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

[DrChinese]

I do not think I have to inform you why these *diehards* have a point. Indeed, this thread is *is QM inherently non-local?*; however most people, including yourself, have turned this question into *Is nature inherently non local* with of course an affirmative answer attached. ... You did not say anthing my comments concerning the violation of causality in the operational sense at all, and this is clearly relevant for the discussion.

My views are fairly common, I suspect: I don't think nature is "inherently" non-local (as ttn does). I don't believe there is any violation of signal locality within oQM. I don't have an explanation for the mechanism for the collapse of the wave function, and I don't know if it should be considered to be a physically non-local. But I am open to new information on the matter.

And I accept that nature is not local realistic, which is also assumed to be true for purposes of this thread. If you want to discuss that particular point, I would request you take it to another thread as it is off-topic here.

 

[DrChinese]

So you have to be very very careful believing what people -- even "experts" -- say on these topics. Except Maudlin. He's right about everything.

That's pretty funny...

 

[NateTG]

DrChinese, would you consider the notion that a physically unmeasurable quantity may be undefined (in the $\frac{0}{0}$ sense) as realistic?

 

[DrChinese]

DrChinese, would you consider the notion that a physically unmeasurable quantity may be undefined (in the $\frac{0}{0}$ sense) as realistic?

Do you mean "realistic" per Bell?

I usually think in these terms: oQM sees the physically unmeasurable characteristics as non-existent or undefined, exactly as you describe them; and that implies "non-realism" to me. There is non-realism to those observables which can be measured individually but not simultaneously. That maps to Bell's definition exactly (what might be called "Bell Reality") because oQM does not meet that definition, and indeed was not supposed to. And Bell Reality also was supposed to map in some way to EPR's "elements of reality"; and indeed they are very close, even if not identical.

Bell wanted oQM on one side of the fence, and EPR's vision of a local reality on the other. He succeeded grandly.

 

[Careful]

My post was in response to the assertion that Bell's theorem cannot be circumvented by local non-realistic theories.
Let's say that I have a neutral electron source and I have a single detector that will randomly measure the electron's spin orientation along one of three axes $a,b$ or $c$ at $0,120$ and $240$ degrees off of the vertical respectively. Then there are six possible results this device can produce: $a^+,a^-,b^+,b^-,c^+,$ and $c^-$.
Now, if I take all of the electrons that gave a result of, say $a^+$ what is the probability that those electrons would have given $b^-$ if they had been measured along the $b$ axis instead, and, more importantly is it possible devise an experiment that will test the prediction?

Let me try to make a suggestion. In quantum mechanics the wave function serves to make statistical predictions about the outcomes of a series of identical experiments. If I shoot the silver atoms (with electrons, you would get a null result, Stern Gerlach experiments with electrons do not provide any direct clue for spin) one by one out of the source and *assume* that they are measured each time when the detector is set on the a direction, I could after sufficiently many times determine the pure state of the electron up to a phase with sufficient accuracy. Now, I can fix the phase by doing a similar series of experiments in the b direction. From that momen on, I can predict whatever I want with fair accuracy.

 

[Careful]

Doctor chinese, you are avoiding again the reference of Sorkin concerning *impossible measurements in QFT* I provided you with (you simply comment that you do not believe it); also you ignore my comments concerning some mismatches between experiment and standard QM predictions. It seems therefore a bit contradictory to you say that you are open to info on the matter while you keep on avoiding the issues I raise. Science is no religion ...

 

[DrChinese]

Doctor chinese, you are avoiding again the reference of Sorkin concerning *impossible measurements in QFT* I provided you with (you simply comment that you do not believe it); also you ignore my comments concerning some mismatches between experiment and standard QM predictions. It seems therefore a bit contradictory to you say that you are open to info on the matter while you keep on avoiding the issues I raise. Science is no religion ...

Huh? I saw a mention of Sorkin and have no idea what it is about or in reference to. I certainly made no comment about it since there is nothing to comment on. Hopefully it is somehow relevant to this thread, perhaps if that is so you will explain it to us (so we will understand how it relates). Disagreement between QM and experiment does NOT belong in this thread, please start a new one if you want to discuss that. This thread is about the non-local nature of QM, and it is poor etiquette to hijack threads for your own purposes. Your participation is very welcome and encouraged at PhysicsForums, but you will not find poor manners tolerated here for long.

As to science being a religion, again I have no idea what you are talking about. And I am pretty sure there are plenty who would get a kick out of the idea that I avoid issues like this.

 

[DrChinese]

Let me try to make a suggestion. In quantum mechanics the wave function serves to make statistical predictions about the outcomes of a series of identical experiments. If I shoot the silver atoms (with electrons, you would get a null result, Stern Gerlach experiments with electrons do not provide any direct clue for spin) one by one out of the source and *assume* that they are measured each time when the detector is set on the a direction, I could after sufficiently many times determine the pure state of the electron up to a phase with sufficient accuracy. Now, I can fix the phase by doing a similar series of experiments in the b direction. From that momen on, I can predict whatever I want with fair accuracy.

Careful:

This post does not relate to this thread. This thread is about non-locality. Please start a new thread to discuss your perception of quantum mechanical statistics. This would be welcome in the proper thread, and is unwelcome in the wrong thread.

I will not participate in a discussion deliberately going out of topic.

-DrC

 

[CJames]

I am joining this discussion as an undergrad with no mathematical knowledge of QM thus far, so keep that in mind. However, I am having trouble seeing why this issue is so difficult to resolve. First off, it seems readily apparent to me that signal-locality is true of QM. I don't think anybody except Careful has said that QM violates special relativity. I can concieve of no method by which a signal can be sent via quantum entanglement.

It also seems readily apparent to me that Bell locality is falsified by experiment. I did not think there was any controversy as to what happens when one member of an entangled pair is observed. You instantaneously know information about the other particle. This does not mean information has traveled outside the light cone, but it does mean you know information about an object outside of the lightcone. In this way the universe is non-local in the sense that a wavefunction can say something about more than one object, or to say it in a more shocking way, the two particles are actually the same "object", despite their physical separation. This is how I understand quantum entanglement. If I am misinformed please let me know.

I was under the impression that both these concepts were well established by experiment. Am I wrong about that?

 

[DrChinese]

I am joining this discussion as an undergrad with no mathematical knowledge of QM thus far, so keep that in mind. However, I am having trouble seeing why this issue is so difficult to resolve. First off, it seems readily apparent to me that signal-locality is true of QM. I don't think anybody except Careful has said that QM violates special relativity. I can concieve of no method by which a signal can be sent via quantum entanglement.

It also seems readily apparent to me that Bell locality is falsified by experiment. I did not think there was any controversy as to what happens when one member of an entangled pair is observed. You instantaneously know information about the other particle. This does not mean information has traveled outside the light cone, but it does mean you know information about an object outside of the lightcone. In this way the universe is non-local in the sense that a wavefunction can say something about more than one object, or to say it in a more shocking way, the two particles are actually the same "object", despite their physical separation. This is how I understand quantum entanglement. If I am misinformed please let me know.

I was under the impression that both these concepts were well established by experiment. Am I wrong about that?

No, I think you have it right.

For me personally, the confusion begins when you talk about an object outside the lightcone. Alice makes a measurement, which causes collapse of the shared wave function. Now you know something about something somewhere else, true, and that is outside the lightcone.

But what has happened that is really so weird? We project the knowledge we have back to the point at which the entangled particle pair was created. This is the same thing that happens when only one particle is involved, nothing strange about that. The particle acts as if it had that orientation from the last point something happened.

Say Alice sees a V orientation with a polarizer at 0 degrees. Naturally, all subsequent measurements will be consistent in EVERY WAY with this knowledge AS IF it was always that way from the creation of the particle. So in that sense there is absolutely nothing happening outside any light cone.

In other words, all quantum measurements find a particle in an eigenstate and its eigenvalue is consistent with the quantum measurement rules. Entangled particles are no different in this respect. So the real question to me is: why does a measurement at time T2 cause the particle to assume a specific value as if it had that value at time T1 (where T1 is before T2) ? Does that make oQM non-local? Or is that a case of backwards causality? I am not sure that anything physical occurs along with the collapse, and I think that is a relevant question too.

Naturally, some of these issues show up in our definition of locality. You can see that there is no information transfer which is FTL, and there is no clear causal effect which is FTL. Yet the Bell Locality condition is violated with a strict application of its definition. So what does that condition actually tell us? Of course, it fits with the Bell Inequality too so that is very important.

Inquiring minds want to know...

 

[Careful]

Huh? I saw a mention of Sorkin and have no idea what it is about or in reference to. I certainly made no comment about it since there is nothing to comment on. Hopefully it is somehow relevant to this thread, perhaps if that is so you will explain it to us (so we will understand how it relates). Disagreement between QM and experiment does NOT belong in this thread, please start a new one if you want to discuss that. This thread is about the non-local nature of QM, and it is poor etiquette to hijack threads for your own purposes. Your participation is very welcome and encouraged at PhysicsForums, but you will not find poor manners tolerated here for long.
As to science being a religion, again I have no idea what you are talking about. And I am pretty sure there are plenty who would get a kick out of the idea that I avoid issues like this.

Sorkin' paper treats the following issue: if you accept QFT and accept that measurment of all gauge invariant observables can be made (local or non local - examples of non local observables are so called Wilson Loops) then the Wightman axiom that two spacelike separated field operators do either perfectly commute or anticommute does indeed imply that two spacelike separated observers cannot signal faster than with the speed of light, but you can carefully select a situation with three obervers A in the past of B , C in the future of B but A not in the past of C in which a measurement at A is going to influence the signalling from B to C. Since this is unacceptable, you need to exclude by hand this kind of situations (note that non local observables are physical and belong to this world) which is the most uninsightful thing you could ever do. To someone with common sense, it seems almost impossible and extremely contrived that nature provides us with non local correlations (ie correlations beyond the lightcone) but forbids us to use them actively. There is much more to say still about the wightman AXIOMS but I leave it for now here. The paper is : Sorkin, impossible measurements on quantum fields, and you can find it on gr-qc (written in 1984).

 

[Sherlock]

Sorkin' paper treats the following issue: if you accept QFT and accept that measurment of all gauge invariant observables can be made (local or non local - examples of non local observables are so called Wilson Loops) then the Wightman axiom that two spacelike separated field operators do either perfectly commute or anticommute does indeed imply that two spacelike separated observers cannot signal faster than with the speed of light, but you can carefully select a situation with three obervers A in the past of B , C in the future of B but A not in the past of C in which a measurement at A is going to influence the signalling from B to C.

A and B are particles which have interacted. We detect A. Since the motions of A and B are somewhat related subsequent to their interaction, then it follows that the detection of A can tell us something about how B might interact with C.

To someone with common sense, it seems almost impossible and extremely contrived that nature provides us with non local correlations (ie correlations beyond the lightcone) but forbids us to use them actively.

But they are used actively, aren't they? Quantum computing? :-)

The view that Nature is local fits the data. It's the simplest, reasonable explanation for why we can't communicate (ie., use the correlations actively in the sense that I take you meant this) superluminally.

Correlations beyond the lightcone are conceptually understood in terms of the conservation laws.

The paper you reference sounds interesting ... I must check it out.

 

[CJames]

No, I think you have it right.
For me personally, the confusion begins when you talk about an object outside the lightcone. Alice makes a measurement, which causes collapse of the shared wave function. Now you know something about something somewhere else, true, and that is outside the lightcone.
But what has happened that is really so weird? We project the knowledge we have back to the point at which the entangled particle pair was created. This is the same thing that happens when only one particle is involved, nothing strange about that. The particle acts as if it had that orientation from the last point something happened.

To me that is the weird part, that the same thing that happens when one particle is involved is what happens when two particles are involved. This is an actual physical example of one thing being in two places at once. That's what I think of when I hear the words non-local. I don't think of superluminal travel or violating special relativity, I think of the total breakdown of the classical concept that an object cannot be in two places at once.

Say Alice sees a V orientation with a polarizer at 0 degrees. Naturally, all subsequent measurements will be consistent in EVERY WAY with this knowledge AS IF it was always that way from the creation of the particle. So in that sense there is absolutely nothing happening outside any light cone.
In other words, all quantum measurements find a particle in an eigenstate and its eigenvalue is consistent with the quantum measurement rules. Entangled particles are no different in this respect. So the real question to me is: why does a measurement at time T2 cause the particle to assume a specific value as if it had that value at time T1 (where T1 is before T2) ? Does that make oQM non-local? Or is that a case of backwards causality? I am not sure that anything physical occurs along with the collapse, and I think that is a relevant question too.

But it shouldn't be possible even in principle to demonstrate whether the collapse is physical, since once you observe it the whole thing collapses.

Naturally, some of these issues show up in our definition of locality. You can see that there is no information transfer which is FTL, and there is no clear causal effect which is FTL. Yet the Bell Locality condition is violated with a strict application of its definition. So what does that condition actually tell us? Of course, it fits with the Bell Inequality too so that is very important.
Inquiring minds want to know...

To me what it actually tells us is that, like I said earlier, something can be in two places at once. But perhaps this is wrong and somehow it is all predetermined at T1. But then wouldn't that violate QM to begin with?

 

[CJames]

carefully select a situation with three obervers A in the past of B , C in the future of B but A not in the past of C in which a measurement at A is going to influence the signalling from B to C.

I'm not going to immediately say you are wrong because I don't know the math well enough. However, if A is in the past of B but the future of C, wouldn't ANY form of communication following the path B->A->C be potentially superluminal? I think that it would and so I don't think that such a situation is allowed by special relativity in the first place.

 

[Careful]

Hi CJ, you must realize that there is usually much more to the things you learn than you teachers tell you. Let me first make some comments and then argue why the predictions of QM are very strange indeed.
(a) It is not SCIENTIFICALLY correct to state that Bell experiments refute local hidden variables/objective local theories. About this issue there is written an impressive book by Franco Selleri, and many papers about how Bell type experiments can be made to violate the orginal Bell and the CHSH inequality have appeared since 1964. Amongst these is the paper of Pearle (1970) in which the key result is that if your detector efficiency does not exceed 70% at relative angles of 90 degrees and 87% at relative angles of zero or 180 degrees, you can reproduce exactly the QM correlations for the pairs which are observed (these efficiencies are extremely high, experiments in which the efficiency is higher such as Rowe 2002 cannot maintain the locality assumption). Many subsequent papers have been written by Caeser (1984-1987), the eminent and late A.O. Barut, Santos, Marshall, Vaidman (and even Wigner before 1970) and many others. Up to date, there exists no experiment which excludes LHVT/OLT theories, you should scan the web upon the number of good papers written in 2005 which propose the next generation of loophole free tests (realize that experimentators are putting their best efforts in this for 35 years now!)
(b) the issue of the thread should be specified to : * is STANDARD QM inherently non local?*. It is actually very easy and much more natural to construct a Wave mechanics in which there is no entanglement at all: these are the Hartree equations. Actually there are at least four types of quantum theories present up to date : Standard, Hartree, GRW spontaneous reduction models, Consciousness nonsense... I do not mention different interpretations (although Bohm differs slightly from standard but makes the same predictions where standard can make them).
(c) I have mentioned before that you can construct causal, but not Bell local, hidden variable models where the predictions match exactly those of QM (so, no dector inefficiencies here), such a theory is backwards causation.

Now, let me argue why the predictions of Standard QM are very weird indeed:
the entangled state remains rotation invariant : what does this mean when both particles are clearly separated (you need a parallelism here)?? That is, the rotional invariance of the source is ``remembered´´ by the *individual* entangled pairs. Make the exercise by assuming that every pair has a definite allignment (ie. spin vector s and - s with the length of s equal to hbar/2), and suppose the v's are uniformly distributed over the sphere (this is what I would mean by rotation invariance). Now figure out a deterministic detection rule and suppose symmetry between both detectors and you will see that your correlations are a straight line if you assume separability of probabilities. So either the particles know (before they leave the cavity) what the dector settings will be in the future, meaning that you assume the world to be fully deteministic and that the particles have the ability to figure out all relevant parameters in this game and compute it, which is of course CRAZY. Or, the particles signal faster than with the speed of light, but we can only see the effect of those signals and not use them (something which I doubt very much - see my previous message). So, (a) a physical process which occurs faster than with the speed of light or (b) which travels backwards in time HAS to occur for the correlations of QM to come out right. This is very strange indeed... (note that standard QM says nothing intelligent about the measurement process which is supposed to provide us with these correlations)

 

[Careful]

A and B are particles which have interacted. We detect A. Since the motions of A and B are somewhat related subsequent to their interaction, then it follows that the detection of A can tell us something about how B might interact with C.
But they are used actively, aren't they? Quantum computing? :-)
The view that Nature is local fits the data. It's the simplest, reasonable explanation for why we can't communicate (ie., use the correlations actively in the sense that I take you meant this) superluminally.
Correlations beyond the lightcone are conceptually understood in terms of the conservation laws.
The paper you reference sounds interesting ... I must check it out.

Hi sherlock, the point is that the interaction between A and B has measurable consequences out of the lightcone of A.
Concerning quantum computing: it is impossibe to tell wether it are CLASSICAL correlations one is using or not (see my latest post).

Cheers,

Careful

 

[ttn]

No, I think you have it right.
For me personally, the confusion begins when you talk about an object outside the lightcone. Alice makes a measurement, which causes collapse of the shared wave function. Now you know something about something somewhere else, true, and that is outside the lightcone.

Wait, that isn't what Bell Locality says. You're forgetting that Bell Locality is defined in terms of conditionalizing all the probabilities on a *complete* description of the state. So it isn't just that the conditional probability changes when you learn something. That's trivial. Consider a simple example. Put a marble into a shoebox, then split the shoebox in half and carry the two halves (one of which contains the marble, but you don't know which one) to distant locations. What's the probability for alice to open her box and find the marble? 50% But suppose we specify that Bob has already looked in his box and has not found the marble. Suddenly the conditional probability for Alice's result (viz, conditional on Bob's outcome) jumps to 100% Is this a violation of Bell Locality? NO! Because the original probabilities (like the 50%) weren't of the right sort. They weren't conditional on the exact complete state of the system before the measurements -- which, for a simple classical example like this, obviously would contain the actual location of the marble. And if you include that, then the probabilities *don't* change when you add this information about Bob's result. If the marble was in Alice's box all along, the probability that she'll find it is 100% whether we specify Bob's outcome or not. So Bell Locality is respected here.

But what has happened that is really so weird? We project the knowledge we have back to the point at which the entangled particle pair was created. This is the same thing that happens when only one particle is involved, nothing strange about that. The particle acts as if it had that orientation from the last point something happened.

You might do this, but then this contradicts OQM, specifically the compelteness doctrine. Say a spin 1/2 particle is in the state |+z>. Then, later, you measure its x-spin-component and get +. Does that mean, "really", the particle had a positive x-spin-component all along, even during that time when the quantum state was |+z>? Not according to OQM! (But maybe according to some kind of hidden variable theory.)

Say Alice sees a V orientation with a polarizer at 0 degrees. Naturally, all subsequent measurements will be consistent in EVERY WAY with this knowledge AS IF it was always that way from the creation of the particle. So in that sense there is absolutely nothing happening outside any light cone.

In other words, all subsequent measurements will be consistent in every way with the assumption that the particle possesses definite spin hidden variables even when not being measured.

No wonder you're having trouble believing that OQM is non-local. You aren't willing to actually *accept* the completeness doctrine! You pay it lip service but then think as if the completeness doctrine were *false*!

In other words, all quantum measurements find a particle in an eigenstate and its eigenvalue is consistent with the quantum measurement rules. Entangled particles are no different in this respect. So the real question to me is: why does a measurement at time T2 cause the particle to assume a specific value as if it had that value at time T1 (where T1 is before T2) ?

Who says it does this? Certainly not OQM! In OQM the state just *is* defined by the quantum state, the wf. That's what the completeness doctrine *means*. And so the state changes -- the particle acquires some definite value for the property measured -- just when the wf collapses, i.e., just when the measurement is made. Not before. To say that this happens before is to say that it had a certain property *before* the measurement was made, i.e., before it was in an *eigenstate* of the operator corresponding to the property in question. And that is to posit hidden variables.

Does that make oQM non-local?

It doesn't make OQM anything, because you're not *talking* about OQM anymore!

Naturally, some of these issues show up in our definition of locality. You can see that there is no information transfer which is FTL, and there is no clear causal effect which is FTL. Yet the Bell Locality condition is violated with a strict application of its definition. So what does that condition actually tell us?

I think you need to get clearer on the definition of Bell Locality. Its violation *does* (at least according to Bell and many others) signal a "clear causal effect which is FTL". That's the whole point of that locality condition -- to test whether a given theory is "locally causal". You *really* need to read Bell's article "La Nouvelle Cuisine" if you want to understand this stuff.

 

[Careful]

A and B are particles which have interacted. We detect A. Since the motions of A and B are somewhat related subsequent to their interaction, then it follows that the detection of A can tell us something about how B might interact with C.
But they are used actively, aren't they? Quantum computing? :-)
The view that Nature is local fits the data. It's the simplest, reasonable explanation for why we can't communicate (ie., use the correlations actively in the sense that I take you meant this) superluminally.
Correlations beyond the lightcone are conceptually understood in terms of the conservation laws.
The paper you reference sounds interesting ... I must check it out.

Moreover, I am glad you state explicitely that you can use faster than light signalling in an operational sense. So standard QM is not local neither in the Bell nor operational sense. Moreover, I do not know precisely which conservation laws you are referring to. If you mean conservation of probability, then this has nothing to do with correlations over the lightcone. You can easily cook up non linear wave theories which still have a probablility current conservation law but give rise only to wave functions with support in the lightcone.

Cheers,

careful

 

[vanesch]

(a) It is not SCIENTIFICALLY correct to state that Bell experiments refute local hidden variables/objective local theories.

This is a correct statement. However, what is usually meant with the statement is that in those situations where quantum mechanics makes idealised predictions that DO violate the Bell inequalities, if we add to that the (quantum-mechanically sound, even though often not derived from first principles) *usual* experimental corrections of apparatus and detectors, then it would be highly surprising that quantum mechanics being wrong, it would have in it a kind of self-correcting mechanism where its ideal predictions are wrong, but its experimental corrections are just as wrong in the opposite sense such as to result in agreement between realistic QM predictions of the experiment (including corrections) and actual experimental results, and that is what is observed: agreement between realistic QM predictions and experimental results. So, barring a conspiracy, it is - to all experimental and scientific standards - a very reasonable working hypothesis that the QM predictions in this area are experimentally verified.

You are however, right, that many of these experiments do not have RAW DATA violating some Bell inequality. The assumption is that when the theory (QM) makes correct predictions concerning its realistic experimental predictions, combined with the overall success of QM in several other domains, that it is very plausible to take as established that QM is also correct in the ideal predictions (which do not correspond to actual experimental setups because of the non-idealities in the apparatus). This is the working hypothesis that is taken in this thread.

Amongst these is the paper of Pearle (1970) in which the key result is that if your detector efficiency does not exceed 70% at relative angles of 90 degrees and 87% at relative angles of zero or 180 degrees, you can reproduce exactly the QM correlations for the pairs which are observed (these efficiencies are extremely high, experiments in which the efficiency is higher such as Rowe 2002 cannot maintain the locality assumption).

This is correct. The experimental data of many experiments (I am not really following up all the latest developments), as such, as raw data, do not violate Bell's inequalities, and as such leave the door open to LR theories - often made up for the purpose.

Many subsequent papers have been written by Caeser (1984-1987), the eminent and late A.O. Barut, Santos, Marshall, Vaidman (and even Wigner before 1970) and many others. Up to date, there exists no experiment which excludes LHVT/OLT theories, you should scan the web upon the number of good papers written in 2005 which propose the next generation of loophole free tests (realize that experimentators are putting their best efforts in this for 35 years now!)

Although I have often seen the argument of LR proponents this way (that experimenters have tried a long time and STILL have no data violating Bell's inequalities), I think they miss the point - understandably, because the "publicity" of these experiments ALSO misleads. True, no RAW DATA excludes the possibility of a future LR theory.
However, ALL these raw data are IN AGREEMENT with the experimental predictions of an overall VERY SUCCESSFUL theory, quantum mechanics, and these experiments are challenging the QM predictions each time, in different situations. Each time, combining STANDARD experimental corrections (also rooted in QM) and predictions of QM, one arrives at agreement. So isn't it very reasonable to presume that after these gazillions of agreements between QM and experiment, the ideal predictions of QM ALSO are correct ?

This is the working hypothesis taken in this thread (and in fact in most of the QM threads here): QM makes experimentally correct predictions, also in those cases where the experiment has not been carried out.

It is actually very easy and much more natural to construct a Wave mechanics in which there is no entanglement at all: these are the Hartree equations.

I don't know exactly what you mean ? Do you mean, the effective potential models ?

 

[DrChinese]

No wonder you're having trouble believing that OQM is non-local. You aren't willing to actually *accept* the completeness doctrine! You pay it lip service but then think as if the completeness doctrine were *false*!
...

And so the state changes -- the particle acquires some definite value for the property measured -- just when the wf collapses, i.e., just when the measurement is made. Not before. To say that this happens before is to say that it had a certain property *before* the measurement was made, i.e., before it was in an *eigenstate* of the operator corresponding to the property in question. And that is to posit hidden variables.

Of course I accept that the WF is a complete description, and of course I don't believe in HV.

(I keep saying that it is "AS IF" and I am not trying to make a literal description. There are several different ways of visualizing what is happening. These are just images, clearly in oQM it is the formalism that rules.)

But there is a mystery about collapse that it would be desirable to know more about. You touch on it above. You say that the WF collapses upon measurement, and sure, this is standard. So when there are 2 entangled particles, which one cause the collapse - measurement of Alice or of Bob? Sure, the results are apparently the same regardless of which one "causes" the collapse. But again, that's the mystery. We have no specific rule that defines this. And again, that is what I am referring to when I say "I am confused about whether WF collapse is physical" etc.

As cjames mentions, the WF is in 2 places at once. So is that a non-local phenomenon? To me, I am not sure it is "non-local" in the sense of Bell Locality. But it might be non-local in another sense.

But I am a bit confused about your marble box example. I think what you are saying is: this example does not violate Bell Locality because adding the information about Bob's outcome does not actually change the probability of the outcome at Alice. Am I close? (Or maybe the example isn't that important, not sure.)

 

[DrChinese]

Hi CJ, you must realize that there is usually much more to the things you learn than you teachers tell you. Let me first make some comments and then argue why the predictions of QM are very strange indeed.
(a) It is not SCIENTIFICALLY correct to state that Bell experiments refute local hidden variables/objective local theories.

Start your own thread if you want to push your personal agenda on Bell tests.

 

[Careful]

This is a correct statement. However, what is usually meant with the statement is that in those situations where quantum mechanics makes idealised predictions that DO violate the Bell inequalities, if we add to that the (quantum-mechanically sound, even though often not derived from first principles) *usual* experimental corrections of apparatus and detectors, then it would be highly surprising that quantum mechanics being wrong, it would have in it a kind of self-correcting mechanism where its ideal predictions are wrong, but its experimental corrections are just as wrong in the opposite sense such as to result in agreement between realistic QM predictions of the experiment (including corrections) and actual experimental results, and that is what is observed: agreement between realistic QM predictions and experimental results.

Hi Vanesh,

You are simply repeating Bell's arguments... In the same way, I could argue that the local realist theories which, when interpreted according to the experimentators data massages, violate the Bell inequalities are merely realistic adjustements to the perfect local realist setup which does not violate these inequalities as long as the setup allows for the separability assumption to be made (such as in long distance correlation experiments). So we are not missing the point as you claim. I do not dispute that QM is successful in calculating the spectrum of the H and He atom (you cannot predict above He) and explaining the Lamb shift. However, I am convinced that these successes have perfect (albeit more difficult and subtle) classical explanations. A good step in this direction is the theory of stochastic electrodynamics which reproduces a good bunch of these so called exclusive results from second quantization in a firstly quantized framework (such as the Casimir effect, and the H atom I believe). Barut and Dowling have done quite some work on this issue ...

My intention is not at all to dispose of standard QM, I am perfectly aware of the fact that it provides an effective way of calculating the statistics of experiments with microscopic objects when applied on the *correct* problems with considerable thought. However, I also gives the wrong answers in some cases and it does not provide any insight into the dynamics of a single particle. I want to obtain *insight* into the microworld which I believe obeys the same laws as the macroworld (that is GR and electromagnetism), therefore entanglement is a crucial issue and one should not take it lightly.

Concerning your remark about the extrapolation of succes, I can only say that people have been looking for over 50 years for a perpetuum mobile; I hope one is not going to look 100 years for entanglement. By the way, Newton theory was also correct for 300 years.

Concerning Hartree, you have to include a classical radiation field determined by the probability current of the particles. In that way, you obtain a QM where each particle has its own wave function and where interactions propagate via a classical maxwell field determined by the sum over all probability currents times the appropriate charges (I think Barut called this the self field approach to QED, it's non-linear of course).

But I appreciate your honesty.

Cheers,

Careful

 

[Careful]

Start your own thread if you want to push your personal agenda on Bell tests.

I have no agenda, I just want people to know the exact scientific status. I think we all agree that this is important

 

[Sherlock]

Hi sherlock, the point is that the interaction between A and B has measurable consequences out of the lightcone of A.

One way to understand how this can happen in a universe which obeys the principle of locality is that the motions of A and B subsequent to their interaction are related. Isn't it?

Concerning quantum computing: it is impossibe to tell whether it is CLASSICAL correlations one is using or not (see my latest post).

I don't know much about quantum computing. I thought it required strictly quantum correlations of the entangled sort.

 

[Careful]

Hi sherlock,

true: A and B are related also in a locally causal universe, but only in the future lightcone of A. In the example I gave you, the influence of A on B travels outside the future lightcone. Moreover, you should not see A and B as ``particles´´ but as observables of a second quantized field.

Cheers,

careful

 

[Sherlock]

Moreover, I am glad you state explicitely that you can use faster than light signalling in an operational sense.

Where did I state that? My current understanding is that evolutions of any sort in our universe are limited by the speed of light.

So standard QM is not local either in the Bell or operational sense.

My understanding is that QM is non-local in the Bell sense, but that this is an artifact of limitations (as are, I assume, essentially correctly specified in the principles of quantum theory) placed on any fundamental theory by our (I assume locality obeying) universe.

I'm not sure what you mean by non-local in an operational sense. But, afaik, the the formal transformations to principle axes were developed in line with the assumption that Nature obeys the principle of locality.

Anyway, as to the question that this thread poses, I think that QM is inherently non-local only in an artificial sense, because it's inherently incomplete in a physical sense -- and I think this inherent incompleteness applies to any theory of fundamental processes (ie., in a universe constrained by the principles of relativity theory and the principles of quantum theory, then no hidden variable theory is possible).

Moreover, I do not know precisely which conservation laws you are referring to.

The classical conservation laws which were taken over directly into quantum theory. Conservation of energy, momentum, angular momentum, etc.

 

[Sherlock]

... true: A and B are related also in a locally causal universe, but only in the future lightcone of A.

Why not in the future lightcone of B also? The precise relationship between the motions of A and B subsequent to their interaction remains until one or the other, or both, are subjected to external influences (such as B interacting with C). The motions of B and C subsequent to their interaction will be, in part, due to B's prior interaction with A.

Moreover, you should not see A and B as ``particles´´ but as observables of a second quantized field.

It's just a convenient way to talk about it. Individual detections are particles.

 

[Sherlock]

This is an actual physical example of one thing being in two places at once.
... I think of the total breakdown of the classical concept that an object cannot be in two places at once.

This isn't a breakdown of the classical concept. It's actually an analogy to our more direct experience of the world. The different parts of the chair I'm sitting on are in many places at once. The different parts of an expanding water wave front are in many places at once.

Your idea of quantum correlations as involving separate parts of the same physical entity are one way to qualitatively conceptualize what's happening. But, it might not be what is actually happening in all cases of quantum correlations.

Suppose that the paired measurements are actually caused by opposite-moving, self-contained, separate wave structures of some sort. In this case, the results, A and B, really aren't caused by different parts of the same physical entity. But, the motions of the wave structures that caused A and B, and hence A and B, can still be related due to a common source or interaction.

 

[ttn]

But there is a mystery about collapse that it would be desirable to know more about. You touch on it above. You say that the WF collapses upon measurement, and sure, this is standard. So when there are 2 entangled particles, which one cause the collapse - measurement of Alice or of Bob? Sure, the results are apparently the same regardless of which one "causes" the collapse. But again, that's the mystery. We have no specific rule that defines this. And again, that is what I am referring to when I say "I am confused about whether WF collapse is physical" etc.

These are all good questions. In OQM, the collapse happens "instantaneously". So whoever measures first (Alice or Bob) collapses the wave function. That's an unambiguous answer as far as it goes.

The problem is, simultaneity is supposed to be relative. So "whoever measures first" isn't so clear after all. Or rather: in order to give a precise meaning to the *dynamics* of OQM, you have to add some extra spacetime structure such as a notion of preferred/absolute simultaneity. This is just another way of seeing the non-locality that is a real part of OQM.

This is of course precisely why MWI people want to get rid of the collapse rule entirely and get along with *just* the unitary dynamics.

But I am a bit confused about your marble box example. I think what you are saying is: this example does not violate Bell Locality because adding the information about Bob's outcome does not actually change the probability of the outcome at Alice. Am I close? (Or maybe the example isn't that important, not sure.)

No, it is important. If you don't understand that example, you won't understand why a violation of Bell Locality means a genuinely problematic "action at a distance" rather than something mundane and physically uninteresting like just learning something you didn't know about some distant thing.

Anyway, what you say is just right: the marble-in-box example involves no violation of Bell Locality. Here we have a change in the conditional probability of an event (Alice finding the marble) when we do/don't conditionalize on Bob's distant outcome. But if that's all Bell Locality required, it would be violated in situations (like this marble example) that *obviously* do not involve any spooky faster-than-light causality. So my point was just to clarify that Bell Locality is *not* in fact violated in this situation. It takes something more than this to violate it. The probability of an event has to change when we conditionalize on some space-like separated info *even though we've already conditionalized on the complete state of the world in the past light cone of the event in question*. In the marble case, *that* probability (namely: the probability for Alice to find the marble conditional on it either definitely already being in the box, or definitely already not being in the box) is either 0 or 100%, and it doesn't change if you *also* specify whether Bob found the marble. That information -- whether Bob found it -- is *redundant* because we've already specified the exact state of things near Alice, so the probabillities don't change.

Now, suppose we aren't talking about finding a marble, but doing some other experiment and getting some particular outcome. And suppose the probability for that particular outcome is different, depending on whether we do or don't conditionalize on some other information pertaining to a spacelike separated region -- and this *even though we've already specified the complete state of things near the experiment in question*. Well then, wouldn't we say that that distant event (information about which we do or don't conditionalize on) is having some kind of effect on the outcome -- an effect which *cannot* be accounted for by *local causes* in the past light cone of the event? This is what a violation of Bell Locality means.

 

[Careful]

Hi sherlock,

You seem to say that (a) QM gives the right predictions and (b) spacetime is real and all processes (which are real by assumption) have speed smaller or equal than light. (b) logically implies that the wave function of (a) must be real which implies that processes exist which go faster than with the speed of light. This is a problem of the Klein Gordon equation for a complex scalar field in first quantization, is remedied by hand for two measurements in quantum field theory, but pops up again in the situation I mentioned to you. Therefore my statement.

Concerning the conservation laws you mention: it is very hard to obtain an anomaly free interacting QFT and these equations do usually not make much sense anymore...

 

[Careful]

Why not in the future lightcone of B also? The precise relationship between the motions of A and B subsequent to their interaction remains until one or the other, or both, are subjected to external influences (such as B interacting with C). The motions of B and C subsequent to their interaction will be, in part, due to B's prior interaction with A.
It's just a convenient way to talk about it. Individual detections are particles.

This should be only so for that part of B which interacted causally with A (which is de facto in the future lightcone of A), not the part that did not interact with A at all. The real problem is that reduction of the state (in B) is an instantaneous non - local (causal) process and that is why the influence of A through B will travel to C.

 

[vanesch]

I do not dispute that QM is successful in calculating the spectrum of the H and He atom (you cannot predict above He) and explaining the Lamb shift.

?? I think that QM has rather more successes on its name than just H and He ! I am even convinced that the "classical field" approaches (the coupled Dirac-Maxwell fields + eventually some noise) have serious problems with higher than He configurations. At best these theories give the same predictions as the Hartree-Fock method with the self-consistent potential, but it is well-known in quantum chemistry that this gives a good approximation, but sometimes not good enough and one needs to add things like "configuration interaction" to get closer to experimental values.

However, I am convinced that these successes have perfect (albeit more difficult and subtle) classical explanations. A good step in this direction is the theory of stochastic electrodynamics which reproduces a good bunch of these so called exclusive results from second quantization in a firstly quantized framework (such as the Casimir effect, and the H atom I believe). Barut and Dowling have done quite some work on this issue ...

Yes, that's fascinating work, I agree. The problem is, however, with most of these approaches, that they tackle ONE SPECIFIC aspect of quantum predictions, and that we can then vaguely hope that they will, one day, be as successfull as standard quantum machinery in all the rest.
It is just not reasonable to accept the quantum machinery for about all the predictions it makes, *except* for those very few predictions that kill off your original belief of how nature ought to be.
The reason why this work is 1) fascinating and 2) probably misguided can be found by "reductio ad absurdum". Indeed, if these classical theories were correct, their computations would be gazillion times simpler than quantum computations. Non-linear partial differential equations in 3 dimensions are, computationally, peanuts as compared to, say the Feynman path integral in QFT, and can be attacked much much easier with finite-element methods than QFT. It would reduce quantum chemistry, and even nuclear physics, to something computationally just as easy as weatherforcasting. Not that weatherforecasting is so simple, but it is doable, while QFT calculations only start to be tractable with lattice techiques. So if it were possible to do so, it would have been done already since a long time.

My intention is not at all to dispose of standard QM, I am perfectly aware of the fact that it provides an effective way of calculating the statistics of experiments with microscopic objects when applied on the *correct* problems with considerable thought. However, I also gives the wrong answers in some cases and it does not provide any insight into the dynamics of a single particle.

I would like to know in what specific cases quantum theory comes up with the wrong experimental predictions which have been falsified by experiment.

I want to obtain *insight* into the microworld which I believe obeys the same laws as the macroworld (that is GR and electromagnetism), therefore entanglement is a crucial issue and one should not take it lightly.

In a way, I *also* adhere to a belief: it is that there are a few fundamental principles on which the entire formalism of physical theory has to be constructed.

Concerning your remark about the extrapolation of succes, I can only say that people have been looking for over 50 years for a perpetuum mobile; I hope one is not going to look 100 years for entanglement. By the way, Newton theory was also correct for 300 years.

If you want my guess, I don't think quantum mechanics in its present form will still be around (except as a useful approximation) 300 years from now - or it will, but then because of lack of progress (for instance, lack of experimental input on quantum gravity phenomena). But as of now, it is still the best thing we have - and it has to be admitted that it is vastly more successful in vastly different fields than anything that tries to rival with it. At best you get *identical* predictions in certain areas. Entanglement is a very standard part of the quantum formalism, and *is* confirmed by many experiments in the sense that these calculations DO correspond to predictions that are verified: see further.

Concerning Hartree, you have to include a classical radiation field determined by the probability current of the particles. In that way, you obtain a QM where each particle has its own wave function and where interactions propagate via a classical maxwell field determined by the sum over all probability currents times the appropriate charges (I think Barut called this the self field approach to QED, it's non-linear of course).

I expected that this is what you meant but wasn't sure. It is indeed the self-consistent field method used in quantum chemistry. A good approximation, but with known deviations from experiment, which is improved upon by configuration interaction techniques which are nothing else but "entanglements" of the different electrons. Even in the H2 molecule, this is experimentally visible (although small). More successes can be found with the H20 molecule, especially the angle between the two bonds, which for the H-F selfconsistent field method gives us 106.1 degrees, while the CI technique gives 104.9 degrees (experiment being 104.5 degrees). Took this from "Modern quantum chemistry" by Szabo and Ostlund.
There are many many examples like this. The problem with the CI technique is of course the huge system of equations that it generates - hence my proof by contradiction of a classical theory doing the same thing: if it worked, it would be done since long.

 

[Careful]

Hi Vanesh,

It is late for me, so I shall treat some part of your comments and shall be back tomorrow for more...
I think your most substantial argument is that the Hartree approximation although it is good is known to deviate slightly from experimental outcome. This is a fact. However, I did not say that Hartree is the full theory, neither did I claim that Fock corrections is what one should be looking for:
(a) your computability argument is incorrect, good computer experiments concerning, say the classical three body problem, have only very recently been performed and obtaining a thorough understanding of it is still on the way. The same comment applies to GR where the post Newtonian approximation is often known not to be adequate and obtaining the full solution (to the nonlinear equations) is a notoriously difficult problem (and picking out the right finite element method can take a considerable amount of time, even for the trained mathematician).
(b) I did not say I accept all predictions of QM except those which kill my belief : I accept all predictions which are confirmed by experiment but this does not imply that QM is the only way to get to these results.
(c) I am fully aware that these aternative approaches are in some sense behind QFT. The reason for this is is easy to think of: just compare the amount of money which is put into both research branches.
(d) I know it is the attitude of most researchers to conclude from Hartree is not equal to, but very close to, experiment implies that entanglement vindicates again. However, here I disagree : could we simply not have forgotten something? Why are these predictions so close while we totally ignore all non-local correlations? One could think now of adding other interaction terms between different wave packages, as you mention, in order fit accurately to the results but this is patchwork. I will come back to this tomorrow.

 

[Sherlock]

You seem to say that (a) QM gives the right predictions ...

It seems to be the most accurate across a wide range of experimental applications. Saying that QM's predictions are "right" is sort of an iffy statement, isn't it? After all, there are limits (due to error and due to fundamental constraints specified by QM itself) to what can be experimentally determined. QM predicts values that experimental runs will approach, and, afaik (I'm just learning), it's calculations agree with experiment.

... and (b) spacetime is real

Space and time are conventions.

... and all processes (which are real by assumption) have speed smaller or equal than light.

Yes, I assume that Nature obeys the principle of locality.

(b) logically implies that the wave function of (a) must be real which implies that processes exist which go faster than with the speed of light.

Space time is a convention. So is the wave function. The wave function is a complete description of what is known about the quantum system it refers to (at least it's one way of describing what is known). However, the wave function is necessarily an incomplete description of the physical reality of the quantum system it refers to. Hence, no superluminality is implied. The assumption that Nature is local is based on strong theoretical arguments which have thus far not been falsified by experiment, so it remains.

Concerning the conservation laws you mention: it is very hard to obtain an anomaly free interacting QFT and these equations do usually not make much sense anymore...

I haven't learned QFT yet, so if your main points depend on this theory, then I must excuse myself from the discussion.

 

[Sherlock]

This should be only so for that part of B which interacted causally with A (which is de facto in the future lightcone of A), not the part that did not interact with A at all.
The real problem is that reduction of the state (in B) is an instantaneous non - local (causal) process and that is why the influence of A through B will travel to C.

I don't understand what you're saying here.