Pressed for Time? Try string theory with a slightly shorter piece of string

Stephen Hawking's A Brief History of Time is perhaps the best-selling popular science book of all time. Published in 1988, it was on The Sunday Times bestseller list for over four years. It has apparently sold "about one copy for every 750 men, women and children on earth". Why then do we need this new, even briefer version?

It was often said that the original was the world's "most bought, but least read, book". Although it began with a gentle review of classical physical concepts, it soon dived into complicated space-time diagrams and a bewildering array of rather difficult technical subjects. It was perhaps not surprising that it spawned a whole new genre of books that attempted to "explain" Hawking's work. This "briefer" version represents Hawking's attempt to present a more qualitative and readable account of the ingredients of modern views of the universe and quantum gravity.

In the original, Hawking states that someone told him that each equation he included in the book would halve his sales. In the event, he decided to include just one, the iconic E=mc2. The new book, co-authored with physicist Leonard Mlodinow, stays true to this diktat. The basic material is still covered but some of the more difficult topics, such as the mathematics of chaotic boundary conditions, are omitted. The space-time diagrams are gone and replaced by glossy colour illustrations, and the book is indeed briefer by some 30-odd pages. Some of the material is more or less word for word the same as before, but other chapters have been compressed or amalgamated.

As before, the book begins with an account of early views of the universe, from Aristotle and Ptolemy to Galileo and Newton. In such a brief account, it is not surprising that history often gets oversimplified. In particular, Tycho Brahe and Johannes Kepler get short shrift. It was the accuracy of Brahe's astronomical measurements that forced Kepler to consider the "ugly" ellipse for the orbit of Mars. Kepler then summarised the results of his analyses in mathematical form, and it was these laws that played a key role in validating Newton's theory of gravity. Similarly, the "Quantum gravity" chapter introduces quantum mechanics with an enormously abbreviated account of the history and key concepts. The presentation focuses on Heisenberg's Uncertainty Principle and uncertainty rather than Schrodinger's equation that predicted the certainties of the energy levels of hydrogen. In a book devoted to a discussion of time and relativity, it seems bizarre that there is no mention of the Einstein-Podolsky-Rosen paradox and Bell's Theorem.

Such "faster than light" signalling implied by quantum mechanical entanglement would seem an essential topic for any serious discussion of quantum mechanics and relativity.

The authors give a qualitative account of the ideas of modern cosmology and attempts by theoretical physicists to formulate a consistent theory combining quantum mechanics and gravity. The result is the conventional picture of the Big Bang theory of an expanding universe undergoing an incredibly fast period of "inflation" to produce the homogeneity of the universe as we see it today. Moreover, as they explain, for consistency with the behaviour of observable matter, theorists need to introduce both mysterious "dark matter" and "dark energy" that apparently make up the majority of the energy in our universe.

"String" theories are then presented as the ultimate unification of the forces of Nature. Unfortunately, these theories - undoubtedly mathematically fascinating in managing to combine gravity and quantum mechanics in a consistent manner - exist only in ten or 26 dimensions.

These string theories are lauded as remedying the inadequacies of the present "standard model" of particle physics, in which the masses of familiar particles -such as the electron - have to be inserted in an ad hoc fashion.

In reality, string theories not only require a whole set of as yet unobserved "spin partners" of particles such as the photon, electron and neutrino, but also introduce the Planck mass, with respect to which all observed masses are in effect negligible.

Far from solving the problem of the masses of everyday particles, present-day string theory predicts these masses to be zero with the observed masses due to as yet undefined and uncalculated "symmetry-breaking" effects. And then, of course, we need to believe that the unwanted dimensions "curl up" to be more or less unobservable. With not a prediction in sight, this sure seems like metaphysics to me.

A Briefer History of Time sets out the conventional picture of the universe and quantum gravity in a much more painless way than its predecessor. Along the way, there is much discussion of the role of God in the evolution of our universe; both books end with the portentous phrase: "Then we would know the mind of God." I doubt the value of such superficial surveys of theoretical physics, and I certainly dislike the introduction of the role of God. However, I have no doubts at all that this book is targeted to appeal to a large number of people and that it will be a commercial success. I just wonder why a great physicist such as Stephen Hawking, with an extraordinary bestseller already behind him, wants to write a book such as this.

Tony Hey is corporate vice-president for technical computing, Microsoft Corporation, and visiting professor, Southampton University.