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Entanglement Question

  1. Jan 4, 2016 #1
    I'm a layman interested in quantum mechanics and I have a few questions. I'm sorry if you receive these types of questions a lot but I can't seem to find the answers.

    My first question lies within the Copenhagen interpretation. In the Copenhagen interpretation, is non locality or counterfactual definiteness rejected? I would assume that non-locality is rejected because to my knowledge that within the confines of the Copenhagen interpretation particles do not have definite attributes until they're observed.
    My last question lies in entanglement. I think I may have a little trouble understanding exactly how counterfactual definiteness solves the correlations within entanglement. My current understanding is that the particles do not have definite attributes until measurement, however they must contain oppsite spins to preserve the conversation of angular momentum once measured. (Is this correct?)
    Anyways, the particles are intrinsically intertwined and the collapse of superposition between the particles is instant. This is where my confusion stems from , because this seems identical to non-locality, except that there are no hidden variables.
  2. jcsd
  3. Jan 4, 2016 #2


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    Gold Member

    It seems you have mixed up meanings of counterfactual definiteness, non-locality and hidden variables. With counterfactual definiteness we understand that we can meaningfully speak about results of unperformed measurements. Idea that particles have definite attributes at all times is more like non-contextual local hidden variables.
    If we speak about perfectly correlated spin measurements of entangled particles we can try to explain it in two different ways. First, we can model such correlations using local hidden variables, but such models are falsified by recent loophole free Bell inequality experiments. Second, we can model such correlations using non-locality.
    In Copenhagen interpretation collapse is non-local so it definitely does not reject non-locality.

    And there are some discussions whether rejecting counterfactual definiteness allows us to model perfect correlations without non-locality. But I can't comment on such viewpoint as how I see it any prediction that a theory makes invokes counterfactual definiteness in some way. So it is meaningless to reject it.
  4. Jan 4, 2016 #3


    Staff: Mentor

    Couterfactual definiteness.

    Once measured - yes - but nor before. Put a red slip of paper in one envelope and a green slip in another. Open one envelope and you immediately know the colour of the slip in the other. The slips are correlated. Its exactly the same with EPR with a twist. It turns out if you assume like the paper they have the property of spin regardless of if its observed or not then there is a certain kind of statistics to the correlations. However for EPR type experiments do conform to those statistics meaning its not like the slips of paper - it does not have the property of spin observed or not. But what if we insist? It turns out there is an out. If you allow FTL communication between the particles.

    That's all there is to it really. Some simply overcomplicate it IMHO.

    Last edited: Jan 4, 2016
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