The title comes from a quote by Albert Einstein. All his life he sought to understand the foundations of physical phenomena. "Something deeply hidden had to be behind things," and he was determined to figure out what it was. So is Sean Carroll.
Sean Carroll, a physicist at Caltech, cares deeply about the foundation of quantum mechanics. Whether quantum mechanics is true is not in question. There's never been a physics experiment that quantum mechanics doesn't correctly predict the outcome. That's more than can be said for Newton's classical mechanics or even Einstein's general relativity, which can't be used to explain things at the subatomic level. But Carroll defies convention in one respect. Most quantum physicists concentrate on specifying the wave function for a quantum-mechanical system and apply Schrödinger's equation to it to predict the system's future state. What they don't do is puzzle over what it all means, what is the real foundation of quantum mechanics. In other words, their attitude is one of "shut up and calculate" instead of worrying about exactly what is deeply hidden.
Carroll promotes the Many Worlds interpretation of quantum mechanics, an alternative to the prevailing Copenhagen interpretation that has ruled quantum studies since the 1920s. In the Copenhagen interpretation, an electron, say, doesn't have a definite position until an observer measures it, at which time its wave function "collapses" into a definite state. In the Many Worlds interpretation, the quantum system "branches" into multiple worlds comprising all possible outcomes of the observation. An observer, being on one and only one branch, sees a single, specific outcome.
Carroll convincingly argues that the Many Worlds model has simplicity and elegance in its favor, but freely admits that it might also have the biggest distance between theory and what we seem to observe in the world. The branches of multiple worlds are, in principle, unobservable, so how can we believe they *really* exist at all? So there's room to argue, room for further study, room for imagination. Whether there's something else deeply hidden, only further research will determine.
Carroll's treatment of quantum physics requires no mathematics, although it's all mathematics at heart, literally. Electrons, photons, and all subatomic particles that make up everything in the universe, are simply vibrating quantum fields, not physical particles. As he says in the epilogue, everything is quantum. In various chapters, he explains quantum mechanics, superpositions, Schrödinger's equation, waveform collapse, quantum entanglement, decoherence, spacetime, gravity, and many other concepts involved in quantum mechanics. But just because there are no mathematics in this book, doesn't mean it's easy to understand. Carroll makes a convincing argument that the Many Worlds intepretation of quantum mechanics really is the simplest and most elegant interpretation, even even if it's not the only model that does that, and even if I can't reproduce his arguments myself. Most importantly, it does accurately predict the results of measurements. In a way, what can be more real than that?