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I Explanation of the EPR paradox

  1. Jan 6, 2017 #1
    I am not sure I understand exactly what was the situation was with the EPR paradox
    Example: for simplicity: two electrons are coupled. One with spin up and the other with spin down.

    Since they are coupled there are one state.

    Separate the electrons one in one galaxy and the other in another galaxy.

    Reverse the spin on one electron and the other immediately reverses.
    Is this true?
    Granted that Einstein lost the point, but this would seem to raise the question
    If true then this occurs with no relevance to space and time. The wave function for this situation pays no intention to space and time.
    This raises the question of what space and time are.
    Everything elsewhere in physical equation seem to have space and time, if only as a matter of convenience,
    except this.
     
    Last edited by a moderator: Jan 6, 2017
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  3. Jan 6, 2017 #2

    phinds

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    There is no action at a distance and no communication of information when one particle of an entangled pair is measured. All that happens is that you now know that if and when the other particle is measured for the entangled characteristics it will be the opposite of the first one measured. Reversing the spin on an electron would measure the spin and thus would tell you what the spin of the other particle would be if measured but it would not cause the other particle to reverse spin.
     
  4. Jan 6, 2017 #3

    PeterDonis

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    No. Reversing the spin on one of the electrons breaks the coupling. More precisely, the coupling assumes that the electrons aren't subjected to any other interactions; but reversing the spin of one of them is an interaction.

    Bear in mind that the model you are using is non-relativistic. A relativistic model, using quantum field theory, still has entanglement (and still with the limitations given above), but it assigns quantum field operators to individual events in spacetime, rather than wave functions. The entanglement shows up as correlations between the field operators.
     
  5. Jan 7, 2017 #4
    I may have misunderstood something along the way but I thought there was communication between the entangled pair of particles and changing the spin state of one electron instantly changed the state of the other? The reason being because it is not possible to know two or more components of spin simultaneously.

    For example, I could measure the Z component of one of the entangled electrons and then Y component of the other. As I know the spin states of the electrons are opposite, then I would know both these components of spin. To overcome this, as I understood it, when the spin state is measured on the first electron, it changes the spin states of both electrons instantly, so when the second electron is measured in a different component of spin, in isn't telling us anything about the original spin states.

    Is that not the case?
     
  6. Jan 7, 2017 #5

    PeterDonis

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    This is an interpretation that is often found in pop science sources, but it's only an interpretation, and is not required by the actual physics. The key point to be aware of is that the correlations between measurements results on the entangled particles, the ones that violate the Bell inequalities and therefore suggest some kind of "instantaneous communication" between them, are only detectable after the results are compared--i.e., after the results from both measurements are communicated by ordinary, light-speed (or slower) information transmission. In other words, there is no way to actually transmit information faster than light usi EPRng entanglement.

    No, you wouldn't. "The spin states are opposite" only applies, as I said, if both electrons undergo no interactions. Measuring the Z component of one and the Y component of the other means they both have undergone interactions. The "opposite spin states" before those measurements are made allow you to predict the correlation between the measurement results, but that's all.

    Again, this is an interpretation, not required by the actual physics.

    The reason EPR considered this situation a "paradox" is that they had an intuitive feeling that the correlations that are possible with quantum entanglement were not consistent with the kind of physical model of what is going on that they were comfortable with. But they didn't really have a way of describing the kind of model they were comfortable with in precise terms. That didn't come until several decades later, when Bell came up with a mathematical criterion--that the function describing the correlations between measurement results must be factorizable in a certain way--that appeared to capture EPR-type intuitions pretty well. Then Bell showed that the predictions of quantum mechanics violate that mathematical criterion. But that doesn't mean "instantaneous communication" must be going on; it just means EPR-type intuitions are wrong. Often the solution to a problem like that is to retrain your intuitions; IMO that is the best response in this case.
     
  7. Jan 8, 2017 #6

    bhobba

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    No.

    Put a red slip of paper in one envelope and a green in another. Move one envelope to the other side of the Galaxy. Open one envelope and you know immediately the color of the other. They are correlated, Nothing weird at all. The same with EPR - they are correlated. The difference is they have some different statistical properties because QM is silent on if objects have properties independent of observation. With the slips of paper they remain red and green at all times and that leads to slightly different statistical properties. Now what if we insist they have properties independent of observation then it turns out there must be ftl communication. But if you dont insist then there is no need for FTL.

    There is more to be said in regard to Quantum Field Theory because it has a slightly different idea of locality:
    https://www.physicsforums.com/threads/cluster-decomposition-in-qft.547574/

    But best to start a new thread if you want to expore it. Basically though since they are correlated then locality (in the QFT cluster decomposition property sense) doesn't really apply. You can define it to apply, but it creates an interesting third alternative of simply saying its a meaningless question - its my personal view. It must be emphasized however its no netter or worse than any other view - its just an interesting one not often discussed.

    Thanks
    Bill
     
    Last edited: Jan 8, 2017
  8. Jan 8, 2017 #7
    I jut want to clarify one thing. The red paper remains red. The green paper remains green. Changing the red paper to green, with the expectation that the green paper changes red is meaningless?
     
  9. Jan 8, 2017 #8

    DrChinese

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    Entanglement does not mean that you can do something to one part of a system and expect that the other part will match the change. Were that true, you could send messages faster than light. That is NOT a prediction of quantum mechanics.
     
  10. Jan 8, 2017 #9
    Does that mean there is no communication between the two entangled particles?

    If so then how should we think of the correlation between the pair of entangled particles? I.e. it's not hidden variables and it's not action at a distance?

    Is it just that that the paired particles have a correlation but that correlation is probabilistic and not something that can be predetermined even by knowing all the variables?
     
  11. Jan 8, 2017 #10

    phinds

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    The pair of entangled particles need to be thought of as a single system that has two characteristics as such that the characteristics have opposite values and when you measure the value of one of the characteristics you automatically know what you will get if/when you measure the other characteristic. The fact that one part of the system is physically removed from the other part is irrelevant.
     
  12. Jan 8, 2017 #11

    PeterDonis

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    How do you define "communication"?
     
  13. Jan 8, 2017 #12
    In very basic layman's terms, can any change (measured or unmeasured) in the properties of one of the entangled particles cause changes to other?
     
  14. Jan 8, 2017 #13
    Maybe it's just my understanding of what a system is. I'd define a system as a set of interacting entities, If the two particles don't interact in any way then I understand them to be two different systems.
     
  15. Jan 8, 2017 #14

    PeterDonis

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    What do you mean by "cause"?

    Do you see the pattern here? You are trying to use ordinary language terms as though they had obvious precise meanings that could be applied to quantum entangment. That's simply not the case. Quantum mechanics is highly counterintuitive; you can't just use your ordinary intuitions and ordinary reasoning using ordinary language and expect to understand it. Entanglement is a property of quantum systems that has no analogue in anything you are familiar with. It's not ordinary "communication", it's not ordinary "one causes changes to the other", nor is it the ordinary lack of either of those things; it's not anything ordinary. So there's no point in trying to fit it into the ordinary categories you are trying to fit it into. It won't work.

    Same problem here; what does "interacting" mean? No ordinary meaning of that word describes quantum entanglement; but no ordinary meaning of "non-interacting" describes it either. It's not like anything you're familiar with, so you can't expect to describe it in familiar terms.
     
  16. Jan 8, 2017 #15

    bhobba

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    The red and green paper is just an example of classical correlation. It is different to quantum correlation for the reason I cited - namely in QM its silent on if the objects involved have that property before observation. That is not the case with the red and green papers - they are red and green at all times. But the key point is its still just a correlation. In EPR due to the arrangement the electrons or photons or whatever are also correlated. That's all it is, just like the red and green papers its just correlation. People are not in any way concerned about other types of correlation - its obvious whats going on - and in fact its exactly the same in QM. But some people get really concerned about it. Why? Its due to Bells theorem. But Bells theorem only applies because the rules of QM are different. I could explain Bell but Dr Chinese does it very well:
    http://www.drchinese.com/Bells_Theorem.htm

    The point is its just a correlation. Don't be confused by side issues that obscure it. That's why I like the idea from QFT where its excluded from the cluster decomposition property so you never run into these issues in the first place.

    Bell is VERY important but please understand what its saying - it is not sayng FTL is necessarily going on. What its saying is if you want it to conform to usual intuitions like the red and green papers you cant have the usual idea of locality ie you need FTL.

    Thanks
    Bill
     
    Last edited: Jan 8, 2017
  17. Jan 9, 2017 #16

    Boing3000

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    On layman's term, exactly how you would think of a correlation between shoes in a shoe box.
    In QM this correlation is a little more spooky, because the correlation depends on the way you open the box (the angle of the measuring device for spin).
    That means that a some angle the correlation vanishes (if Alice gets a "right shoe", Bod has a only 50/50 chance to get a left shoe)
    At the some identical angle, both observers always get an perfectly correlated result (with is not surprising, that's the way shoes are suppose to be packed :wink:)

    Bell's showed mathematically that you'll have two different statistical result by considering the "shoe box"/entangled state as, either copied/cloned/local to each particle (locality), or that there is only one shoe box all along (non-locality)

    Experiment shows that it is the later. Thus you clearly cannot "change" anything, in another "shoe box", because there seems to be only one. You can only open it and observer (passively) the random outcome. The correlation can only be verified later one if Alice an Bob will get in "touch" an compare all their random results...
     
  18. Jan 9, 2017 #17
    I understand your point, but I might not have made mine very well. We can obviously make certain predictions as regards the correlations between entangled pairs of particles. For example the experiments that show the violation of Bell's theorem still predict very accurately a probably of 0.25 of finding a match when selecting the 0, 120 and 240 degree measuring angles. The underlying quantum principles may not be intuitive or easily understood, but IMO the results still tell us something about the quantum world and that doing the same thing (in this case) always yields the same results.

    So the crux of matter for me was trying to get a better understand of what drives this behaviour, or in this case, try and eliminate the what doesn't. And from what I understand is it isn't "action at a distance" or in other words, when one particle is measured it doesn't communicate with the other and tell it what to do. The correlations that we see at different angles is something else that is not clearly understood.

    Thanks, I like the analogy. But as I mentioned above I think where my reasoning keeps taking me is that if we can do the same experiments over and over again and always get the same results (i.e. quantum predictions match experiments), then there is some underlying principle driving that behaviour albeit something we don't understand fully.
     
    Last edited: Jan 9, 2017
  19. Jan 9, 2017 #18

    DrChinese

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    Past the quantum formalism itself, no one can (scientifically) describe it any further. That is what the interpretations attempt to accomplish. As they all currently make the same predictions, there is no way to experimentally distinguish them.

    The upshot: take your pick. :smile:
     
  20. Jan 10, 2017 #19

    PeterDonis

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    Some off topic, speculative posts and responses have been deleted. Please bear in mind the PF rules on personal theories, and please keep posts on the thread topic. Thanks!
     
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