The classic sci-fi novels of H.G. Wells, Robert Heinlein, and Arthur C. Clarke, along with hundreds of sci-fi movies and TV shows, have explored an array of exotic, at times mind-bending themes, among them the existence of intelligent aliens, time travel, and parallel universes. One often-asked question is whether such things are scientifically plausible. In this volume, modern physicists and other scientists weigh in and try to answer that question.