Speed restrictions need not apply

The title of this book says it all. The assumption that the velocity of light in a vacuum is constant, at 299,792.5km a second, is one of the cornerstones of the theory of relativity. If it can be shown to be wrong, and that the speed of light has changed since the birth of the universe, then many of our most deeply entrenched ideas will have to be dramatically revised. And this is precisely what is being suggested by 37-year-old Portuguese theoretical physicist João Magueijo. Heresy? Undoubtedly - but can there be any evidence in its favour?

It is now almost a century since Einstein published his special theory of relativity, followed later by the general theory. It has been challenged many times, but with no success; it has survived every test that has been made. Whether Magueijo's variable speed of light (VSL) theory will lead to a different result remains to be seen.

According to most cosmologists, the universe as we know it began with a "big bang" 13.7 billion years ago; everything - space, time, matter - came into existence at the same moment. Expansion began and has continued ever since.

However, there are some aspects that are hard to explain, and one of these relates to what is known as the "horizon problem". Using suitable equipment, look at a galaxy 10 billion light-years away; then turn and look at another galaxy, also 10 billion light-years away, in exactly the opposite direction. There can be no contact between the two galaxies because it would take light 20 billion years to travel from the one to the other, and the universe is less than 14 billion years old. We must ask how it can be that the universe appears to be the same in all directions, both in content and in temperature.

The problem is now solved simply by going back to the time when the universe was very young and comparatively small; the same restrictions would apply. The preferred solution is that at a very early stage there was a brief period of very rapid inflation, when the universe expanded to many times its original size; subsequently, the rate of expansion slowed to its current value. Yet this is not entirely satisfactory, and important questions remain unanswered.

But suppose that, in the early life of the universe, light travelled faster than it does now? Systems in opposite directions would then have been able to make contact, and there is less difficulty in accounting for the strange uniformity wherever we choose to look.

Initially, there are bound to be misgivings about any concept as revolutionary as this, and it must be said that in some ways Magueijo does not help, because here and there he gives the impression of being an eccentric. The whole idea of VSL came to him "in a flash" as he was walking across the playing fields of St John's College, Cambridge, on "a damp winter morning"; this would solve everything - his equivalent of Archimedes' "Eureka". He feels that he is surrounded by stupidity and implies that critics of his ideas are beneath contempt.

He has hard words to say about Cambridge, and maintains that "Brits" have been doubtful about inflation theory because it was first proposed by Americans. Even past pioneers are not spared: Edwin Hubble was "an insufferable snob, disliked by all", though this was certainly not my impression of him (and I knew him, whereas Magueijo did not). Some of his words and phrases strike a jarring note and are quite unsuitable for inclusion in a scientific book. The text would have benefited enormously from the attentions of a skilful sub-editor.

Yet, despite all this, Magueijo is no eccentric. He is a highly respected academic, and his theories, remarkable though they may be, must be taken very seriously indeed. Basically, he writes extremely well, presents his case with admirable clarity and makes no attempt to gloss over the difficulties and uncertainties of VSL. In evaluating it, there is no room for compromise. It may well be that the whole concept will be discredited and will be regarded as simply yet another attempt to undermine Einstein.

But there is also a very distinct chance that Magueijo is right - and, if so, he will be ranked with the greatest cosmologists of the age, while his book will become a classic. Time will tell.

Sir Patrick Moore, FRS, is the author of more than 60 books, mainly on astronomy.