The remit of this book is staggering — it is no less than the coverage of the development of cosmology from its beginnings in antiquity right up to the theory of the multiverse, and the fact that our universe may be just one of an infinite number, each possessing physical forces and constants with different strengths to ours. The fact that the publication is targeted at the lay person (who may have little knowledge of cosmology and astronomy), makes the remit even more remarkable.

Kaku is one of the co-founders of a branch of String Theory and as such, one may start to read the book with the misconception that its contents will be biased towards the perspective of this particular theory. However, this is not the case, and the reader is firstly treated to a commendably objective history of astronomy, classical physics and the Copernican/Galilean Revolution, relativity, quantum mechanics, string theory, and the discovery of dark energy and dark matter, along with some extremely well written explanations and diagrams. The overriding power of modern cosmology in explaining the universe — the marriage of the study of large scale objects such as galaxies groups, with that of very small scale subatomic particles is a growing theme throughout the book, and includes a superb explanation of the Cosmic Microwave Background radiation, and culminates in a discussion of the Standard Model, Inflation, and the five eras of the development of our universe.

Kaku gives an elegant account of the breaking of symmetry of the physical forces a fraction of a second after the Big Bang, and how String Theory can reconcile gravity with the electro-weak and strong nuclear forces, hence providing a theory of everything. Like many other physicists, he is hopeful that the evidence for supersymmetry and many of the sub-atomic particles predicted by String Theory may be forthcoming when the Large Hadron Collider (LHC) at CERN reaches full power in 2010 or 2011. He certainly hopes so — the discovery of the graviton, the Higgs particle, and minute black holes will mean many physicists have not been traversing a blind alley for the past forty years!

Of course, an integral part of String Theory are extra spatial dimensions and Kaku develops this, and speculates on whether it will be possible to detect these. The book puts our everyday notions of time and reality to the test, and examines the fate of the universe as it expands exponentially, speculating that after trillions of years of such expansion and subsequent cooling, conditions will be unable to sustain intelligent life. Kaku also speculates on the possibility of whether given enough time, and if we don’t destroy ourselves first, we can ascend through the types of advanced civilisation to reach a point where we can harness the power of stars and galaxies and enter another universe where conditions are once again favourable for life.

Perhaps the most startling revelation is that the Copernican Theory of Mediocrity may apply to our universe. The values of the universal constants and forces may, after all, be arbitrary, and a random result of symmetry breaking in a certain way at the time of the Big Bang. For example, Kaku shows how if one decreases the general strength of gravity by an infinitesimally small amount clouds of hydrogen will not coalesce into stars, planets and galaxies and life would not exist. Increase gravity and stars will burn and die too quickly, never allowing the time for planets and life to evolve.

Kaku documents a stark choice facing physicists, either our universe has been created in a way conducive to the development of complexity and ultimately life itself (the anthropogenic principle), or we live in a multiverse of universes — by an infinitesimally small chance we just happen to live in one of the few habitable universes. Kaku eloquently shows how astro-physics, philosophy and even religion are drawn together at this point.

In conclusion, Parallel Worlds is a fascinating insight into current cosmological theory and models, and sheds light on many of the dilemmas and discoveries with which astrophysicists and cosmologists are now grappling. By its very nature, any book involving cosmology will become dated rapidly, and this book will probably be no exception (especially when the LHC comes up to full power). However Dr Kaku should be complemented in a superb attempt to portray contemporary cosmological and physical theories in a highly interesting and readily understandable way.

The book contains an excellent glossary of cosmological and physical terms, and there is little mathematics. For me anyway, it’s the sort of book which, once started, you simply can’t put down.