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Mr Al-Khalili explains that time is thought to have started along with the universe in the big bang, about 15 billion years ago (for comparison, the sun is about halfway through its life of about 10 billion years). Time also has a direction: although most physical processes are reversible, there is roughly speaking a tendency (familiar to parents) for the world to become ever less organised.

However, time does not always flow at the same rate: travelling close to the speed of light or falling into a black hole slows the passage of time relative to an observer. These are complex ideas that physicists usually express in mathematical language. Mr Al-Khalili avoids mathematics, yet gives an admirably clear account of some of the concepts involved.

The Physicist, October 1999

Jim Al-Khalili has written a splendid popular book. It begins where it must, with geometry, curved space and all that. The book is authoritative and yet reasonably simple. It contains some excellent insights and analogies, but always warns you of the limits to the analogies.

The book would be an excellent resource for school teachers in both Maths and Physics to enrich their teaching, and to enthuse their students. I especially liked the story of the hotel with an infinity of rooms, and the procedures for accommodating further infinities of guests. Finally the book delves into time and time travel and the debate on whether time travel is theoretically possible. This brings into focus the contradictions between quantum physics and general relativity. The reader is left well aware of what we do know and most importantly, what we don't know, and the exciting theoretical challenges ahead.

Many physicists will enjoy this easy to read book, but they will have to be patient with many of the easy bits. I highly recommend it for teenagers with an interest in science and for non-scientists interested in the deep questions of our universe.

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