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Now a hot research topic, the physics of time travel attracts many a relativist convinced that there is nothing in the laws of physics to forbid travelling backwards in time. Barry Chapman is one of this band, and in Reverse Time Travel (Cassell, £16.99, ISBN 0 304 524 5) he reveals his favourite solution to the familiar time-travel paradoxes, the notion of parallel worlds. Time travellers would slip sideways in time. A surprisingly dull treatment of an exciting subject, but lots of background on Einstein’s theories.