Ultimately, the arguments over quantum mechanics have much bigger stakes: what reality is. The basic problem is that the theory tells us what we can expect to observe if we make measurements of a quantum system such as an atom or an electron. It doesn’t tell us how the world is, only what we’ll see if we look. Quantum uncertainty, the physicist and philosopher Jeffrey Bub of the University of Maryland told me, “doesn’t simply represent ignorance about what is the case, but a new sort of ignorance about something that doesn’t yet have a truth value, something that simply isn’t one way or the other before we measure.”