Time is one of the most familiar aspects of human experience and one of the least understood in physics. We feel it passing. We measure it constantly. And yet some of the most rigorous theories of the universe suggest it may not exist at a fundamental level at all. The Wheeler-DeWitt equation, proposed in the 1960s, describes the universe as a single, static quantum state with no built-in time parameter. In this picture, there is no external clock ticking in the background. The experience...