(Non-relativistic) quantum mechanics provides a continuous, deterministic formula for the evolution of a quantum state, via the Schrödinger equation. Until a “measurement” happens. Then everything changes. But there is no general agreement on when or how a measurement occurs. In fact many people suspect that a “state changing” measurement is a fiction. Many of the various “interpretations” of QM are the result of trying to solve this problem in various ways.

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