[In the excerpt below, Reeves argues that while A Brief History of Time is well-written, Hawking fails to communicate that his ideas are based on assumptions which have yet to be proven.]

A Brief History of Time is a document, both scientific and human, about a man who has fought against a terrible illness (motor neurone disease) to become one of the leading figures in contemporary astrophysics. Stephen W. Hawking is well on his way to matching the popularity of Einstein among the general public. The book tells us about the evolution of his thinking, which has deeply influenced the face of contemporary cosmology. It should be read by everybody interested in physics and astronomy. It is also highly readable, the style being brisk, sharp and often witty. The arguments, reduced to the essential, are clearly made and convincing, and Hawking's comparisons and analogies are very much to the point. He reveals himself here as a master of scientific popularization. He asks, too, the fundamental question of cosmology: "Even if there is only one possible unified theory [of the universe], it is just a set of rules and equations. What is it that breathes fire into the equations and makes a universe for them to describe?"

After presenting a summary of contemporary cosmology, Hawking introduces us to his own contribution to the physics of black holes. Quantum mechanics has reversed the diktats of General Relativity; after Hawking, "Black holes ain't so black" any more. Could it play the same trick on the classical "singularity" of the Big Bang? Hawking and his collaborators hope that it can. Their programme is grandiose, being nothing less than to give a complete theory of the universe, without a singularity and without any beginning to time.

Physical theories are made of two essential ingredients: a set of laws and a set of "arbitrary initial conditions". But, in the case of the universe, what can we say about the initial conditions? How were they chosen? Do we have to appeal to a divinity? Hawking's dream is to show that these initial conditions are not arbitrary but can themselves be derived from his cosmological model. Interesting results have already been published on the subject. But the model is still at an early stage. No one knows how far these fascinating views will be taken. Hawking warns us that the final results are still far away, but the warning is perhaps not expressed strongly enough. I have asked a number of unscientific readers of the book and they, generally, were under the impression that Stephen Hawking's views already were the new cosmology, that we need no initial singularity and no beginning to time.

Both the cosmologies advanced here are based on an assumption, which is not mentioned in either book and which may well turn out to be wrong: the assumption that the universe has just the so-called "critical" density, between an ever-expanding universe and a later, contracting one. This assumption (which, as far as I can see, is essential to both models) is not supported by observations. The present best estimates of universal density yield a value of 10 to 20 per cent only of this critical density, so despite the wishes of many theorists, and despite the efforts of many observers, there is no sign that the universe has the critical density. This should have been stated clearly in both books. "I hate it when I see beautiful theories trampled down by ugly facts", said Mark Twain. "Trampled" may be too strong here, "threatened" would be more appropriate.