= Wave function collapse
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Similar to <quantum jump> in the <Bohr model>, but for the <Schrödinger equation>.
The idea the the wave function of a small observed system collapses "obviously" cannot be the full physical truth, only a very useful approximation of reality.
Because then are are hard pressed to determine the boundary between what collapses and what doesn't, and there isn't such a boundary, as everything is interacting, including the observer.
The <many-worlds interpretation> is an elegant explanation for this. Though it does feel a bit sad and superfluous.