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I Quantum entanglement information

  1. Aug 9, 2017 #1
    It is said that the measurement done on a particle instantly affects its entangled pair because Bell's theorem excludes a hidden variable. That means there is a cause and an instant effect at a distance. Say we have two entangled particles A and B. If there is no hidden variable then the state of the particles are not definite until measured. After we measure the first particle and find it with spin |up> for example, does it mean now particle B is in a definite state |down>?
    If we consider both particles as a single system, after the measurement of the first particle is the system state determined on the measurement axis?
     
    Last edited: Aug 9, 2017
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  3. Aug 9, 2017 #2

    DrChinese

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    Yes, it acts as single system. It is "as if" there is a distant "effect". As far as anyone can tell, the nature of a measurement "here" leads to a matching change "there".

    I don't know I would characterize it as a "cause" though, as the specific result is random. Also, there is no causal direction to speak of, as the sequence of measurements seems to make no difference.
     
  4. Aug 9, 2017 #3

    PeterDonis

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    No, it doesn't. It just means that quantum entanglement cannot be modeled using local hidden variables.

    The proper definition of "causality" is that spacelike separated measurements must commute, i.e., the results can't depend on the order in which the measurements are made. Spacelike separated measurements made on entangled particles satisfy this requirement. Note that this requirement says nothing about "instant effect at at distance"; there is no need to invoke that. Such a need would only be present if the results of the measurements depended on which one was done first.
     
  5. Aug 10, 2017 at 3:22 AM #4
    If the events are spacelike separated they cannot be cause and effect and since they states are ways opposite there should have a common cause which points to a hidden variable. If there is no hidden variable then there is an instant effect at a distance. It is clear that there is no effect at a distance. It is also clear that there is no hidden variable. It is also clear that the above conclusions are contradictory.
    No communication theorem doesn't demonstrate information doesn't travel faster than light but only we cannot send information (either faster than light or not )using entanglement.
     
  6. Aug 10, 2017 at 3:35 PM #5

    DrChinese

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    A reasonable assumption (perfect correlations imply HV), but one which is consistent only with instantaneous action at a distance.

    Your statement 2. is incorrect (you have it backwards). If there ARE Hidden Variables, then there must be instantaneous action at a distance.
     
  7. Aug 11, 2017 at 1:25 AM #6

    bhobba

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    Put a green slip in an envelope and a red slip in another - open one and you instantly know the other. No communication, spooky action at a distance and any of the other stuff touted about entangled systems. Entangled systems, like the pieces of paper are simply correlated.

    Here is the real issue. Quantum correlation has some statistical properties different to classical correlation. The reason is we know at all times in the paper slip example that the paper is red or green. In QM we do not know what properties it has unless actually observed. This leads to the different statistics - but again - its just a correlation - no more, no less.

    Now what does Bell say. Well first Bell was a genius in the same class as Einstein, Feynman, Von-Neumann etc - he saw to the heart of the matter when others did not. And that heart is simple - want it to be like the slips of paper - then there must be non local influences going on. But just accept QM as is then no problem. That's it, that's all. Nothing too complicated really - but of course it took the genius of Bell to see it.

    He even wrote a paper explaining it that way except he used the amusing Bertlmann's socks analogy, rather than slips of paper:
    https://cds.cern.ch/record/142461/files/198009299.pdf

    BTW please don't post about the math - he leaves out some steps - they are not hard to do but are tedious and you can pretty much see they are true without even doing the math.

    Thanks
    Bill
     
  8. Aug 11, 2017 at 5:10 AM #7

    m k

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    Can't we see a statistical difference between polarizations if other half of stream of polarization entangled pair is put thrue a slit?
     
  9. Aug 11, 2017 at 5:43 AM #8

    bhobba

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    Yes - but the slit is an observation.

    Thanks
    Bill
     
  10. Aug 11, 2017 at 10:02 AM #9
    "Einstein, Podolsky, and Rosen argued that "elements of reality" (hidden variables) must be added to quantum mechanics to explain entanglement without action at a distance. Later, Bell's theorem suggested that local hidden variables of certain types are impossible, or that they evolve non-locally."Wikipedia.
    I suppose you meant non local variables.
     
  11. Aug 11, 2017 at 10:40 AM #10

    Nugatory

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    There is a reason why wikipedia is not, in general, an accepted source here.... Often it's good, sometimes it's not so good, and sometimes it's just confusing. If you believe that it's in conflict with what @DrChinese posted, then we have an example of it being either wrong or confusing.

    If you want to know whether a particular theory is precluded by Bell's theorem, you can consider whether it conforms to all the assumptions that Bell made when he derived his inequality. If it does, then it must obey the inequality and therefore we reject it as not consistent with experimental results. If it does not, then as far as Bell's theorem is concerned it may or may not be a valid theory and we have to look elsewhere for reasons to accept or reject it. You can drag words like "reality", "locality", "hidden variables" into the discussion, but unless you clearly connect these words to the assumptions that go into the derivation of the inequality you're unlikely to find any new insight and likely to find yourself in a sterile argument about which words to apply to any given theory.
     
  12. Aug 11, 2017 at 2:35 PM #11
    I'm sorry.
    I only wanted to clarify this. Does "No communication theorem" alone demonstrate information cannot travel faster than light?
     
  13. Aug 11, 2017 at 2:46 PM #12

    Nugatory

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    No. It does prove that measurements on an entangled system do not transmit information at any speed, whether faster than light or not.

    It says nothing about other possible ways of transmitting information, but of course relativity has a lot to say on that question.
     
  14. Aug 12, 2017 at 4:45 AM #13
    Thank you. I still don't understand why this conclusion. I can only deduce that we cannot send information using entanglement (there is nothing we can do on a entangled particle to be detectable by measuring its entangled pair), not even at lower speed than c. I'm not arguing that information can travel faster than light.
     
  15. Aug 16, 2017 at 6:56 PM #14
    does the effect of observation have an effect upon the observed partial ?
     
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