The central question of this book is 'what are space and time'. Although these are familiar concepts to us, they have always been a source of interest to philosophers and scientists. Newton assumed that space was a fixed framework in which events took place, whereas Leibniz and later Mach claimed that to talk of space separately from the events within it didn't make any sense. In this book Greene explains what more recent scientific theories have to say on this question.

The first half of the book is a look at what modern physics tells us about the nature of space and time. Thus the book starts with special and some general relativity. Greene examines various philosophical perspectives. He looks at Mach's ideas - how distant matter can have an influence on the water in the rotating 'Newton's Bucket'. He also explains how relativity suggests the notion of the block universe, and leads to the speed of light being the ultimate speed limit. We are then introduced to quantum theory, in particular how this speed limit is challenged by entanglement and Bell's inequality. Moving on to Time, there is the crucial question of why it appears to have a preferred direction, when physics is mostly time symmetric. Greene looks various ideas explaining this 'arrow', in particular he considers thermodynamics and the possibility that the universe might have been a low entropy fluctuation, but shows how this leads to the conclusion that you probably came into existence - but with all your current memories ready formed - a few seconds ago.

Greene describes current thoughts on the nature of the universe - how it started with the big bang, and is described using Einstein's General relativity. This book has plenty of illustrations which help the reader to understand such things as curved spacetime, and the properties of finite and infinite models of the universe. The universe is expanding, and furthermore this expansion is accelerating. Greene looks at ideas of what might cause this acceleration - both today and in the presumed inflationary era. This means dealing with some fairly esoteric stuff - Higgs and Inflaton fields, and so particle physics, but Greene manages to convey it all in a way that makes it seem clear. Of course there is quite a bit on string theory, but for a fuller treatment you should read The Elegant Universe. In the current book Greene is more concerned with moving on to more speculative ideas such as branes, and showing how these might be used to explain the origin of the universe we know. I was interested to read explanations of why the entropy of the early universe was so low - which is recognised as a fundamental question of physics - and how this relates to inflation.

This book is aimed at readers of popular science, and it does contain a lot of the claims and visualisations of such books which I find rather dubious. The balloon (stretchy space) model of cosmology, the indentation model of gravity and others are there. I won't dwell on these here as you can find elsewhere on this website, and I have to admit that such visualisations do have their uses, its just that I'd like to see some acknowledgement of their flaws. I'll restrict myself to looking at the analogy of travel through space with travel through spacetime (see box). Although the analogy with space is useful, it is necessary to realise that the metric structure of spacetime is different from that of space.

In The Elegant Universe Greene gave us a guide to string theory. This book is more general, and at first I thought that it was a 'beginners guide to physics', but in fact although it is non-technical, it is intellectually deep, and is likely to have something of interest even for those experienced in the subject - there are plenty of notes at the end for those wanting more details. This is a much more philosophical book than The Elegant Universe, but don't let that put you off - it's still very readable. You may have seen Greene's TV appearances, and will realise that he has a talent for explaining advanced material in an easy to understand manner. In fact he manages to include a great deal of modern physics without resorting to equations, and I would recommend this book to anyone wanting to know about current ideas regarding the universe in which we live