Ever since Plato first told his students the allegory of the cave, people have wondered whether dimensions exist beyond the three we immediately perceive. An extra dimension—time—played a role in Einstein's work, although he saw it only as a necessary evil to get his equations to work. Other scientists were more receptive: mathematical physicists Oskar Klein and Theodor Kaluza made higher dimensions an integral part of their attempts to discover a "theory of everything" that would tie together strong and weak nuclear forces, electromagnetism and gravity. Halpern explains that over the past century gravity has been the shadow flickering on the walls of the cave hinting at other realms. Why is it so weak compared with electromagnetism? With string theory, and its successor, M-theory, physicists speculate that gravity "leaks" back and forth between our reality, an 11-dimensional "brane" (or membrane) and other branes, perhaps as close as a millimeter away. Halpern masterfully creates word pictures to illustrate mind-bending scientific theories, and he paints highly detailed sketches of the scientists involved—sometimes too detailed, leading readers to lose the thread of the narrative. Science buffs won't find much new here, but for average readers, this is an accessible account of the search for what lies behind our dim perception of reality.