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Thread: A key concept: superposition of states.

  1. #1 A key concept: superposition of states. 
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    Bell's inequality is valid if "local realism" is accepted: what's happening here doesn't determine what's happening there. This is completely different from the "conservation laws" that impose to a system spliting in two parts to conserve the same total value for, say, momentum or energy. With entangled states it is what is "measured" here that determines what's happening there, thus contradicting local realism and hence Bell's inequality. This is a very far reaching conclusion and its experimental verification has been a major achievement of the physics of these last decades.
    At the root of this quantum behavior lies the "superposition principle": states of a system are linear combinations of eigenvectors. This, I feel, was perceived more as a useful mathematical description than reflecting a deep physical reality. Today, don't you think, my fellow physicists, that we are convinced that the state "collapse" onto an eigenvector occuring during a measurement is not just a nice mathematical description but rather corresponds to what is "actually" happening. The spin is neither up or down (like Schrodinger's cat) it becomes a spin when we measure it here since it is what happens there.
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  2. #2  
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    Quote Originally Posted by frogeraie View Post
    Today, don't you think, my fellow physicists, that we are convinced that the state "collapse" onto an eigenvector occuring during a measurement is not just a nice mathematical description but rather corresponds to what is "actually" happening.
    In the many-worlds interpretation, there is no state "collapse". The eigenvectors of the quantum state simply become entangled with the corresponding macroscopic states of the measuring device.
    A tensor equation that is valid in any coordinate system is valid in every coordinate system.
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    Absolutely right! Spin is not defined until you measure it. That's why you will find it completely aligned ( or misaligned) against any axis you choose to measure it along. It's a crazy thing.
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