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  1. Feb 2023
    1. Instead of trying to resolve in general this problem of how macroscopic clas-sical physics behavior emerges in a measurement process, one can adopt thefollowing two principles as providing a phenomenological description of whatwill happen, and these allow one to make precise statistical predictions usingquantum theory

      To resolve the measurement problem from quantum mechanics into the classical realm, one can use the observables principle and the Born rule.

    2. Principle (The Born rule). Given an observable O and two unit-norm states|ψ1〉 and |ψ2〉 that are eigenvectors of O with distinct eigenvalues λ1 and λ2O|ψ1〉 = λ1|ψ1〉, O|ψ2〉 = λ2|ψ2〉the complex linear combination statec1|ψ1〉 + c2|ψ2〉will not have a well-defined value for the observable O. If one attempts tomeasure this observable, one will get either λ1 or λ2, with probabilities|c21||c21| + |c22|and |c22||c21| + |c22|respectively.