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 Mentor A measurement is an interaction between the system and its environment (which includes the measurement device), so the state of the system can obviously be changed by the measurement. So this "observer effect" (as defined by the article) is clearly present in QM, but no one uses that term. I don't remember seeing it in any of the QM books I've read. It's true that the measurement device obeys the rules of QM, but in realistic situations, the interactions between the device and its environment will ensure that the "pointer" (the component that indicates the result of the measurement) behaves in a way that's indistinguishable from classical behavior. In fact, if it behaved in any other way, we wouldn't consider it a measurement.