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This question already has an answer here:

After measuring a quantum state for a particular measurable, the state collapses to one of the eigenstate of that measurable. If we measure for momentum in an infinite square well, the system will collapse to a momentum eigenstate. But uncertainty in position is not arbitrary large (it's $2L$). So we are left with finite uncertainty in momentum and position at the same time. Does this mean that we aren't supposed to make such measurement? Or something else.

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marked as duplicate by Qmechanic quantum-mechanics Oct 28 '16 at 13:28

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