What precisely happens when a quantum measurement is performed? That’s the quantum measurement problem, in broad strokes. There are optimistic folks, like David Mermin (see Physics Today, June 2022, page 62), who believe there is no measurement problem, but that’s because they think they know the answer to it. Unfortunately, despite almost a century of effort, no one solution has been completely accepted by a majority of physicists. The fairest thing is to admit that the measurement problem is still with us.

The awkwardness of the measurement problem is only enhanced by the undeniable empirical success of textbook quantum theory. According to that treatment, quantum systems are described using wavefunctions. Wavefunctions evolve according to the Schrödinger equation, at least when the system is not being observed. Upon measurement, the wavefunction collapses to an eigenstate of the measured observable.

That textbook version of quantum mechanics fits a wide variety...

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