Is Quantum Theory About Reality or What We Know?

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.

One of the most striking features of quantum theory is that its predictions are, under virtually all circumstances, probabilistic. If you set up an experiment in a laboratory, and then you use quantum theory to predict the outcomes of various measurements you might perform, the best the theory can offer is probabilities—say, a 50 percent chance that you’ll get one outcome, and a 50 percent chance that you’ll get a different one. The role the quantum state plays in the theory is to determine, or at least encode, these probabilities. If you know