
#1
Feb613, 12:03 AM

P: 14

I know Bell has been discussed ad nauseam on these forums, but there's something I'm having trouble coming to terms with: the wording and usage of Bell's theorem seems to be nearsighted. "No physical theory of local hidden variables can ever reproduce all of the predictions of quantum mechanics." How is it so that no sufficiently complex model could ever be conceived which produces the same predictions? For example: in he case of measuring the polarization or spin of entangled photons, QM predicts a correlation proportional to the cosine of the difference between the two angles in which the photons are measured. It's claimed that ANY local realist model would predict a linear correlation. How can this be said with any real confidence? Is it possible that we simply don't fully understand how spin and polarization are quantized? Is it possible that there are hidden variables, but the machinery is so complex as to appear nonlinear?




#2
Feb613, 05:48 AM

Homework
Sci Advisor
HW Helper
Thanks ∞
PF Gold
P: 11,009

Probably enough to call into question the mathematical foundation of the standard model in particle physics. This is big league stuff. That may be the case  we certainly do not understand everything  but proving it would be bigger than the Nobel prizes. Look through the literature and you'll see that people have been trying really complex systems and trying to catch a hidden variable. If it helps  think of it as a statement conditional upon a particular model holding true. It's a prediction of what you'd have to do to prove the model false. 



#3
Feb613, 09:01 AM

Sci Advisor
PF Gold
P: 5,146

a) Matches the predictions of QM for a difference of 120 degrees. b) Provides perfect correlations at the same angles. c) Is realistic, ie has values for angles regardless of whether they are actually tested. Example: 0 degrees / 120 degrees / 240 degrees + +   +  etc. (after you put together about 10 or 15 you will see the effect, which is that you cannot get sufficiently close to what the quantum mechanical prediction is). You cannot prepare such a data set by hand, even knowing what you want to accomplish! That is why no model can exist  there is no possible result set that fits the above bill unless you allow some kind of nonlocal communication between Alice and Bob. *I would strongly recommend that you familiarize yourself with the core of the Bell proof first. Or you can ignore nearly 50 years of research and analysis on the matter. This has been hashed through on many posts here, you can look back at some of those too. 



#4
Feb613, 11:46 AM

P: 79

Is Bell's theorem nearsighted?
What Simon Bridge and DrChinese said. But I'll add my two cents.
The observed correlations are nonlinear. There's nothing else to refer to. 



#5
Feb613, 04:38 PM

P: 14

Thanks for the responses. But so far these answers are of the "just because" form. I am familiar with the theorem. Indeed, DrChinese, I've ready your own writeups on both Bell and EPR in the past and found them helpful. But perhaps the central question was missed. The theorem is predicated on local hidden variables producing a linear correlation in measurement with regard to the measuring angle. Why is this so? I understand quite well that QM predicts a variable/nonlinear correlation and that observation verifies it. I understand that the proofs are thorough regarding the ways in which linear correlation =/= the types of correlations in measurements we see in QM. But specifically, why can there never be a LHV model which produces nonlinear correlations with regard to measurement angle?
Consider the attached image. It represents a scenario in which you are trying to measure the color of a point on a single sphere (which is divided in half into two colors and which is in a random orientation) and a corresponding point at some angle relative to the first point. A is the first measured point, B a the second, and C a the third. The angle between A and B is θ, as is the angle between B and C, with respect to a 2 dimensional measuring plane shown in green. The probably of getting a matching color on B is proportional to the portion of the circumference marked in blue which is the same color as A. If you run this for all possible alignments of D (the axis of the sphere) then you get the same correlation between A and B and between B and C, but because the portion of correlated points on the illustrated circumference does not vary linearly with the angle difference, you will find that the probability of correlation between A and C varies nonlinearly with respect to θ. 



#6
Feb613, 05:10 PM

Sci Advisor
PF Gold
P: 5,146

So you will see that your example cannot be both realistic and match QM. Again, try to generate triplets at A=0/B=120/C=240 degrees and you will see it cannot be done. And don't forget you need perfect correlations too! So AB=BC=AC=.25 (or .75 depending on whether you are modeling Type I or Type II PDC). The best you will be able to do is AB=BC=AC=.333 (or .666). Normally, when this subject comes up, the first thing you want to do is to ignore the realism requirement. That requirement is, to be precise, the idea that counterfactual outcomes can be presented (i.e. you can provide an value for any hypothetical measurement). Then the locality requirement is that such outcomes are independent of actual choice of measurement elsewhere. What Bob chooses to measure does not affect Alice's outcome. If you deny that counterfactual outcomes exist, or need to be supplied, then you are simply accepting the standard viewpoint and agreeing with Bell. 



#7
Feb613, 05:18 PM

Sci Advisor
PF Gold
P: 5,146

Or to put it another way: the linear idea is not a stringent requirement. More anecdotal than anything. It is absolutely not part of the formalism. So objecting to that is not an undercut to Bell.




#8
Feb613, 07:06 PM

PF Gold
P: 670

As an aside, Christopher Fuchs has pointed out that some physicists do interpret Bell's to imply nonlocality because Bell's inequality only assumes locality not realism:




#9
Feb613, 08:06 PM

Sci Advisor
P: 1,185

www.iafe.uba.ar/e2e/phys230/history/moon.pdf 



#10
Feb813, 11:13 PM

PF Gold
P: 363

coincidence rate must be linear or less: a = a_{1}a_{2}= difference in polarization direction. R_{0} = aligned perfectly. Coincident rate , D_{1} = R_{0}R(a) And D_{2} =R_{0}R(2a) Bell required D_{2}≤ 2(D_{1}) QM predicts D_{1} = R_{0}[1cos^{2}(a)] D_{2}=R_{0}[1cos^{2}(2a)] So while linearity is not an assumption it is an expectation , when not met , is given to demonstrate inequality violation as in case where a = 20^{o} and D_{2} is about 4x D_{1} 



#11
Feb913, 02:37 AM

P: 14

so... what am I missing? The key difference between what I'm suggesting and the example you use is that given the spherical geometry, the proportion of possible correlations to anticorrelations varies nonlinearly as θ. This doesn't match QM exactly, but that isn't my point. If using a spherical interpretation can generate an unexpected prediction, then how (and please be thorough) can we be sure that no one would ever discover an lhv model which matches observation? 



#12
Feb913, 12:23 PM

Sci Advisor
Thanks
P: 2,957

All this discussion of whether the correlation is linear or not is besides the point, as Bell's argument is not "predicated on local hidden variables producing a linear correlation in measurement with regard to the measuring angle". Indeed, in the discussion leading up to equation #7 in the paper, Bell shows how a local hidden variable model can lead to a cosθ relationship between measurements; and with your ball you've constructed another lhv model with nonlinear correlations. Instead, the flow of the argument is roughly as follows: A) If we assume the truth of equation #2 in the paper, mathematical logic leads us to Bell's inequality. B) Quantum mechanics, under some circumstances, makes predictions that disagree with Bell's inequality. C) Therefore, if QM is correct, then the assumption behind equation #2 in the paper must be false. Now that experiment has verified the quantum mechanical prediction, we pretty much have to admit that equation #2 in the paper is not correct. Thus, LHV theories have only two ways forward: 1) Find a flaw in the experimental procedures, in which case maybe equation #2 has not been proven false after all. This leads us into the endless (and increasingly futile) search for "loopholes" in the experiments. 2) Find a theory that doesn't imply the truth of equation #2, but that would still be generally considered a realistic LHV theory (an informal definition would be "would have satisfied Einstein, Podolsky, and Rosen"). This is a very tough challenge, as equation #2 is pretty much a restatement of what an EPR local realistic theory would be. (There is room for endless discussion about exactly which classes of theories are excluded if Bell's equation #2 does not hold in general.... and I have no doubt that this thread in general, and this post in particular, will provoke more of this discussion... But that does not change the fact that a broad class of theories are excluded, including all those that would have been generally accepted as LHV in the preBell world). 



#13
Feb913, 02:21 PM

Sci Advisor
PF Gold
P: 5,146

You can hand pick the values yourself, and then build a model to suit. But it won't make any difference because the values will still not be able to match an observer independent model. Clearly, you cannot have an expectation of .23275 for the angle settings 0/120/240 degrees. If you have 0/120 match that percentage, and 120/240 match that percentage, the likelihood of 0/240 matching will be something like 50%. Oops, that means it is an observer dependent result because it is dependent on which pair is actually being measured. To help you see this, and I will use the QM value of .25 instead of your value of .23275 to make it clear: 0 120 240 +  +   + +    +  0/120 matches 25%, 120/240 matches 25%, so all good. But when that happens, you end up with 50% for 0/240 matches! That means it is observer dependent and therefore no realistic. Once you adjust your model to be observer independent, your best ratio will be .333. Now I know what you are thinking, "my model IS observer dependent". Well, sorry, no, it isn't as I am showing you. The test I have given above is standard. If it is realistic, you must be able to produce values for any angle setting independent of which other is selected. By the way, there have been a number of attempts to exploit ideas like yours to produce a local realistic model (of course they all are failures). See for example Caroline Thompson's "Chaotic Ball" model. 



#14
Feb913, 08:04 PM

PF Gold
P: 670





#15
Feb913, 08:37 PM

Sci Advisor
Thanks
P: 2,957

I believe that most Bellquestioners haven't made through that stage yet. They're not ready for a discussion of whether to give up locality or realism because they haven't yet accepted that we can't have both. 



#17
Feb1013, 03:20 PM

P: 14

That said, thanks to everyone for your responses. One difficulty here may be that theoretical physics has advanced to such a point that it truly is natural philosophy in the classical sense. And along with any philosophy, we easily get tied up in terminology. http://arxiv.org/pdf/quantph/0607057v2.pdf I'm also curious if a oneelectron type theory, perhaps expanded to other particles (one photon, one quark, etc) could function as a nonlocal realist explanation to bell violation. 



#18
Feb1013, 05:55 PM

Sci Advisor
PF Gold
P: 5,146

*And I mean in the sense that he thinks he is right and everyone else is wrong. 


Register to reply 
Related Discussions  
Bell's Theorem, nonlocality  Quantum Physics  16  
Bell's Theorem  Advanced Physics Homework  2  
Bell's Theorem  Quantum Physics  0  
Bell Theorem  Quantum Physics  115  
Bell's theorem  General Physics  27 