Time travel involves those ultimate concentrations of matter and gravity, black holes. A black hole is, among other bizarre things, 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. The book begins with the 17th-century 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 a regular contributor to "The Times", "The Guardian" and "New Scientist". His other books include "The Hole in the Sky", "Hothouse Earth" and the award-winning "In Search of Schroedinger's Cat".