In this groundbreaking book, the renowned theoretical physicist Lee Smolin argues that fundamental physics—the search for the laws of nature—is losing its way. One of the major problems, according to Smolin, is the growing gulf between the speculations of theorists and reality as revealed by experiment. Ambitious ideas about extra dimensions, exotic particles, multiple universes, and strings have captured our imaginations. But these ideas have not been tested experimentally, and some—string theory, for example—seem to offer no possibility for experimental tests. And because these ideas have tied up funding and talent to the detriment of other pursuits, they threaten to impede the progress of science. With clarity, passion, and authority, Smolin charts the rise and fall of string theory and takes a fascinating look at what will replace it.