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 Feb 2023


Instead of trying to resolve in general this problem of how macroscopic classical 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.

Principle (The Born rule). Given an observable O and two unitnorm statesψ1〉 and ψ2〉 that are eigenvectors of O with distinct eigenvalues λ1 and λ2Oψ1〉 = λ1ψ1〉, Oψ2〉 = λ2ψ2〉the complex linear combination statec1ψ1〉 + c2ψ2〉will not have a welldefined value for the observable O. If one attempts tomeasure this observable, one will get either λ1 or λ2, with probabilitiesc21c21 + c22and c22c21 + c22respectively.
