Spin difference between entangled and non-entangled

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  • #176
atyy
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Ok. So before Bell came along, what was the excuse to even begin entertaining the idea EPR argument in both cases would not actually hold true?
The EPR argument is that local hidden variables exist. One reason hidden variables were not widely considered was that von Neumann produced an influential but wrong proof that hidden variables, either local or nonlocal, could not exist. The error in the proof was probably known to Einstein and some others, but not the general community. In the 1950s, Bohm produced a nonlocal hidden variable theory that explicitly demonstrated that von Neumann's proof was wrong in a way that could not be corrected. Einstein knew Bohm's hidden variable theory, but it was not the sort he was looking for, since Bohm's theory was nonlocal. Bell later showed that the local hidden variable theory Einstein had hoped for was not possible.

Although the question of hidden variables is related to EPR and locality, the deeper question that Einstein was trying to address was the issue of reality. The problem is that in quantum mechanics, it is very difficult to consider the wave function "real". In typical quantum mechanics, the outcomes of measurements are real in the usual commonsense way, but the wave function is not necessarily real in that sense, and is just a way to calcuate the probabilities of experimental outcomes. This lack of reality means that QM cannot answer the question of whether the moon is there when we are not looking. So if QM is truly fundamental, it seems difficult to retain our naive notion of reality. But if hidden variables exist, then QM is not truly fundamental, and we don't have to give up our naive notion of reality.
 
  • #177
stevendaryl
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QM was working well. So opinion split, some thinking QM was "complete" (really as complete as it gets) and others (such as EPR) thinking QM was a stopgap. This debate continued for decades.
I think that there is a sense in which Bell's theorem really didn't do much to the practice of physics. The sort of theory that was proved to be impossible, according to Bell, was not the sort of theory that anyone had any idea for developing, anyway. Einstein may have hoped for one, but nobody had a clue what such a theory might look like.
 
  • #178
DrChinese
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Einstein died shortly before Bell discovered his theorem, and for me one of the most tantalizing unanswerable questions is what Einstein would have done with Bell's theorem if he had been around to see it.
Me as well! I think he would have acknowledged it quickly and moved on to ponder its impact. :)
 
  • #179
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Me as well! I think he would have acknowledged it quickly and moved on to ponder its impact. :)
That's what I'd expect... And what might have come out of the pondering? That is the tantalizing part.
 
  • #180
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I think that there is a sense in which Bell's theorem really didn't do much to the practice of physics. The sort of theory that was proved to be impossible, according to Bell, was not the sort of theory that anyone had any idea for developing, anyway. Einstein may have hoped for one, but nobody had a clue what such a theory might look like.
It certainly doesn't seem that anyone ever much looked back to a classical approach as envisioned by EPR. QM led to QED and ultimately the Standard Model we have today, which relates the Weak and Strong forces as well.
 
  • #181
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Thanks to responses I received in this thread I satisfied my curiosity regarding solid line on the graph below.


MalusQC.png


Fig. 1 The realist prediction (solid curve) for quantum correlation in an optical Bell test.
The quantum-mechanical prediction is the dotted curve


Now I would like to understand why the result of Aspect experiment (doted line) is a proof for entanglement.

What are the most DIRECT evidences that the “entangled” photons in this experiment are indeed entangled - have perfectly opposite polarization?
 
  • #182
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What are the most DIRECT evidences that the “entangled” photons in this experiment are indeed entangled - have perfectly opposite polarization?
Are you asking what the evidence is that they have perfectly opposite polarization after they've been measured?

Every time anyone has ever set two polarizers at 90 degrees so that we're checking for "perfectly opposite" and sent one photon into each one, they've always both passed or neither passed. That was a well-known experimental observation decades before the Aspect experiment.
 
  • #183
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Thanks to responses I received in this thread I satisfied my curiosity regarding solid line on the graph below.


MalusQC.png


Fig. 1 The realist prediction (solid curve) for quantum correlation in an optical Bell test.
The quantum-mechanical prediction is the dotted curve


Now I would like to understand why the result of Aspect experiment (doted line) is a proof for entanglement.

What are the most DIRECT evidences that the “entangled” photons in this experiment are indeed entangled - have perfectly opposite polarization?
The obvious thing is that the quantum predictions are matched for entangled pairs, and that is different than the predictions for other hypotheses. What more do you want? :)
 
  • #184
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Are you asking what the evidence is that they have perfectly opposite polarization after they've been measured?
Yes, this is how I should ask.
Every time anyone has ever set two polarizers at 90 degrees so that we're checking for "perfectly opposite" and sent one photon into each one, they've always both passed or neither passed
So how this result is translated into conclusion that EACH photon in the pair has "perfectly opposite" to each other (90 degrees difference) polarization.
The Aspect's (and others') experiment is statistical in nature (performed on series of photons). The wave function, is also, as I understand, could be interpreted statistically - say it applies only to the average result. In this case, in absence of perfect correlation (entanglement) for EACH photon pair we still may be ended up with the statistically perfect correlation - doted line and will be in agreement with the statistical interpretation of QM.

If this is true, would it undermine "spooky" action on distance?
 
  • #185
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The obvious thing is that the quantum predictions are matched for entangled pairs, and that is different than the predictions for other hypotheses. What more do you want
Could this match, between QM and the experimental results for entangled pairs, be limited to the statistical average only? Do we have any ground to extend this statistical result into behavior of individual pairs?
 
  • #186
DrChinese
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... The wave function, is also, as I understand, could be interpreted statistically - say it applies only to the average result. In this case, in absence of perfect correlation (entanglement) for EACH photon pair we still may be ended up with the statistically perfect correlation - doted line and will be in agreement with the statistical interpretation of QM.

If this is true, would it undermine "spooky" action on distance?
No. When the angles are integer multiples of 90 degrees, there is perfect correlation (or anti-correlation) for every pair. This demonstrates entanglement very nicely. At other angles, the statistics support quantum non-locality via agreement with predictions. As to action at a distance: this is dependent on your interpretation.
 
  • #187
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When the angles are integer multiples of 90 degrees, there is perfect correlation (or anti-correlation) for every pair
Before follow your explanation I have a question about Fig.1.
I thought that at 90 degrees between polarizer QM prediction should be the same as realistic prediction. Why the realistic prediction on the Fig. 1 is only 50%?
 
  • #188
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Why the realistic prediction on the Fig. 1 is only 50%?
By talking to my self I understood why it is 50%.
 
  • #189
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Every time anyone has ever set two polarizers at 90 degrees so that we're checking for "perfectly opposite" and sent one photon into each one, they've always both passed or neither passed. That was a well-known experimental observation decades before the Aspect experiment.

If the perfect correlation of entangled photons that passes two polarizers at 90 degrees was observed before Aspect experiment, do we really need Bell's theorem, rotating polarizers, and other complex experiments to prove entanglement. Apparently even for polarizers at 90 degrees the realistic prediction would never explains 100% correlation ?
 
  • #190
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If the perfect correlation of entangled photons that passes two polarizers at 90 degrees was observed before Aspect experiment, do we really need Bell's theorem, rotating polarizers, and other complex experiments to prove entanglement. Apparently even for polarizers at 90 degrees the realistic prediction would never explains 100% correlation ?
That is not the case. It depends on the particular realistic theory. As mentioned previously, that solid line is one possibility and it has certain advantages. Others do other things well. None of them match QM. So you have asked different questions: a) what proves or demonstrates entanglement; and b) why don't local realistic theories match QM vis a vis entanglement. The answer to a) is perfect correlations demonstrate entanglement. Matching the predictions of QM at other angles (where there are no perfect correlations) demonstrates entanglement too.

The answer to b) is that in local realism, there is no ongoing state of entanglement. It might be something, but it isn't entanglement because separated particles can have no ongoing physical connection. And not surprisingly, local realism cannot mimic entanglement (as we now know).
 
  • #191
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The answer to a) is perfect correlations demonstrate entanglement.
So far I able to follow your explanation, but I probably missing something.

If the result of the the perfect correlation of entangled photons (that passes two polarizers at 90 degrees) was a well-known decades before the Aspect experiment, what was the explanation for this phenomenon at that time? How long ago the first experiment of this kind was performed? Did Einstein know about result of this experiment?
 
  • #192
DrChinese
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So far I able to follow your explanation, but I probably missing something.

If the result of the the perfect correlation of entangled photons (that passes two polarizers at 90 degrees) was a well-known decades before the Aspect experiment, what was the explanation for this phenomenon at that time? How long ago the first experiment of this kind was performed? Did Einstein know about result of this experiment?
As far as I know, the first entanglement experiments were in the 70's. So Einstein did not know. Entanglement of photons and entanglement in general was not well understood for many years after EPR. The perfect correlations of EPR were not really specific, more general and theoretical.

Keep in mind that the conflicts between local realism and QM were not known until after 1965. So for many years, no thought was given that entanglement statistics disprove local realistic theories.
 
  • #193
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the conflicts between local realism and QM were not known until after 1965. So for many years, no thought was given that entanglement statistics disprove local realistic theories.

In case of experiment with two polarizers at 90 degrees it isn't just a statistic; it should be demonstrated (practically) for each individual pair passes polarizer.
So, if at the time of EPR paper, someone confront Einstein with the Gedankenexperiment about entangled photons that passes two polarizers at 90 degrees his prediction (most probably) would be 50% correlation, while the supporters of the mainstream QM would come up with 100% correlation?

How difficult would be performing this experiment in 1935?
 
  • #194
DrChinese
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In case of experiment with two polarizers at 90 degrees it isn't just a statistic; it should be demonstrated (practically) for each individual pair passes polarizer.
So, if at the time of EPR paper, someone confront Einstein with the Gedankenexperiment about entangled photons that passes two polarizers at 90 degrees his prediction (most probably) would be 50% correlation, while the supporters of the mainstream QM would come up with 100% correlation?
Einstein would say that (100% correlation) was consistent with a local hidden variables theory (and proof of entanglement, which it is). The 50% would be a different local hidden variables theory (no entanglement though). There are an infinite number possible, and they all disagree with experiment at one angle or another. The 100% ones needed Bell to show us the flaw.
 
  • #195
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Einstein would say that (100% correlation) was consistent with a local hidden variables theory (and proof of entanglement, which it is).
Because Einstein didn't believe in entanglement he would be in difficult position to offer (not exotic) realistic explanation for the 100% correlations. I am still puzzling why Bohr and others didn't challenge EPR paper with this simple Gedankenexperiment. I guess I still missing some important details.

Let's conceder this Gedankenexperiment and to follow all events that lead to the 100% correlation for the detectors set at 90 degrees. I will conceder two scenarios:

1). When the entangled pair interacts with the closer polarizer the wave function collapses yielding two independent photons having perfectly opposite polarization that also perfectly aligned with corresponding polarizer. In this case both photons will be detected and this contributes to the 100% correlation.

2). However if after the wave function collapse the first photon is not fully aliened with the detector it still may pass detector (with some probability) however I am not sure that this insure the 100% pass for second photon. This pair may erode "picture perfect" 100% correlation.

It is why I don't see how we can observe 100% correlation in this experiment even entanglement is true. What I am missing?






If the polarisa that is closer to the source interact with entangled particle
In the his prediction (
As I understand the 100% correlation result from the fact that when one photon
 

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