EPR Paradox Failure Explained for High Schoolers

[jobsism]

EPR paradox-failure?!

Can anyone please explain to me why the EPR paradox failed to bypass the uncertainty principle? I would appreciate it if minimal maths is used, because I am still a high-schooler and don't know much about higher math.

 

[DrChinese]

The short answer is that the Heisenberg Uncertainty Principle is a fundamental consequence of quantum physics, while the assumptions of EPR are not. And quantum physics is experimentally supported in all respects to the best of my knowledge.

 

[ThomasT]

Can anyone please explain to me why the EPR paradox failed to bypass the uncertainty principle? I would appreciate it if minimal maths is used, because I am still a high-schooler and don't know much about higher math.

You might find this paper interesting:
http://arxiv.org/PS_cache/arxiv/pdf/0706/0706.2097v2.pdf

 

[zonde]

Can anyone please explain to me why the EPR paradox failed to bypass the uncertainty principle? I would appreciate it if minimal maths is used, because I am still a high-schooler and don't know much about higher math.

EPR paradox was not intended to bypass uncertainty principle.
EPR paradox was intended to demonstrate incompleteness of quantum mechanics.
It is more concerned with cases where you can predict something with certainty rather than cases where you can't do that.

 

[nismaratwork]

The short answer is that the Heisenberg Uncertainty Principle is a fundamental consequence of quantum physics, while the assumptions of EPR are not. And quantum physics is experimentally supported in all respects to the best of my knowledge.

To the best that any modern measurements can be made, I think you've made an accurate statement.

Zonde: EPR was concerned with challenging the notion of action-at-a-distance.

 

[jobsism]

EPR paradox was not intended to bypass uncertainty principle.
EPR paradox was intended to demonstrate incompleteness of quantum mechanics.
It is more concerned with cases where you can predict something with certainty rather than cases where you can't do that.

But predicting something with certainty in QM, itself violates the uncertainty principle, doesn't it?

Thanks, Thomas T for the pdf, but it used rather technical terms and i could understand little of it...

Maybe I should rephrase my doubt: As far as i understand, in the EPR paradox, the motion of one particle "somehow" affects the other. I would like to know the theory behind this "somehow" effect in detail(only the theory, not the math). Am i understand that it basically is due to the wave nature of matter?

 

[nismaratwork]

But predicting something with certainty in QM, itself violates the uncertainty principle, doesn't it?

Thanks, Thomas T for the pdf, but it used rather technical terms and i could understand little of it...

Maybe I should rephrase my doubt: As far as i understand, in the EPR paradox, the motion of one particle "somehow" affects the other. I would like to know the theory behind this "somehow" effect in detail(only the theory, not the math). Am i understand that it basically is due to the wave nature of matter?

Quick sidenote: I may be wrong here, but I believe that Zonde is one of those who in other threads, has rejected non-locality and Bell tests. Maybe you don't want to take his word on... anything?

 

[nonequilibrium]

But predicting something with certainty in QM, itself violates the uncertainty principle, doesn't it?

No, I think a popular misconception is that QM ensures everything is uncertain (if that were true, this sentence would be contradictory). Like for example, QM may say you don't exactly know where the particle is, but it ensures you, for example, of the fact that the particle is somewhere. It doesn't forbid more than it forbids, which is FAR from everything.

 

[DevilsAvocado]

Can anyone please explain to me why the EPR paradox failed to bypass the uncertainty principle?

As you may already have noticed, there are several opinions around this. Some even claims that Einstein did not completely indorsed the http://plato.stanford.edu/entries/qt-epr/" (Einstein & Podolsky had a dispute after the publication). What we can say is that Einstein was not happy about the evolution of QM, and when Heisenberg and Born declared at the Solvay Conference in 1927 that the revolution was over and nothing further was needed – Einstein's skepticism turned to dismay. Einstein could not accept that space and time was removed from any underlying reality, and that QM was to be understood as a probability without any causal explanation.

From this I think we can say that Einstein with the EPR paradox aimed to show that QM was incomplete and fundamentally inadequate. According to Einstein there was a need for something else, as QM was not the appropriate starting point for constructing the new theory he thought was needed.

To grasp the "problem" with proving the uncertainty principle wrong, you can think of this macroscopic example:

Assume you have a time-variant signal, a sound wave, and you want to know the exact frequency of the signal at a given moment. This is impossible. To determine the exact frequency it’s necessary to resample the signal over time and thus lose a degree of precision in the position. In other words a sound cannot be both, the exact time (as in a short pulse) and a precise frequency, as in a continuous tone. Phase and frequency of a (sound) wave in time is analogous to the position and momentum of a (QM) wave in space.​

Einstein was of course intelligent enough to realize this, and he was not especially interested in the question of simultaneous values for incompatible quantities like position and momentum, and Einstein told Schrödinger "ist mir wurst" – literally, it's sausage to me; i.e., he couldn't care less.

Einstein was concerned with an underlying reality that had a causal explanation.

But in 1964 John Bell showed that Local Hidden Variables (LHV) is incompatible with the predictions of QM in "[URL Theorem[/URL]:

No physical theory of local hidden variables can ever reproduce all of the predictions of quantum mechanics.​

But predicting something with certainty in QM, itself violates the uncertainty principle, doesn't it?

No. If you where a QM particle designed to come to work every day at eight o'clock, and this was repeated for 10 years, we could say that we have a pretty good prediction of your arrival at work, right? But this doesn’t tell us anything about what time you got out of bed, or which way you took to work. Okay? (Einstein wanted to know when you got out of bed! )

Maybe I should rephrase my doubt: As far as i understand, in the EPR paradox, the motion of one particle "somehow" affects the other. I would like to know the theory behind this "somehow" effect in detail(only the theory, not the math).

That’s the Million Dollar Question!

The solution to http://en.wikipedia.org/wiki/Bell_test_experiments" is not settled. We only know that either Locality and/or Reality have to go. That’s all.

If non-locality is proven, then my guess is that it has to have some relation to the QM http://en.wikipedia.org/wiki/Wave_function" .

My personal guess is that we need to merge QM + GR and maybe also find the solution for Quantum Gravity (QG), before we find the final solution to EPR-Bell... I guess... sort of...


P.S. nismaratwork’s sidenote is worth reading...

 

[zonde]

Zonde: EPR was concerned with challenging the notion of action-at-a-distance.

This is incorrect of course.
From EPR paper:
"For this purpose let us suppose that we have two systems, I and II, which we permit to interact from the time t=0 to t=T, after which time we suppose that there is no longer any interaction between the two parts."
So EPR uses locality as condition for their own example.

Quick sidenote: I may be wrong here, but I believe that Zonde is one of those who in other threads, has rejected non-locality and Bell tests. Maybe you don't want to take his word on... anything?

I do not reject Bell tests. I reject fair sampling assumption used in photon Bell tests. And without it they are not conclusive.

 

[zonde]

But predicting something with certainty in QM, itself violates the uncertainty principle, doesn't it?

Not necessarily.
Look you have one particle and you describe it with two non-commuting variables (A1 and B1) i.e. you have uncertainty between them. Now you have entangled particle with variables A2 and B2. There is no uncertainty between A1 and A2 and likewise between B1 and B2.
So your conclusion seems quite natural that this situation violates uncertainty principle. But there is another possibility that both A and B are not descriptions of individual particle. Say A describes individual particle but in this case B doesn't and it is description of slightly different thing.
As an example, photon polarization is property of individual particle but photon phase is not because we don't have reference for measurement of photon phase and it can be measured only relative to another photon.

If you look at technical details of photon Bell experiments you can find there that necessary condition for observation of entanglement is coherence between H and V photons. If this condition isn't met you observe correlation only between H/V polarization measurements but correlation between +45/-45 measurements (it is this other non-commuting variable for photon polarization entanglement experiments) disappears.

 

[nismaratwork]

This is incorrect of course.
From EPR paper:
"For this purpose let us suppose that we have two systems, I and II, which we permit to interact from the time t=0 to t=T, after which time we suppose that there is no longer any interaction between the two parts."
So EPR uses locality as condition for their own example.


I do not reject Bell tests. I reject fair sampling assumption used in photon Bell tests. And without it they are not conclusive.

Nothing personal Zonde, but the point is that you're hardly the mainstream view, and someone who isn't familiar with your views should be made aware of that... especially once they express a failure to get your point. The bottom line is that you reject the results of the BSMs, which is the same thing as rejecting the test themselves. Most believe they're conclusive, you don't... that's your choice but you're one of two people I've met (ThomasT being the other) who believes this with any conviction once challenged with a wealth of evidence to the contrary.

Bottom line: someone who is new here deserves to know where you stand; it's not as though I called your knowledge or intelligence into question, I just pointed out that you hold a minority opinion in an arena that is related to his question. Note that I'm not going after your second post after the one I'm currently responding to, but when you hold a relatively unique opinion on a mainstream educational site, it's probably a good idea to lead with that caveat.

 

[ZapperZ]

I do not reject Bell tests. I reject fair sampling assumption used in photon Bell tests. And without it they are not conclusive.

So does that mean that you have written a rebuttal to D. W. Berry, et al., "Fair-sampling assumption is not necessary for testing local realism" Phys. Rev. A 81, 012109 (2010)?

Zz.

 

[DrChinese]

So does that mean that you have written a rebuttal to D. W. Berry, et al., "Fair-sampling assumption is not necessary for testing local realism" Phys. Rev. A 81, 012109 (2010)?

Zz.

Here is a link to the above:

http://arxiv.org/abs/0712.2490

By the way, I think there has been ample evidence that fair sampling has nothing to do with experimental results. I realize that a so-called "loophole free" test is desirable. But it will certainly be an anti-climax after the wonderful work that has been performed in recent years.

 

[DevilsAvocado]

I do not reject Bell tests. I reject fair sampling assumption used in photon Bell tests. And without it they are not conclusive.

Would you say that this test, using trapped ⁴⁰Ca⁺ ions (20 protons and 20 neutrons) and a qubit state fidelity of 99.5%, demonstrating conflict with non-contextuality according to the Kochen-Specker theorem, is conclusive?

http://arxiv.org/abs/0904.1655"[/URL]

[SIZE="3"][B]State-independent experimental test of quantum contextuality[/B]
G. Kirchmair, F. Zähringer, R. Gerritsma, M. Kleinmann, O. Gühne, A. Cabello, R. Blatt, C. F. Roos

[SIZE="1"](Submitted on 10 Apr 2009 (v1), last revised 5 May 2009 (this version, v2))
Journal reference: Nature 460, 494 (2009)
DOI: http://www.nature.com/nature/journal/v460/n7254/full/nature08172.html"

The question of whether quantum phenomena can be explained by classical models with hidden variables is the subject of a long lasting debate. In 1964, Bell showed that certain types of classical models cannot explain the quantum mechanical predictions for specific states of distant particles. Along this line, some types of hidden variable models have been experimentally ruled out. [B]An intuitive feature for classical models is non-contextuality: the property that any measurement has a value which is independent of other compatible measurements being carried out at the same time. However, the results of Kochen, Specker, and Bell show that non-contextuality is in conflict with quantum mechanics.[/B] The conflict resides in the structure of the theory and is independent of the properties of special states. It has been debated whether the Kochen-Specker theorem could be experimentally tested at all. Only recently, first tests of quantum contextuality have been proposed and undertaken with photons and neutrons. Yet these tests required the generation of special quantum states and left various loopholes open. Here, using trapped ions, we experimentally demonstrate a state-independent conflict with non-contextuality. [B]The experiment is not subject to the detection loophole and we show that, despite imperfections and possible measurement disturbances, our results cannot be explained in non-contextual terms.[/B][/QUOTE]

 

[nismaratwork]

So does that mean that you have written a rebuttal to D. W. Berry, et al., "Fair-sampling assumption is not necessary for testing local realism" Phys. Rev. A 81, 012109 (2010)?

Zz.

In the long and ongoing non-locality thread it's pretty clear that, no, Zonde has done no such thing, but that does not sway him. To be fair, I don't think he's ever claimed to have refuted it.

 

[zonde]

Nothing personal Zonde, but the point is that you're hardly the mainstream view, and someone who isn't familiar with your views should be made aware of that... especially once they express a failure to get your point. The bottom line is that you reject the results of the BSMs, which is the same thing as rejecting the test themselves. Most believe they're conclusive, you don't... that's your choice but you're one of two people I've met (ThomasT being the other) who believes this with any conviction once challenged with a wealth of evidence to the contrary.

You are not very careful with your statements.
It is quite a stretch to say that most believe photon Bell tests are conclusive. It might be reasonable to say that most believe results are convincing but it is not the same as conclusive.

Another thing is that the way you put it one might get impression that I reject raw data of Bell experiments and that is of course wrong. I am just questioning interpretation of results and nothing more.

 

[zonde]

So does that mean that you have written a rebuttal to D. W. Berry, et al., "Fair-sampling assumption is not necessary for testing local realism" Phys. Rev. A 81, 012109 (2010)?

Zz.

No, but I have counter example that satisfies their additional assumptions and still produces QM prediction for ~10% coincidence rate.
Do you want to look at it? I have posted it couple of times but I believe attachments are deleted after some time period so I will have to post it one more time.

EDIT: It turned out that second attachment is still there. Here is the https://www.physicsforums.com/showpost.php?p=2617303&postcount=439" with attachment.

 

[zonde]

Would you say that this test, using trapped ⁴⁰Ca⁺ ions (20 protons and 20 neutrons) and a qubit state fidelity of 99.5%, demonstrating conflict with non-contextuality according to the Kochen-Specker theorem, is conclusive?

No, it is not conclusive. The effect of manipulation crosstalk is not rigorously explored. On the good side they do some manipulations to prevent measurement crosstalk but still they treat photons (their actual measurement equipment) as classical particles and therefore you can not fully relay on their reasoning.

And it is funny that they define non-locality as contextuality:
"An intuitive feature for classical models is non-contextuality: the property that any measurement has a value which is independent of other compatible measurements being carried out at the same time."
When usual meaning of "context" is surroundings and environment. So I would say that context would be appropriate word for description of detector's state that is involved in measurement of photon under question. And non-contextuality would mean that detector's state doesn't play any role in photon measurement.

But of course formally I can define red as blue and blue as red and within some text it should be taken as valid method.

 

[ThePhysicsGuy]

Can anyone please explain to me why the EPR paradox failed to bypass the uncertainty principle? I would appreciate it if minimal maths is used, because I am still a high-schooler and don't know much about higher math.

They deal with two completely different issues (as far as anyone at the moment knows). EPR deals with non-local transmission of quantum state, while the uncertainty principle deals with the limits of obtainable information. So, you've got two distinct issues, obtaining information vs. transmitting quantum state.

 

[nismaratwork]

You are not very careful with your statements.
It is quite a stretch to say that most believe photon Bell tests are conclusive. It might be reasonable to say that most believe results are convincing but it is not the same as conclusive.

Another thing is that the way you put it one might get impression that I reject raw data of Bell experiments and that is of course wrong. I am just questioning interpretation of results and nothing more.

Yeah, but your post in response to Zapperz is most telling, and encapsulates the problem. You don't have a sufficient rebuttal, just your opinion bolstered with what I personally (careful enough) feeble 'logic'.

 

[ZapperZ]

No, but I have counter example that satisfies their additional assumptions and still produces QM prediction for ~10% coincidence rate.
Do you want to look at it? I have posted it couple of times but I believe attachments are deleted after some time period so I will have to post it one more time.

EDIT: It turned out that second attachment is still there. Here is the https://www.physicsforums.com/showpost.php?p=2617303&postcount=439" with attachment.

How do you know your "counter example" is valid? If you think it is, then you should submit it as a rebuttal, rather than argue it here on some public forum. Or do you think it won't pass careful scrutiny by those who are experts in such a field?

Zz.

 

[nismaratwork]

How do you know your "counter example" is valid? If you think it is, then you should submit it as a rebuttal, rather than argue it here on some public forum. Or do you think it won't pass careful scrutiny by those who are experts in such a field?

Zz.

Well he posted it, if not for peer review in a journal then at least for your review. What do you think of it?

 

[DrChinese]

Well he posted it, if not for peer review in a journal then at least for your review. What do you think of it?

This forum (Quantum Physics) is not really intended for posters to present their original ideas. There is a section here for that called "Independent Research" which is moderated & reviewed.

 

[nismaratwork]

This forum (Quantum Physics) is not really intended for posters to present their original ideas. There is a section here for that called "Independent Research" which is moderated & reviewed.

Yeah, I know... I'd call my last post "entrapment" in the strictest legal sense. I'm REALLY tired of the endless lack of traction in a particular thread due to what amounts to that inappropriate use of personal theories (hint... you're not the problem.)

 

[ThomasT]

Most believe they're conclusive, you don't... that's your choice but you're one of two people I've met (ThomasT being the other) who believes this with any conviction once challenged with a wealth of evidence to the contrary.

nismaratwork, you'd better get your facts straight. Zonde (whose presentations I've found interesting and worthy of respect, even if I might not entirely agree with all his conclusions, but then I might not entirely understand them) and I have been exploring somewhat different considerations regarding Bell. He's concerned with the validity of the science. I'm concerned with the validity of the logic. I'm not sure what you're saying, or what you think, that I believe. Regarding what you think, I don't care. Regarding what you're saying, state it clearly and then back it up with some quotes or else retract it.

Regarding jobsism's questions, there have been a few answers to the point. However, nobody has explained it satisfactorily yet. If the heavyweights looking at this thread don't explain it to the OP in sufficient detail, then I will. And, yeah that's right, you don't really want that. So, let's go people -- chop chop, explain, elaborate!

 

[zonde]

How do you know your "counter example" is valid?

I tried to make it maximally lucid. There are not much where an error can hide. The model is split in four worksheets - source, Alice, Bob and result (coincidence counting). It's easy to check that locality condition is satisfied.

If you think it is, then you should submit it as a rebuttal, rather than argue it here on some public forum. Or do you think it won't pass careful scrutiny by those who are experts in such a field?

Counterexamples usually are rather very specific to argument. So my example has limited to no use outside the context of specific argument.
As I see the major problem with different arguments against local realism (including the paper mentioned) is that they are aimed at LHV models that do not justify uncertainty principle or rather simply contradict it.
My example is of the same kind of LHV models so among other considerations I see no point in promoting such approach.

If we talk about that paper you mentioned it just replaces fair sampling assumption with other assumption: "the detection efficiency factorises as a function of the measurement settings and any hidden variable"
If you look in more details it turns out that this "any hidden variable" of single particle can affect only the measurement of that single particle. That clearly ignores all the possible LHV models where collective behavior of ensemble is considered.
That way it doesn't give any arguments against direction that I consider perspective.

 

[ZapperZ]

I tried to make it maximally lucid. There are not much where an error can hide. The model is split in four worksheets - source, Alice, Bob and result (coincidence counting). It's easy to check that locality condition is satisfied.


Counterexamples usually are rather very specific to argument. So my example has limited to no use outside the context of specific argument.
As I see the major problem with different arguments against local realism (including the paper mentioned) is that they are aimed at LHV models that do not justify uncertainty principle or rather simply contradict it.
My example is of the same kind of LHV models so among other considerations I see no point in promoting such approach.

If we talk about that paper you mentioned it just replaces fair sampling assumption with other assumption: "the detection efficiency factorises as a function of the measurement settings and any hidden variable"
If you look in more details it turns out that this "any hidden variable" of single particle can affect only the measurement of that single particle. That clearly ignores all the possible LHV models where collective behavior of ensemble is considered.
That way it doesn't give any arguments against direction that I consider perspective.

Really!

If I have something that either falsifies, or shows a slight flaw in something that was published, I would submit a rebuttal because:

(1) the community needs to know since this is the way science works;
(2) I get an extra publication.

For something that has "... limited to no use.. ", you are certainly milking it pretty often. If you wish to keep using it, you must have it published. If not, I would strongly suggest that this should be the last time you refer to it, per our PF Rules.

Zz.

 

[zonde]

Really!

If I have something that either falsifies, or shows a slight flaw in something that was published, I would submit a rebuttal because:

(1) the community needs to know since this is the way science works;
(2) I get an extra publication.

For something that has "... limited to no use.. ", you are certainly milking it pretty often. If you wish to keep using it, you must have it published. If not, I would strongly suggest that this should be the last time you refer to it, per our PF Rules.

Zz.

I get your point.

But it seems it wouldn't be necessary to consider anything like that. With some delay I finally understood what this article is about.
It gives necessary condition that justifies fair sampling assumption. And this condition is experimentally verifiable!
So you don't have to blindly assume fair sampling, you can test it.

From conclusions of discussed paper:
"In the case where hidden variables are allowed, then the relevant condition is that the efficiency factorises as in Eq. (7). Any condition that depends on the hidden variables can not be proven to hold, because it is possible that it might be violated for values of the hidden variable that it is not possible to prepare. However, it is possible to falsify it. The great advantage of providing a necessary condition, as we have done, is that if it can be shown not to hold, then the sampling is shown to be of a form that invalidates the CHSH-Bell inequality. In contrast, if the condition that is tested is not necessary, then testing it is not useful. Showing that it does not hold does not show that the sampling is of a form that invalidates the CHSHBell inequality, and it cannot be conclusively shown to hold. Thus our results put testing of the sampling in Bell experiments [43] on a rigorous basis."

 

[nismaratwork]

nismaratwork, you'd better get your facts straight. Zonde (whose presentations I've found interesting and worthy of respect, even if I might not entirely agree with all his conclusions, but then I might not entirely understand them) and I have been exploring somewhat different considerations regarding Bell. He's concerned with the validity of the science. I'm concerned with the validity of the logic. I'm not sure what you're saying, or what you think, that I believe. Regarding what you think, I don't care. Regarding what you're saying, state it clearly and then back it up with some quotes or else retract it.

Regarding jobsism's questions, there have been a few answers to the point. However, nobody has explained it satisfactorily yet. If the heavyweights looking at this thread don't explain it to the OP in sufficient detail, then I will. And, yeah that's right, you don't really want that. So, let's go people -- chop chop, explain, elaborate!

In my view, the notion of you explaining your own views on the subject of EPR and non-locality to Zapperz is also a win, so please I would urge you to do so. This is, in essence, your chance to resolve over 80 pages of cyclical discourse with an impartial arbiter... I for one am confident that your view is not one which is correct, but beyond that it certainly doesn't meet PF requirements, anymore than Zonde's. Fire away ThomasT, I don't expect even the view of staff to change your behavior or arguments, but at least it might spare those who read and participate in related threads from dealing with identical interjections every few pages.

You're the one who has a love affair with Malus' Law, right?... god, please, explain that.

 

[nismaratwork]

I get your point.

But it seems it wouldn't be necessary to consider anything like that. With some delay I finally understood what this article is about.
It gives necessary condition that justifies fair sampling assumption. And this condition is experimentally verifiable!
So you don't have to blindly assume fair sampling, you can test it.

From conclusions of discussed paper:
"In the case where hidden variables are allowed, then the relevant condition is that the efficiency factorises as in Eq. (7). Any condition that depends on the hidden variables can not be proven to hold, because it is possible that it might be violated for values of the hidden variable that it is not possible to prepare. However, it is possible to falsify it. The great advantage of providing a necessary condition, as we have done, is that if it can be shown not to hold, then the sampling is shown to be of a form that invalidates the CHSH-Bell inequality. In contrast, if the condition that is tested is not necessary, then testing it is not useful. Showing that it does not hold does not show that the sampling is of a form that invalidates the CHSHBell inequality, and it cannot be conclusively shown to hold. Thus our results put testing of the sampling in Bell experiments [43] on a rigorous basis."

Your argument is an Ouroboros of failed logic, old arguments, and personal prejudice eating the tail of ThomasT's own (see above adjectives) arguments. Do you REALLY think this forwards your viewpoint, to simply repeat the same statements in slightly different configurations? After reading dozens and dozens of pages where Dr Chinese, Devils Avocado, RUTA and others tear it apart, I'm just amazed that you continue so blindly.

 

[DevilsAvocado]

No, it is not conclusive. The effect of manipulation crosstalk is not rigorously explored. On the good side they do some manipulations to prevent measurement crosstalk but still they treat photons (their actual measurement equipment) as classical particles and therefore you can not fully relay on their reasoning.

This is interesting. Are you claiming that all measurements of QM phenomena, to be valid, must be non-classical measurements? Only "QM measurements" for QM phenomena??

This is revolutionary... what happened to decoherence...??

I have one question though – If only "QM measurements" are valid measurements, how can we be sure of what we have finally measured??

(I take it you are not rejecting the Heisenberg uncertainty principle, also.)

And it is funny that they define non-locality as contextuality:

Noncontextuality – If a QM system possesses a property (value of an observable), then it does so independently of any measurement context, i.e. independently of how that value is eventually measured.

 

[ZapperZ]

I get your point.

But it seems it wouldn't be necessary to consider anything like that. With some delay I finally understood what this article is about.
It gives necessary condition that justifies fair sampling assumption. And this condition is experimentally verifiable!
So you don't have to blindly assume fair sampling, you can test it.

From conclusions of discussed paper:
"In the case where hidden variables are allowed, then the relevant condition is that the efficiency factorises as in Eq. (7). Any condition that depends on the hidden variables can not be proven to hold, because it is possible that it might be violated for values of the hidden variable that it is not possible to prepare. However, it is possible to falsify it. The great advantage of providing a necessary condition, as we have done, is that if it can be shown not to hold, then the sampling is shown to be of a form that invalidates the CHSH-Bell inequality. In contrast, if the condition that is tested is not necessary, then testing it is not useful. Showing that it does not hold does not show that the sampling is of a form that invalidates the CHSHBell inequality, and it cannot be conclusively shown to hold. Thus our results put testing of the sampling in Bell experiments [43] on a rigorous basis."

Er.. I don't get it. Isn't this the whole point of what I'm trying to convey, that YOUR questioning about fair sampling is actually moot if some conditions are met? And it has nothing to do with requiring 100% detection loop-hole free either. You questioned the inherent fair-sampling criteria in Bell-type measurements. This article has shown that it isn't so.

Zz.

 

[zonde]

Er.. I don't get it. Isn't this the whole point of what I'm trying to convey, that YOUR questioning about fair sampling is actually moot if some conditions are met? And it has nothing to do with requiring 100% detection loop-hole free either. You questioned the inherent fair-sampling criteria in Bell-type measurements. This article has shown that it isn't so.

This article replaces fair sampling with testable condition. Until this condition is tested interpretation of Bell tests have the status of "if". When it will be tested we will get rid of that "if". Situation will be resolved by experiment - exactly the way it should be in physics.

Fine until this condition is tested we can replace fair sampling assumption with weaker assumption that we can call ... (?) "sampling that is equivalent to fair sampling".

Is this ok?

 

[ZapperZ]

This article replaces fair sampling with testable condition. Until this condition is tested interpretation of Bell tests have the status of "if". When it will be tested we will get rid of that "if". Situation will be resolved by experiment - exactly the way it should be in physics.

Fine until this condition is tested we can replace fair sampling assumption with weaker assumption that we can call ... (?) "sampling that is equivalent to fair sampling".

Is this ok?

Yes, and what does this have anything to do with the your "counter example" that you've been trying to sell? Recall that you used it as the centerpiece as an INHERENT problem in all Bell-type experiments, even those testing the CHSH violation.

Zz.

 

[zonde]

Yes, and what does this have anything to do with the your "counter example" that you've been trying to sell?

Nothing. I thought that this paper was a no-go theorem and as a result completely misinterpreted it. And therefore I tried to argument against it using this "counter example".
But as this theorem is not a no-go theorem my "counter example" have nothing to do with it.

Recall that you used it as the centerpiece as an INHERENT problem in all Bell-type experiments, even those testing the CHSH violation.

Sorry, can't recall that.

 

[zonde]

This is interesting. Are you claiming that all measurements of QM phenomena, to be valid, must be non-classical measurements? Only "QM measurements" for QM phenomena??

No. I said classical particles not classical measurements. Please read carefully.
Classical particles don't have inherent phase. Photons have.

Noncontextuality – If a QM system possesses a property (value of an observable), then it does so independently of any measurement context, i.e. independently of how that value is eventually measured.

Then I will avoid "Noncontextuality". It seems that this term is too ambiguous.

 

[nismaratwork]

Nothing. I thought that this paper was a no-go theorem and as a result completely misinterpreted it. And therefore I tried to argument against it using this "counter example".
But as this theorem is not a no-go theorem my "counter example" have nothing to do with it.


Sorry, can't recall that.

Zonde... did you SERIOUSLY just play the "I don't recall senator" card?

 

[ThomasT]

In my view, the notion of you explaining your own views on the subject of EPR and non-locality to Zapperz is also a win, so please I would urge you to do so. This is, in essence, your chance to resolve over 80 pages of cyclical discourse with an impartial arbiter... I for one am confident that your view is not one which is correct, but beyond that it certainly doesn't meet PF requirements, anymore than Zonde's. Fire away ThomasT, I don't expect even the view of staff to change your behavior or arguments, but at least it might spare those who read and participate in related threads from dealing with identical interjections every few pages.

What are you talking about, nismaratwork? I asked you to state clearly what you think it is that I believe, since you seem so bent on associating it with Zonde's consideration(s). But you've responded in a very vague and seemingly inflammatory way. You say you're confident that my view is incorrect and yet you don't seem to know what my view is. You also say that my view "certainly doesn't meet PF requirements". What does that mean? You insult Zonde by telling the OP not to believe anything that he says about anything. Now what kind of talk is that? If you think that Zonde's concern about an aspect of the science in certain experiments is unfounded, then engage him in discussion about it.

Your post ends with the following statement/question:

You're the one who has a love affair with Malus' Law, right?... god, please, explain that.

And I again have to wonder what you're talking about. Yes, Malus Law is an empirical law that's an important component of classical and quantum optics, but what does it have to do with the OP's questions?

Anyway, can we get back on topic?

I provided a link to a paper that dealt, somewhat, with jobsism's question regarding "why the EPR paradox failed to bypass the uncertainty principle". It was a bit technical. So, let's start again with jobsism's original question.

He asked:

Can anyone please explain to me why the EPR paradox failed to bypass the uncertainty principle? I would appreciate it if minimal maths is used, because I am still a high-schooler and don't know much about higher math.

Ok, I'm not exactly sure what jobsism means by "the EPR paradox failed to bypass the uncertainty principle". So, hopefully, jobsism or somebody will clarify that.

I'm not sure what, if anything, the uncertainty principle (hup) has to do with EPR. Hopefully somebody will clarify that also.

Does the following statement by EPR depend on an application of some formulation of the hup: "when the operators corresponding to two physical quantities do not commute the two quantities cannot have simultaneous reality"? If so, is it a correct application of the hup?

If qm in general and the hup in particular are taken to apply only to experimental preparations and recorded data (the mainstream interpretation) and not to the existence or properties of an underlying reality, then how does the hup facilitate EPR's above-quoted statement?

Anyway, EPR or not, there's just no way to ascertain precisely how formal qm corresponds to an underlying reality. An underlying reality can't be talked about objectively, scientifically. This is the problem that the Copenhagen Interpretation (which includes the hup) addresses. The existence and properties of some proposed underlying reality are a matter of speculative inference and can't be definitively evaluated scientifically. When Bohr or someone else says that qm is a complete description of physical reality, I take them as referring to the physical reality that's amenable to objective, scientific study (ie., the material, instrumental preparations and recorded data). And it does seem that qm gives as complete an accounting as can be given of that physical reality.

But predicting something with certainty in QM, itself violates the uncertainty principle, doesn't it?

I don't think so. The hup expresses a quantitative, proportional relationship, mediated by h (the quantum of action) between certain, associated measurements like position and momentum, time and energy, angular position and angular momentum, etc. It says that the product of the uncertainty (the deviation from the average value of a set of measurements) of, say, a set of position measurements, and a set of momentum measurents (wrt similarly prepared systems) can't be less than h.

Wrt just position or just momentum measurements, or measurements associated with certain filter settings in Bell tests, etc., of entangled particles, then it's possible, via applicable conservation laws, to predict with certainty the outcome at A if the outcome at B is known, and vice versa.

Maybe I should rephrase my doubt: As far as i understand, in the EPR paradox, the motion of one particle "somehow" affects the other.

That was the hypothetical alternative that they dismissed, wasn't it?

I would like to know the theory behind this "somehow" effect in detail (only the theory, not the math). Am i understand that it basically is due to the wave nature of matter?

There's no mainstream theory about this, afaik. There is the de Broglie-Bohm 'theory' which exhibits certain 'nonlocal' formal transformations. But there's no way to know if this corresponds to an underlying reality. The formal nonlocality is, prima facie, a mathematical convenience that accords with knowledge of certain statistical results and ignorance of underlying mechanisms.

 

[nismaratwork]

ThomasT, having seen you play footsie for over 60 pages of the EPR thread started by Deepak Kapur, you may understand why I have no desire to engage with you AT ALL in this subject. Perhaps I have the points you make mixed with Zonde's; I admit that after reading a few dozen pages I found them nearly interchangeable... one of you continually referred to Malus' Law as though it were somehow pertinent to the issue of non-locality, BSMs and EPR being discussed.

The issue here is that now we're outside of that particular thread, and it's no longer a relatively knowledgeable group who can easily dismiss your view, and Zonde's lack of support for his. I felt it was wise to give a friendly 'heads up" as to the source of the information, baseless as it was... your entrance here was not intended or desired... at least you don't cite material you've written, but fail to submit for peer review. Was that clear enough?

You end by mentioning deBB, and refer to formal non-locality as a mathematical convenience... I believe the OP and others deserve better than such a rough and inaccurate treatment of the subject material. If you believe that, then by all means present your alternative, or evidence against non-locality which renders this a mathematical exercise. Certainly there are interpretations of QM and features of it such as non-locality, and that's predicated on a lack of understanding as to the "underlying mechanisms" to quote you. Unfortunately, that's not very informative, and would tend to lead away from the crux of the issue: Either mathematical formalism for the sake of results, or useful interpretations to form a valid ontology. You offer neither here, and without Dr. Chinese, or RUTA or others to offer a more... seasoned... approach, I felt it would be amiss to let such assertions slide.

 

[ThomasT]

ThomasT, having seen you play footsie for over 60 pages of the EPR thread started by Deepak Kapur, you may understand why I have no desire to engage with you AT ALL in this subject. Perhaps I have the points you make mixed with Zonde's; I admit that after reading a few dozen pages I found them nearly interchangeable... one of you continually referred to Malus' Law as though it were somehow pertinent to the issue of non-locality, BSMs and EPR being discussed.

The issue here is that now we're outside of that particular thread, and it's no longer a relatively knowledgeable group who can easily dismiss your view, and Zonde's lack of support for his. I felt it was wise to give a friendly 'heads up" as to the source of the information, baseless as it was... your entrance here was not intended or desired... at least you don't cite material you've written, but fail to submit for peer review. Was that clear enough?

Uh ... no. So far, the only on-topic thing you've offered, in post #5, is:

EPR was concerned with challenging the notion of action-at-a-distance.

Which Zonde countered, in post #10, with this point:

This is incorrect of course.
From EPR paper:
"For this purpose let us suppose that we have two systems, I and II, which we permit to interact from the time t=0 to t=T, after which time we suppose that there is no longer any interaction between the two parts."
So EPR uses locality as condition for their own example.

Which point you, apparently, acquiesced to, and then proceeded to write some more off-topic, off-point, and incorrect (wrt what you say I believe) stuff, and continued, in a somewhat derisive tone, with your comments about Zonde. Zonde and ZapperZ have been having a nice discussion about Zonde's concern.

As for the OP taking anybody's word for anything, I agree that he shouldn't take anybody's word for anything. But in asking questions at PF, he'll get a number of perspectives, and different ways of thinking about his concerns.
And regarding your critique of Zonde, well maybe he's wrong, but he's certainly not wrong for questioning the science involved in certain experiments. We're supposed to do that. He's concerned that certain sampling assumptions might not be well founded. I always just assumed that they were well founded, considered things in the ideal, and wondered what can be definitively inferred about an underlying reality from BI violations, as well as the form in which models of entanglement can be rendered. It does seem that we're going to have to do without realism -- at least a certain sort of explicitly local realism, and at least for the foreseeable future.

As for your contention that the contributors to this thread aren't knowledgeable enough to easily dismiss my views, well, I'd consider ZapperZ, DrChinese, ThePhysicsGuy, Zonde, mr. vodka, and DevilsAvocado to be knowledgeable enough. And while a few in that group have undoubtedly, wrt past threads, dismissed certain of my views, they've also, in the process, helped clarify lots of things.

You end by mentioning deBB, and refer to formal non-locality as a mathematical convenience... I believe the OP and others deserve better than such a rough and inaccurate treatment of the subject material. If you believe that, then by all means present your alternative, or evidence against non-locality which renders this a mathematical exercise.

There isn't, and afaik there can't be, any evidence for or against nonlocality. There are interpretations. If the more or less explicit nonlocality of deBB isn't just a mathematical contrivance, then what, in nature, does it refer to? Gravity used to be considered a nonlocal phenomenon. Now it's local.

Certainly there are interpretations of QM and features of it such as non-locality, and that's predicated on a lack of understanding as to the "underlying mechanisms" to quote you. Unfortunately, that's not very informative ...

The following is a quote from this paper, Experimental Study of A Photon as A Subsystem of An Entangled Two-Photon State, located here: http://arxiv.org/PS_cache/quant-ph/pdf/9811/9811060v1.pdf

Following the creation of the pair, the signal and idler may propagate to different directions and be separated by a considerably large distance. If it is a free propagation, the state will remain unchanged except for the gain of a phase, so that the precise momentum (energy) correlation of the pair still holds. The conservation laws guarantee the precise value of an observable with respect to the pair (not to the individual subsystems). It is in this sense, we say that the entangled two-photon state of SPDC is nonlocal. Quantum theory does allow a complete description of the precise correlation for the spatially separated subsystems, but no complete description for the physical reality of the subsystems defined by EPR. It is in this sense, we say that quantum mechanical description (theory) of the entangled system is nonlocal.

... and would tend to lead away from the crux of the issue: Either mathematical formalism for the sake of results, or useful interpretations to form a valid ontology. You offer neither here, and without Dr. Chinese, or RUTA or others to offer a more... seasoned... approach, I felt it would be amiss to let such assertions slide.

The "crux of the issue" regarding "mathematical formalism for the sake of results, or useful interpretations to form a valid ontology" might be phrased as the following question: How are you going to evaluate a proposed ontology of an underlying reality? Why do you think it is that the mainstream interpretation of qm is the instrumentalist/probabilistic interpretation?

 

[dx]

EPR's criterion of 'physical reality' is the following: "If, without in any way disturbing a system, we can predict with certainty a value of a physical quantity, then there exists an element of physical reality corresponding to this physical quantity."

Let two bodies A and B interact. If we know the momentum of A and the momentum of B before the interaction, then the momentum of A after ther interaction can be determined by a measurement of B's momentum after the interaction, without disturbing A. Thus EPR would want to assign a 'physical reality' to the momentum of A. On the other hand, If the body B is sufficiently heavy to serve as a measuring instrument, then we can determine the position of A after the interaction by measuring the position of B. Since again we have not interfered with A, this position also has an element of 'physical reality'. So apparently, the conjugate variables P and Q both have 'an element of physcial reality', even though the quantum mechanical description does not allow such a simultaneous fixation of conjugate quantities.

The solution of the paradox requires the recognition that the above measurements refer to two mutually exclusive experimental arrangements. In the arrangement suited to predict the momentum of A, there must be a latitude in the position of B which is not compatible with its use in the other experiment as a position measuring device, where it must be assumed to be heavy enough that an exchange of momentum with A does not affect its velocity. Thus, the two quantities p and q cannot be simultaneously be given unambiguous meaning, but only as part of two experimental arrangements which are mutually exclusive.

 

[DevilsAvocado]

EPR's criterion of 'physical reality' is the following: "If, without in any way disturbing a system, we can predict with certainty a value of a physical quantity, then there exists an element of physical reality corresponding to this physical quantity."

Very true. Einstein tried to show that there is an underlying reality that has a causal explanation. The Heisenberg uncertainty principle states that this is impossible. John Bell showed that Einstein was wrong, and that QM predictions and EPR-Bell experiments without any doubts verifies that Local Realism is false.

We can have non-local realism, or local non-realism, or non-local non-realism, but not local realism. This is a fact that is accepted by scientific community. Anyone giving a different view is just advocating not peer reviewed personal theories and speculations.

The Heisenberg uncertainty principle is correct. Einstein was wrong.

 

[nismaratwork]

Very true. Einstein tried to show that there is an underlying reality that has a causal explanation. The Heisenberg uncertainty principle states that this is impossible. John Bell showed that Einstein was wrong, and that QM predictions and EPR-Bell experiments without any doubts verifies that Local Realism is false.

We can have non-local realism, or local non-realism, or non-local non-realism, but not local realism. This is a fact that is accepted by scientific community. Anyone giving a different view is just advocating not peer reviewed personal theories and speculations.

The Heisenberg uncertainty principle is correct. Einstein was wrong.

Well said, and it makes me really hope that the foundations community can put something together. I can't help but believe that these debates would be less cyclical if there was something concrete to point to as an alternative to having proved what cannot be true. Unless a theory emerges which matches or exceeds QM's predictions with LHVs (impossible), the need for a framework beyond formalism may be necessary if only to calm some fractious elements.

ThomasT: I didn't intend to respond, but this is simply incorrect:

As for your contention that the contributors to this thread aren't knowledgeable enough to easily dismiss my views, well, I'd consider ZapperZ, DrChinese, ThePhysicsGuy, Zonde, mr. vodka, and DevilsAvocado to be knowledgeable enough. And while a few in that group have undoubtedly, wrt past threads, dismissed certain of my views, they've also, in the process, helped clarify lots of things.

I never said any such thing; I am referring to the list of people who ARE knowledgeable enough to easily dismiss your views, as well as Zonde's. I should also be clear that virtually anyone who is familiar with Aspect's, Bell's, and Zellinger's work is also capable of the same feat... the issue is making you or another understand that in less than 40 pages of text with you running in circles all the while. To be fair, I made a shorter list, but the point remains... please don't misrepresent what I said, especially to the point of reversing it entirely. Beyond that, I have no desire (as I said in my previous thread), to engage with you in the slightest.

 

[DrChinese]

EPR's criterion of 'physical reality' is the following: "If, without in any way disturbing a system, we can predict with certainty a value of a physical quantity, then there exists an element of physical reality corresponding to this physical quantity."

Let two bodies A and B interact. If we know the momentum of A and the momentum of B before the interaction, then the momentum of A after ther interaction can be determined by a measurement of B's momentum after the interaction, without disturbing A. Thus EPR would want to assign a 'physical reality' to the momentum of A. On the other hand, If the body B is sufficiently heavy to serve as a measuring instrument, then we can determine the position of A after the interaction by measuring the position of B. Since again we have not interfered with A, this position also has an element of 'physical reality'. So apparently, the conjugate variables P and Q both have 'an element of physcial reality', even though the quantum mechanical description does not allow such a simultaneous fixation of conjugate quantities.

The solution of the paradox requires the recognition that the above measurements refer to two mutually exclusive experimental arrangements. In the arrangement suited to predict the momentum of A, there must be a latitude in the position of B which is not compatible with its use in the other experiment as a position measuring device, where it must be assumed to be heavy enough that an exchange of momentum with A does not affect its velocity. Thus, the two quantities p and q cannot be simultaneously be given unambiguous meaning, but only as part of two experimental arrangements which are mutually exclusive.

Very nicely stated! It is the *simultaneous* existence of the non-commuting elements of reality which is at question - and is NOT embedded in quantum theory. EPR thought it was "reasonable" to assume they exist simultaneously. Reasonable, yes, but still incorrect as we now understand (a la Bell, Aspect). If you accept their incorrect assumption, you would conclude QM is incomplete. Otherwise, QM appears to be "complete" in the EPR sense.

 

[ThomasT]

If you accept their incorrect assumption, you would conclude QM is incomplete. Otherwise, QM appears to be "complete" in the EPR sense.

I know what you mean, and agree. However, just to clarify, the "EPR sense" of completeness has to do with an underlying reality, doesn't it? And, the 'qm sense' of completeness has to do with material, instrumental preparations and the resulting data, doesn't it? So, can we say that qm is complete insofar as it refers unambiguously to preparations and data, but that we have no way of knowing if it's actually a complete description of an underlying, measurement independent, reality, or even if such a reality exists?

Bringing this around to jobsism's first question, "Can anyone please explain to me why the EPR paradox failed to bypass the uncertainty principle?". I'm not sure if he's asking why EPR violated the hup, or why it didn't violate the hup, or what. I'm not even sure if the hup is applicable to the EPR scenario (a couple of posters seemed to indicate that it isn't). I'm also not sure if the situation dx described is identical to the EPR scenario. If you, or dx, or somebody, would answer these questions, it would be much appreciated.

In the meantime, although you're probably already familiar with them, the following papers might be interesting to those who aren't:

Measuring Position and Momentum Together
http://arxiv.org/PS_cache/arxiv/pdf/0804/0804.4333v1.pdf

The Standard Model of Quantum Measurement Theory: History and Applications
http://arxiv.org/PS_cache/quant-ph/pdf/9603/9603020v1.pdf

 

[ThomasT]

I never said any such thing; I am referring to the list of people who ARE knowledgeable enough to easily dismiss your views, as well as Zonde's. I should also be clear that virtually anyone who is familiar with Aspect's, Bell's, and Zellinger's work is also capable of the same feat... the issue is making you or another understand that in less than 40 pages of text with you running in circles all the while. To be fair, I made a shorter list, but the point remains... please don't misrepresent what I said, especially to the point of reversing it entirely. Beyond that, I have no desire (as I said in my previous thread), to engage with you in the slightest.

What you said, in post #40 of this thread was:

The issue here is that now we're outside of that particular thread, and it's no longer a relatively knowledgeable group who can easily dismiss your view, and Zonde's lack of support for his.

And I replied that I think that the contributors to this thread are knowledgeable enough to easily dismiss my view.

Anyway you still haven't stated what view you, apparently, think needs to be dismissed. I've asked some questions in this thread. If you can help answer them, it would be appreciated.

 

[ThomasT]

The Heisenberg uncertainty principle states that this (that there is an underlying reality that has a causal explanation) is impossible.

Not that I disagree with you DA, but the deeper meaning (beyond the statistical interpretation) of the hup has been, and still is afaik, a subject of some debate. I'm just curious where you read this, or how you (independently) came to this interpretation.

 

[ThomasT]

EPR's criterion of 'physical reality' is the following: "If, without in any way disturbing a system, we can predict with certainty a value of a physical quantity, then there exists an element of physical reality corresponding to this physical quantity."

Let two bodies A and B interact. If we know the momentum of A and the momentum of B before the interaction, then the momentum of A after ther interaction can be determined by a measurement of B's momentum after the interaction, without disturbing A. Thus EPR would want to assign a 'physical reality' to the momentum of A. On the other hand, If the body B is sufficiently heavy to serve as a measuring instrument, then we can determine the position of A after the interaction by measuring the position of B. Since again we have not interfered with A, this position also has an element of 'physical reality'. So apparently, the conjugate variables P and Q both have 'an element of physcial reality', even though the quantum mechanical description does not allow such a simultaneous fixation of conjugate quantities.

The solution of the paradox requires the recognition that the above measurements refer to two mutually exclusive experimental arrangements. In the arrangement suited to predict the momentum of A, there must be a latitude in the position of B which is not compatible with its use in the other experiment as a position measuring device, where it must be assumed to be heavy enough that an exchange of momentum with A does not affect its velocity. Thus, the two quantities p and q cannot be simultaneously be given unambiguous meaning, but only as part of two experimental arrangements which are mutually exclusive.

Thanks for weighing in, dx. My solution to the 'paradox' has been that the hup doesn't apply to the EPR scenario. But then I never really put much stock in EPR and might have been operating under a misapprehension. Anyway, in EPR isn't there a joint unambiguous measurement, p at A and q at B, then the subsequent inclusion of deduced attributes, and then on to their argument? What am I missing?

 

[dx]

I'm also not sure if the situation dx described is identical to the EPR scenario.

A situation identical to EPR can be treated as follows: Take a diaprhram with two narrow slits, and let two particles of known momentum pass through them. If the momentum of the diaphragm is known exactly before and after the particles pass through, then A = p₁ + p₂ and B = q₁ - q₂ are exactly known, which is compatible with quantum mechanics since [A, B] = 0. Therefore, if we measure p₁, then we know that p₂ = B - p₁. Or, if we measure q₁, then we know q₂ = q₁ - A. So even though we are presented with a free choice of determining either p₂ or q₂, of the second system by measuring only the first system, q₁ - A and B - p₁ do not commute.

The point is that the criterion formulated by EPR is revealed to be ambiguous in light of the actual conditions that we are faced with in atomic physics, where concepts such as 'state' and 'behavior' cannot retain their usual meaning due to the existence of the quantum of action. The feature of individuality that underlies the comprehension of atomic phenomena is irrational within the scope of classical visualization and mode of explanation. However, any attempt of extrapolation of our causal spacetime description into the atomic domain must ultimately rest on the heavy scales and clocks, whose behavior is and must be accounted for classically. Thus in judging the form that such an extrapolation can take, we are essentially involved in an analysis of the possibilities of definition and observation, with due attention paid to the quantum of action, whose consideration is inevitable in any such analysis. Such an analysis, performed by Bohr, has shown that any situation which permits a causal account of a quantum process excludes a spacetime account of that process, and vice versa. Thus, the description of physical reality provided by quantum mechanics cannot be a causal-spacetime description, but a 'complementary' description, where the role of the measuring instruments is central. In fact, the quantum mechanical formalism must be viewed simply as a tool for such a complementarity description, whose well-defined application must always refer to the exact conditions of the experiment.