The current belief concerning the fate of the universe is that it faces a 'heat death', when everything reaches equilibrium, and so life will no longer be possible. Not an appealing prospect for any life forms still around - so maybe they can escape to a different universe, or even build their own. A wacky sci-fi idea? Yes - but in 'Parallel Worlds' Michio Kaku shows that there is some real science behind the fiction.

The book is in three parts, the first one being a summary of the development of cosmology. Starting from early ideas, Kaku moves on to Einstein's General theory of relativity, with his greatest blunder - the cosmological constant - and then on to the expansion of the universe and the big bang. This leads up to the current ideas of an inflationary phase of the universe and the presence of dark matter and dark energy - the return of the cosmological constant. The second part of the book describes some of the areas of modern physics which relate to the parallel worlds. This isn't just about sci-fi ideas of 'dimensional portals and time travel', there is also a chapter on recent and upcoming experimental results and how they relate to the ideas discussed. In addition to this Kaku explains about branes and string theory - areas of research to which he is a major contributor. He also looks at anthropic arguments, noticing how the parameters of the universe seem to be 'just right' to make life possible.

The third part of the book is the most sci-fi like, describing the ultimate fate of the universe and the giving a twelve point plan for escape. Five stages of the universe are described starting with the big bang and moving through the present era into the degenerate era when stars have all gone out. This is followed by the black holes era, but that isn't then end - eventually the black holes will all evaporate and the dark era will commence with the universe approaching its final heat death. A bleak prospect, so Kaku goes on to look at the technological abilities which our civilisation may develop and how we will be able to use them to get to a new universe. His plan includes technology such as a warp drive and a particle accelerator several light years long, so I don't think that it's likely to happen any time soon.

There are several different ideas as to the meaning of the term 'Parallel Worlds'. Firstly, General Relativity allows space twist around and thus connect up in different ways. If you fall into a spinning black hole then you might end up in a different universe. Alternatively, if you could construct a wormhole then having it lead to somewhere else in the universe might lead to time travel paradoxes, so maybe its better if it goes to a different universe altogether. In this way new universes can 'bud off' the existing one. The second meaning of the term is based on quantum theory - in the case of Schrödinger's cat the Many-Worlds interpretation of quantum theory says that the cat is indeed both dead and alive, but that the universe has split into two to accommodate both possibilities. Thirdly, ideas such as string theory require that there be more than the usual four dimensions of spacetime. Thus a different universe might exist just a fraction of a millimetre away from ours in one of these extra dimensions. Now many books fail to distinguish between these different meaning of 'Parallel Worlds', but I had hoped that Kaku's book would do better. Unfortunately, I don't feel that it does so. In fact this is rather a long book and I found that there is a tendency to present a large number of ideas without giving the readers a chance to get them clear in their minds.

Kaku has written a book which is easy to read - maybe too easy. Even in the chapter on paradoxes in cosmology there never seemed to be anything leading the reader to think 'maybe I'd better stop and mull over that for a while' Oddities tend to be presented for their gee-whiz factor rather than to encourage the reader to look further into the subject. The book has been shortlisted for the 2006 Aventis prize, but I wouldn't tip it as a winner, as it is too science-fictiony at the expense of providing a true starting point for the subjects considered (for which I would suggest the works of Brian Greene instead). That said, if you're into science fiction, or you want an overview of modern physics which isn't too taxing, then this book is ideal.