TIME TRAVEL has become, if not respectable, then certainly fashionable in some quarters of the physics world over the past decade or so. Much of the blame can be laid at the door of the astronomer Carl Sagan, who was writing a science fiction novel in the summer of 1985, and asked the relativist Kip Thorne, of CalTech, to come up with some plausible sounding scientific mumbo-jumbo to "explain" the literary device of a wormhole through space which could enable his characters to travel between the stars. Encouraged to look at the equations of the general theory of relativity in a new light, Thorne and his colleagues first found that there is nothing in those equations to prevent the existence of such wormholes, and then realised that any tunnel through space is also, potentially, a tunnel through time. The laws of physics do not forbid time travel.
This realisation had two consequences. When Sagan's novel, Contact, appeared in 1986 it contained a passage that read like pure Sf hokum, but which was (although few readers realised it at the time) a serious science factual description of a spacetime wormhole. And as Thorne and his colleagues began to publish scientific papers about time machines and time travel, the spreading ripples have stimulated a cottage industry of similar studies.
Curiously, this anecdote does not feature in Paul Nahin's otherwise remarkably comprehensive account of the fact and fiction of time travel. Nahin is a professor of electrical engineering at the University of New Hampshire, and the author of several published science fiction stories, some dealing with the puzzles and paradoxes of time travel. He tells us how he discovered, and "devoured" science fiction stories at the age of ten, and this book is clearly a labour of love. The approach is scholarly, with 36 pages of footnotes, nine technical (but not overly mathematical) appendices, and a no-holds-barred bibliography. Nahin's style is distinctly more sober than the material he deals with, but what he lacks in sparkle he certainly makes up for in comprehensiveness.
The approach, in line with the author's background, is from the fiction and towards the fact. Old favourites, such as H. G. Wells and Frank Tipler, make their expected appearances, as do less familiar time travel fictions from the nineteenth century (comfortably predating Albert Einstein's theories) and more obscure scientists and philosophers. And, of course, the familiar time travel paradoxes get a thorough airing.
There are, though, two major weaknesses in Nahin's treatment of the science. The lesser is his discussion of black holes, which is weak and sometimes a little confused. Much more importantly, though, he fails to appreciate how the "many worlds" interpretation of quantum mechanics allows a time traveller to go back in time and alter the past without producing problems such as the notorious grandfather paradox. In the conventional version of the paradox, a traveller goes back and murders his grandfather as a young boy, so the traveller could never have been born, so grandfather never died -- and so on. But in the many worlds version (championed today by David Deutsch, of the University of Oxford), the act of killing grandad creates a new reality, so that when the traveller then goes forward in time he is no longer in his own world, but in the universe "next door". This explains, for example, some of the more subtle touches in the "Back to the Future" trilogy of movies, which Nahin comments on while missing their point entirely. But although the book is flawed, it is still welcome. It does not lend itself to being read from front to back like a novel, but is ideal to dip in to and hop around in, like a time traveller dipping in to history. It is also a first class reference book for anyone interested in the Sf side of time travel, and one that will be welcomed by the fans -- at least, they will welcome it when and if it becomes available in paperback at a sensible price.