J Richard Gott's Time Travel in Einstein's Universe explores humanity's long-held dream of travelling in time. He explains how Einstein's theories of relativity offer a faint glimmer of hope that was not available under Newtonian physics. It's an interesting and informative read, but anyone looking for the blueprints for a time machine, especially for travel to the past, will be sorely disappointed as time travel is strictly a pastime for super-civilisations. However, thinking about time travel is useful when exploring the extreme limits of current theories. Gott is a physicist himself, and has shown that a time machine operating for tiny fractions of a second could have led to the universe creating itself, side-stepping the problem of where it came from. When it comes to practical applications, special relativity allows travel to the future by accelerating up to an appreciable fraction of the speed of light. This has already happened to a modest extent; cosmonauts and astronauts who have orbited the Earth many times at high speeds are a few milliseconds younger than they would otherwise be. Travel to the past is much harder and involves manipulating wormholes, black holes or cosmic strings, feats possible only with galaxy-spanning technology. The ideas are not always easy to grasp, but Gott illustrates the concepts with references to popular fiction and film wherever he can. Although his writing sometimes lapses into a dry, textbook-like style that can be hard going, for anyone interested in the frontiers of cosmology it's well worth persevering. --Elizabeth Sourbut