Reality as we know it is bound by a set of constants—numbers and values that dictate the strengths of forces like gravity, the speed of light, and the masses of elementary particles. In , Cambridge Professor and bestselling author John D.Barrow takes us on an exploration of these governing principles. Drawing on physicists such as Einstein and Planck, Barrow illustrates with stunning clarity our dependence on the steadfastness of these principles. But he also suggests that the basic forces may have been radically different during the universe’s infancy, and suggests that they may continue a deeply hidden evolution. Perhaps most tantalizingly, Barrow theorizes about the realities that might one day be found in a universe with different parameters than our own.