Stephen Hawking's insights are old hat to physicists, so why do they provoke such furious debate?

Even before his latest best-seller hit the bookshops, the wheelchair-bound physicist had provoked worldwide debate about the deepest issues in science, philosophy and religion.

Stephen Hawking lectures in June at Canada's University of Waterloo, where he holds a distinguished research chair.

No academic can whip up a controversy faster than Stephen Hawking. Even before his latest immediate best-seller The Grand Design hit the bookshops last week, the wheelchair-bound theoretical physicist had provoked worldwide debate about the deepest issues in science, philosophy and religion. Are scientists now able to explain the origin of the cosmos? Is the notion of a cosmic creator no longer necessary? Are the views of philosophers on such matters now redundant? The former holder of Sir Isaac Newton's chair of mathematics at Cambridge University and arguably the world's most famous scientist insists that the answer to all the questions is - yes.

It is some feat to seize the attention of the lay public, academia and religious leaders at the same time, but then Prof Hawking is used to doing the seemingly impossible. To be a world-class theoretical physicist is hard enough; to succeed while dealing with a progressive neurodegenerative disorder is nothing short of astonishing. Yet for anyone familiar with Prof Hawking's career, the biggest surprise is that he can still conjure up a furore. After all, he has been telling us pretty much the same thing ever since his original best-seller A Brief History of Time appeared more than 20 years ago - and even then most of what he was saying was years old.

Take his supposedly startling claim that the universe may have burst into existence of its own accord. The first hints of this possibility emerged in the early 1970s, with a celebrated paper in the journal Nature entitled "Is the universe a vacuum fluctuation?". Its author was a physicist named Edward Tryon, who pointed out that some features of the universe are consistent with it emerging from the so-called quantum vacuum.

Put simply, this is a consequence of the famous Uncertainty Principle of quantum theory, according to which matter and energy can emerge spontaneously from nowhere, as long as they do not hang around too long. This is no mere theoretical possibility: since the 1940s physicists have detected so-called vacuum particles emerging literally from nowhere and vanishing again. The Uncertainty Principle demands that the particles do not last very long - less than a billion-billionth of a second - but during their brief existence they give rise to effects which tally with theory to many places of decimals.

But the fleeting existence of sub-atomic particles is one thing; could the entire universe also appear out of nowhere, and continue to exist for at least 14 billion years? That is a big ask, as the Uncertainty Principle requires that the bigger the amount of mass or energy conjured up, the less time it can endure - and the universe is both hefty and old. Prof Tryon pointed out a possible loophole, however, based on a quirk of the force of gravity, which imbues it with negative energy. He argued that this could more or less exactly cancel out the mass-energy of the universe, allowing it to appear from nowhere and persist for billions of years.

Prof Tryon's idea was admittedly rather sketchy. How, for example, did it explain the fact that the universe not only exists, but is expanding? Even so, the basic idea that the universe may owe its origins to quantum vacuum effects has been mainstream for almost 30 years. Small wonder, then, that many cosmologists will struggle to understand the furore over Prof Hawking's claim last week that "because there is a law such as gravity, the universe can and will create itself from nothing".

They will doubtless not be surprised, however, by the facile misunderstanding by the media of his claim that "it is not necessary to invoke God to light the blue touchpaper and set the universe going". This has been widely interpreted as implying that - as one leading newspaper put it - "God did not create the universe, says Stephen Hawking". Of course, it implies no such thing. Whatever Prof Hawking's belief in God, he undoubtedly knows the difference between a proof of redundancy and an outright disproof. All he has done is espouse an alternative explanation for the origin of the universe that seems not to require a creator - which falls far short of actually proving a creator was not involved.

What does seem to have surprised some academics are Prof Hawking's pronouncements on philosophy. His antipathy towards its practitioners is well-known. In A Brief History of Time he declared that philosophers "have not been able to keep up with the advance of scientific theories". Last week he dusted off this assertion, adding: "Their discussions seem increasingly outdated and irrelevant". Some leading philosophers, such as Anthony Grayling of the University of London, have responded by claiming they do take on board the latest cosmological theories, and amend their arguments accordingly. But this misses the real irony of one of the few apparent about-turns in Prof Hawking's thinking since his first best-seller.

In The Grand Design he makes much of M-theory, a loose collection of mathematical ideas developed by theorists since the mid-1990s. Prof Hawking is among those who believe M-theory may turn into the "Theory of Everything" that sums up all the properties of the universe and its contents in a single set of equations. But this belief comes at a very high price. While little is known about the final form of M-theory, it appears to demand the existence of not one but a myriad different universes, all of which may be part of one grand "multiverse".

As ever, the idea that we may live in just one of many possible universes has been around for decades, and for decades was rejected as simply too outlandish. Only now are theorists starting to take it seriously. Not that they can point to any compelling observational evidence for their belief; indeed, many suspect it will never be possible to prove the existence of these alternative universes. All that has happened is that Prof Hawking and his colleagues have overcome their initial revulsion at the idea, and decided it might have some advantages after all. In short, they have simply swapped one philosophical view of reality for another.

It is always fascinating to have insights into the minds of great thinkers, and for that reason alone The Grand Design deserves a wide readership. But many readers may well come away suspecting that when it comes to creating something out of nothing, few can do it better than a 21st-century theoretical physicist. Robert Matthews is Visiting Reader in Science at Aston University, Birmingham, England