Time travel involves those ultimate concentrations of matter and gravity, black holes. Among other bizarre properties, a black hole is a place where time literally comes to a stop - an edge of time. In this book John Gribbin takes the reader over that edge, to a place where time runs backwards and everyday rules of common sense no longer apply. This book begins in the 17th century, with the cleric who first suggested that black holes might exist, and along the way it involves meetings with key historical characters, including the lazy philosopher who found inspiration in the flight of a fly, the mathematicians who invented a new kind of geometry in which parallel lines can cross each other, and physicists who didn't believe their own theories until confronted by new astronomical discoveries which they should have predicted but didn't. John Gribbin is the author of "The Hole in the Sky" and "Hothouse Earth".