The latest attempt to describe the “holy grail of modern physics.” Although in theory it works brilliantly, no one fully understands quantum mechanics. However, Carroll (Theoretical Physics/Caltech; The Big Picture: On the Origins of Life, Meaning, and the Universe Itself, 2016, etc.) works hard—and somewhat successfully—to deliver an accessible explanation. “Quantum mechanics,” he writes, “is unique among physical theories in drawing an apparent distinction between what we see and what really is….If we free our minds from certain old-fashioned and intuitive ways of thinking, we find that quantum mechanics isn’t hopelessly mystical or inexplicable. It’s just physics.” This doesn’t bother most physicists, who belong to the shut-up-and-calculate school, and searching for a deep meaning is unfashionable. Carroll swims against the tide, explaining several theories that attempt to describe what is happening, with an emphasis on his favorite, the many-worlds theory. He begins by pointing out that in our everyday world, the world of classical mechanics, every object has two features: a location and a velocity. Everything is transparent; whatever happens to that object is explained by classical laws of physics—essentially Newton’s. In contrast, every quantum object has one feature: a wave function defined by Schrödinger’s 1926 equation, which explains what happens when one measures it. Although true for objects of any size, quantum mechanics becomes essential at the atomic and subatomic levels. Some popular writers proclaim that this demonstrates our ignorance or perhaps a mysterious spiritual element in the universe. The author disagrees but admits that, as a description of how reality works, it makes no sense. Eschewing mathematics, Carroll labors mightily to reveal the meaning behind quantum mechanics with a major detour into general relativity, both of which might benefit from at least a little math. Readers who remember freshman college physics will be intrigued; others will struggle.