
Now what does it mean to accept this extra premise? Is this some kind of weird thing to believe in? Is it tantamount to adding extra variables to QM or believing in epicycles or cold fusion or homeopathy? I'd be curious what others think.
 

vanesch

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ttn said:
It's pretty simple and, Patrick, I'm pretty sure you and I agree about this: the other premise that you need is the idea that for each photon pair in the experiments, the measurements on both sides *have definite outcomes*.
I do agree with this, that this is the only extra hypothesis that you need and which can be used to "save" us potentially from the blunt rejection of locality as such.
Well, except of course the other hypothesis, that QM is NOT valid and that the loopholes in all these experiments ARE conspiring to make us believe so, as says the LR crowd. But without any indication of *failure* of QM, I find this highly highly improbable and not a fruitful working hypothesis.
And I can tell you that I do not find it comfortable to reject this very reasonable hypothesis of the existence of the other measurement, but nevertheless I do ! Because I'm a d**khead :smile: and still refuse to let locality go :bugeye: as of now.
Where comes my d**kheadedness from ? (ok, my mom will say: from your dad, but that's not what I mean :rofl:). It comes from 2 points:
1) we already accepted the "not having definite values until you measure it" idea for the microworld, in a way. It is only because now we could (potentially) apply the same reasoning to the quantity "outcome seen by my remote friend" and not only to "position of the electron in the atom" that we start having problems with this ; maybe because suddenly what we were willing to accept in the microworld didn't struck us as so weird as when you apply it to your remote friend ; but that's just a matter of scale.
2) I hate to give up relativity ; the space-time concept. And you have to, when you screw up the locality condition. It simply works too well: all that requirement of the Lagrangian having to be a lorentz scalar and so on, it's hard to let this go.
However, I recognize that this is somehow a personal choice, and ttn has convinced me now that the Bohmian view is not so outlandish, after all. Nevertheless, I stick with my view, but I respect his.
 

ttn

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vanesch said:
I do agree with this, that this is the only extra hypothesis that you need and which can be used to "save" us potentially from the blunt rejection of locality as such.
Good.
Well, except of course the other hypothesis, that QM is NOT valid and that the loopholes in all these experiments ARE conspiring to make us believe so, as says the LR crowd. But without any indication of *failure* of QM, I find this highly highly improbable and not a fruitful working hypothesis.
Yes, of course. It's possible the QM predictions are just wrong and the apparent experimental support for those predictions is due to some kind of systematic error in the experiments. But I don't think this is likely or fruitful.
1) we already accepted the "not having definite values until you measure it" idea for the microworld, in a way. It is only because now we could (potentially) apply the same reasoning to the quantity "outcome seen by my remote friend" and not only to "position of the electron in the atom" that we start having problems with this ; maybe because suddenly what we were willing to accept in the microworld didn't struck us as so weird as when you apply it to your remote friend ; but that's just a matter of scale.
There's a difference between "not having definite values for spin components" at the microlevel and "not having a clear ontology at all" at the microlevel. It's of course true that in the Copenhagen approach we do give up both of these things -- following Bohr/Heisenberg we basically don't think it's possible to talk about reality at the microlevel at all, and it just follows that, in particular, we shouldn't assign particular real values to spin components.
But for someone who rejects the Copenhagen approach (in favor, say, of de Broglie's old pilot wave approach that was later rediscovered by Bohm), this first argument doesn't work. In the de Broglie - Bohm theory, we always had a clear micro-ontology, but recognized that spin is a contextual property so that it doesn't make sense to assign definite pre-measurement values to spin components. But then there is no valid extrapolation to the macro-level. To reject the idea of experiments having definite outcomes is to reject that (for example) Bob either ran home to tell his mom that he got "spin up" as opposed to staying in the lab and crying that he got "spin down" -- that is, it is to reject statements about the positions of (huge collections of) particles. And that is the very kind of thing we Bohmians never rejected even at the micro-level.
But this is just a point about the ease of swallowing your point 1. I grant of course that no matter how easy or hard it is for someone to swallow, it is possible to avoid the conclusion of non-locality if you do swallow the idea that Bob's experiment didn't have a definite outcome (so he now doesn't have a definite position, etc.....).
2) I hate to give up relativity ; the space-time concept. And you have to, when you screw up the locality condition. It simply works too well: all that requirement of the Lagrangian having to be a lorentz scalar and so on, it's hard to let this go.
The interesting question to me is whether or not you've really saved locality this way. You may still retain some kind of formal Lorentz invariance, yes. But is the resulting theory really local in the sense of respecting the principle of relativity? I'm inclined to think that it isn't. Part of relativity is the idea that physics looks the same for all observers. But in this version of MWI, physics isn't the same for all observers. There's one special observer who is dynamically special -- this is Alice in the standard example, since her experiment really *does* have a definite outcome (it having happened right where she is), while Bob's really doesn't have a definite outcome. The whole thing turns into a kind of solipsism for Alice, in which really all that exists is "information" in Alice's head. And, yes, the mathematical laws governing the influx of information are lorentz invariant... but have we really preserved the spirit of relativity here? Not only are Alice and Bob non-equivalent observers, but one of them doesn't even really *exist* as a conscious scientist. (That's why I say this turns into solipsism.)
Of course, at this point I stop caring whether or not there's some way of claiming to have respected relativity. It's just too crazy to even take that question seriously anymore.
However, I recognize that this is somehow a personal choice, and ttn has convinced me now that the Bohmian view is not so outlandish, after all. Nevertheless, I stick with my view, but I respect his.
Fair enough. But just to be clear, it's not like what I'm saying about locality is some part of "the Bohmian view." What I'm saying about locality is, I think, just plain true. The connection to Bohm's theory is that if you accept the truth of what I'm saying about locality, you have a hard time not becoming a Bohmian! If the only way to avoid rejecting locality is to accept something like solipsism (and if one is unwilling to go there), then you might as well opt for the non-local theory which makes the most intuitive sense, which helps you understand QM as much as possible, which doesn't suffer from any unprofessional vagueness and ambiguity like Copenhagen, which is known to be consistent with experiment, etc., etc. In short, as soon as you accept that non-locality is a fact which has to be incorporated into one's theory, it is practically impossible *not* to recognize that Bohm's theory is far and away the best option.
 

vanesch

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ttn said:
The interesting question to me is whether or not you've really saved locality this way. You may still retain some kind of formal Lorentz invariance, yes. But is the resulting theory really local in the sense of respecting the principle of relativity? I'm inclined to think that it isn't. Part of relativity is the idea that physics looks the same for all observers. But in this version of MWI, physics isn't the same for all observers. There's one special observer who is dynamically special -- this is Alice in the standard example, since her experiment really *does* have a definite outcome (it having happened right where she is), while Bob's really doesn't have a definite outcome.
This, on the other hand, is not correct. There is still a symmetry between the observers, and what you just described is because we described everything from Alice's point of view. But you can repeat the story from Bob's point of view, and now, TO HIM AS AN OBSERVER, it is Alice who didn't have definite outcomes. The only difference is that the "Bob-observer" might have seen different results than the Bob-who-was-seen-by-Alice-observer, and this is the point where things get mind-boggling :smile:
The price to pay for that is that each of us lives then in his own little world with different outcomes, but also with copies of all the others which DID have outcomes which are consistent with ours.
The whole thing turns into a kind of solipsism for Alice, in which really all that exists is "information" in Alice's head. And, yes, the mathematical laws governing the influx of information are lorentz invariant... but have we really preserved the spirit of relativity here? Not only are Alice and Bob non-equivalent observers, but one of them doesn't even really *exist* as a conscious scientist. (That's why I say this turns into solipsism.)
Of course they exist BOTH as conscious scientists, but not necessarily in the same branch, in which case each of them is in contact with a "clone" of the conscious version of the other one - and I leave it up to your taste to declare that clone also a conscious one or not.
Of course, at this point I stop caring whether or not there's some way of claiming to have respected relativity. It's just too crazy to even take that question seriously anymore.
I will not disagree with you that it sounds crazy. The question is if it is crazy enough :smile:.
My point of view is that we shouldn't care about the "crazyness" of an explanation if it fits the formalism it is supposed to explain. Because one day, that formalism is going to change, and then the crazy explanation will go in the dustbin. And this is my main reason to prefer "crazy" MWI over "intuitive" Bohmian mechanics: "crazy" MWI is closer to the formalism of current QM and relativity than Bohmian mechanics (in which the spacetime manifold as a geometrical object doesn't make sense).
In short, as soon as you accept that non-locality is a fact which has to be incorporated into one's theory, it is practically impossible *not* to recognize that Bohm's theory is far and away the best option.
I fully agree with that. I'd say that if all we had was non-relativistic QM, then it would almost be obvious that Bohmian mechanics is a superior explanation. But as of today, I don't want to toss out GR. And that is what you do when you accept non-locality. So if I want to save GR, *I have no other option* as to consider that Bob, according to Alice, didn't have a definite outcome - and that Alice, according to Bob, didn't have one either, and that when they meet, each of them meets with ONE VERSION of the other, and as such, each of them is happy that way, each one in his/her own branch.
I think that the issue can only be settled if we have a full integration of GR and QM.
 

ttn

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vanesch said:
This, on the other hand, is not correct. There is still a symmetry between the observers, and what you just described is because we described everything from Alice's point of view. But you can repeat the story from Bob's point of view, and now, TO HIM AS AN OBSERVER, it is Alice who didn't have definite outcomes. The only difference is that the "Bob-observer" might have seen different results than the Bob-who-was-seen-by-Alice-observer, and this is the point where things get mind-boggling :smile:
The price to pay for that is that each of us lives then in his own little world with different outcomes, but also with copies of all the others which DID have outcomes which are consistent with ours.
I was under the assumption that there was only one world. I mean, for the sake of discussion, I am happy to allow that this world look quite crazy, that big macroscopic things are in crazy superpositions and entangled states, etc. But I don't know what you're talking about if you are literally saying that there is now a world associated with each person. "Real" ceases to have a meaning, and there is now only "real for me" and "real for you" and "real for Alice", etc.
Here's why this bothers me. You said in a previous post that you thought it was extremely unlikely that the apparent experimental confirmation of the QM predictions (that is, the experimental evidence that Bell's inequalities are violated) is due to some kind of systematic error, as the local realists say/want. I entirely agree with you. After all, a number of different experiments have been done at a number of locations around the world by independent people, etc.... Well now you're saying that really there's no such thing as "the world" -- just personal fantasies that each of us create for ourselves that are radically inconsistent with each other's. So did those Bell test experiments even *happen*? That isn't even a meaningful question anymore, under this version of MWI that you're advocating.
My point is really that there's a kind of hierarchy to knowledge. Certain statements/conclusions rest on others such that if you give up one thing, you must also give up (as now meaningless) the other things that depend on it. So how can you claim that the experimental evidence supporting the QM predictions is strong, when in the next breath you say something that renders that statement totally meaningless? That's my fundamental problem with this approach. You talk as if you're making a choice from among several things to give up, but the fact is those several things are not all at the same level hierarchically. And you end up "opting" to give up one that means, really, you've given up all the others as well. As soon as you deny that there's one world, out there, independent of us, and it's science's job to figure out what that world is like, you render meaningless any debate about whether that world is as described by relativity, whether a certain theory's experimental predictions are correct, whether a given experiment even happened, etc., etc. So I just don't see the rationality of the option you're making here.
I fully agree with that. I'd say that if all we had was non-relativistic QM, then it would almost be obvious that Bohmian mechanics is a superior explanation. But as of today, I don't want to toss out GR. And that is what you do when you accept non-locality. So if I want to save GR, *I have no other option* as to consider that Bob, according to Alice, didn't have a definite outcome - and that Alice, according to Bob, didn't have one either, and that when they meet, each of them meets with ONE VERSION of the other, and as such, each of them is happy that way, each one in his/her own branch.
I think that the issue can only be settled if we have a full integration of GR and QM.
I disagree with this. I think it's you who's got a serious problem with relativity, not me. It's easy enough to keep the whole formalism of relativity (both S and G) but add some kind of preferred foliation to spacetime so that one can give meaning to the non-local interactions in Bohm's theory. There's a whole textbook that shows how to do this for GR. The book is by Janossy, and it's cited by Bell in, I think, "How to Teach SR". Basically what I'm talking about here is a kind of Lorentz Ether Theory -- something with a preferred rest frame, i.e., a notion of absolute simultaneity, but which otherwise shares the same formalism and empirical predictions as relativity. Such theories *exist* and they *work*. And one can easily embed a Bohmian theory on this kind of space-time background, and everything works fine. There are instantaneous action at a distance type interactions going on among all the particles, but this turns out to be masked by uncertainty about the particles' initial conditions -- in (rather amazingly, but it works out) just such a way so that all the empirical predictions come out to be Lorentz invariant, and you can never detect the ether.
Now, is this kind of theory consistent with relativity? Yes and no. It makes all the same predictions, and everything at the level of observations comes out Lorentz invariant. So far so good. But behind the scenes, the fundamental laws are not Lorentz invariant. (There's a preferred frame, or in GR a preferred foliation into spacelike hypersurfaces.) So that does conflict with the principle of relativity (which just basically asserts that there is no such preferred frame). But who cares? There's no empirical evidence for this principle anyway and, I say, some evidence against it (namely the empirical violations of Bell's inequalities).
So it's at least clear how to integrate Bohm's theory with relativity. What about MWI? Well, take GR. Energy density is related to spacetime curvature. Well what happens to Einstein's field equations in the situation I outlined a while ago -- Bob runs home to momma if he gets "spin up" but stays in the lab for a nap if he gets "spin down". What does the spacetime curvature look like? Well, if we go with the "one world" version of MWI (with everything "as seen by Alice") then Bob is just in a superposition of being in two different places. So what is the energy density associated with that? Not clear. So the whole thing is rather ambiguous. And it only gets worse if you go with the truly many worlds version of MWI. Then in Alice's fantasy world, there's this crazy ambiguity about gravitational fields over near Bob, while in Bob's fantasy world the gravitational field is perfectly sensible near him but has this crazy ambiguity over near Alice.
How do you resolve any of this?
 
ttn said:
... the other premise that you need is the idea that for each photon pair in the experiments, the measurements on both sides *have definite outcomes*. That is, for any incoming photon pair, both Alice and Bob see definitely either spin up or spin down (along whatever direction is being measured at that moment) for their photon. In particular, Alice has to say: I know Bob just made a measurement and I don't know yet what the outcome of that measurement was, but I know it had some one particular definite outcome.
If one accepts this, then there is no way around the conclusion that locality is false. Anyone disagree with that?
The "definite outcomes" of individual measurements are detection or nondetection. I don't see how one could conclude anything about the locality or nonlocality of nature from this.
 

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