James Owen Weatherall, professor of logic and philosophy of science at U.C. Irvine, [writes]: Physicists know how to use quantum theory—your phone and computer give plenty of evidence of that. But knowing how to use it is a far cry from fully understanding the world the theory describes—or even what the various mathematical devices scientists use in the theory are supposed to mean. One such mathematical object, whose status physicists have long debated, is known as the quantum state.

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