Quantum theory is one of the most successful theories in the history of physics. The accuracy of its predictions is astounding. The breath of its application is impressive. Quantum theory is used to explain how atoms behave, how elements can combine to form molecules, how light behaves, and even how black holes behave. There can be no doubt that there is something very right about quantum theory.

But at the same time, it is difficult to understand what quantum theory is really saying about the world. In fact, it is not clear that quantum theory gives any consistent picture of what the physical world is like. Quantum theory seems to say that light is both wavelike and particlelike. It seems to say that objects can be in two places at once, or even that cats can be both alive and dead, or neither alive nor dead, or—what? There can be no doubt that there is something troubling about quantum theory.