Gribbin sets out to explain how time travel is theoretically possible and looks at the various different ways which might allow it. He pays particular attention to the most likely time-machine: that provided by the concept of the "wormhole".
Like a tunnel through space and time, a wormhole could connect different regions of the Universe which occupy both different spaces and different times.
The book works through the history of theories of time: Newton, gravity, the pioneers of Black Hole theory. Then geometrical theories: from Euclid and Descartes onto ideas of non-flat space-time; Einstein's ideas on gravity and Schwarzchild's singularities.
Chapter 3 is about dense stars - how concentrations of mass can warp spacetime - white dwarfs, neutron stars and similar.
Chapter 4 progresses to the logical conclusion of the existence of black holes from which no signal (light or otherwise) can ever escape.
Chapter 5 is about black holes with spin, exploding horizons and centrifugal confusion.
Chapter 6 introduces hyperspace - timeloops, wormholes, etc.
Chapter 7 explores the possibilities of time machines.
Chapter 8 mentions the latest and wildest ideas. Black holes that bounce, oscillating universes etc.
Good for the grant proposals, but shows a distinct lack of social relevance and artistic creativity!