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I'm a little confused when it comes to measuring the momentum in a infinite or finite square well. Is this even possible? One of the postulates of quantum mechanics states that measuring an observable will leave it as an eigenstate of that observables corresponding operator just after measurement. Fine and dandy but for an infinite square well this would result in the wavefunction existing beyond the boundaries. For a finite square well this would also result in the wavefunction being an eigenstate of the momentum operator and therefore a free particle. Can someone explain where I am going wrong/what I'm missing. Thanks.

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# Measuring momentum in a square well

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