There are three types of quantum physicists: (1) those who think quantum mechanics is defaced by a so-called measurement problem; (2) those who think, as I do, that there is no measurement problem; and (3) those who think the issue is not worth serious thought. You can find the diverse views of 17 physicists and philosophers from the first two groups in chapter 7 of Maximilian Schlosshauer’s Elegance and Enigma.
Most people in all three groups would agree on the following: Quantum mechanics describes a physical system entirely in terms of states. A state is a compendium of probabilities of all possible answers to all possible questions one can ask of the system. Quantum mechanics is inherently statistical. There is no deeper underlying theory that gives a fuller description.
The state assigned to a system can change in time in two ways. If no question is asked of the system,...
