Cosmic doughnuts followed by superstring fries?

Television advertisers have taken to heart the fact that we live in a big world full of big banks and fast-food outlets that sell big hamburgers. So it is perhaps just as well that the universe seems to have been created in a big bang.

These two popular books examine ideas from the front line of theoretical physics, where mysterious and fundamental questions about the origin of the universe - and the big bang - are being addressed. For example, did the big bang herald the very first moment of time or did it signal a transition from a different world that existed previously? Such questions may even help us to answer whether the universe was fated to contain life or whether our existence is the result of a lucky accident.

James Lidsey takes readers on a cosmic journey and concludes that the bang might have been much bigger than previously thought. The pilgrimage begins with a pedestrian approach to the structure of the universe and how stars such as the Sun shine. He then outlines the evidence that the universe is expanding, before introducing some of the latest theories in particle physics, including the idea that we may be living in a ten-dimensional universe where the extra dimensions are rolled up like tiny cosmic doughnuts.

After a quick history lesson on what happened to the universe from a fraction of a second after the big bang through to the present day, Lidsey explains the shortcomings of the big-bang model. He then describes how this model was rescued by a theory called inflation, developed in the early 1980s. This theory predicts that the early universe underwent a period of rapid expansion that may account for the structure of galaxies observed today.

The author then moves into territory uncharted by experiments and discusses the idea that the world as we know it is just a small part of a much larger universe. Hidden parts of space may still be expanding at an incredible rate and may have many more dimensions than the four with which we are familiar. Lidsey also speculates that our universe is part of a network of "baby" universes that are continually being generated inside black holes.

As a physicist, Lidsey's research interests focus on the early universe and, in particular, on inflation and the cosmological aspects of so-called superstring theory. On the whole he has made an admirable attempt to avoid confusing readers with jargon. He has also made a real effort to think of new analogies, which many cosmology lecturers are likely to steal. But good analogies are not enough. The Bigger Bang lacks the personal reflections and tales about major players in the field that make popular science books so compelling. Its style also suggests that Lidsey is not entirely comfortable writing for the lay person. Unfortunately, Lidsey's rather dry prose makes it harder to follow his account of the latest developments in cosmology, including the idea that our own universe currently sits inside a black hole. As a result The Bigger Bang is likely to go down better with physicists and students who want to learn more about the cutting edge of cosmology without ploughing through any mathematics, than with the general reader.

Meanwhile, theorist Lee Smolin is a self-styled optimist. He believes that the framework to solve one of the outstanding challenges in physics - uniting Einstein's theory of relativity and quantum mechanics - will be found by 2015 at the latest.

When this new theory, known as quantum gravity, has been developed, it will provide answers to questions of what space and time are. It will also contain insights into the elementary particles and the forces that govern them, and it will be a theory of cosmology. Discovering quantum gravity is a tall order, but Smolin sticks his neck out further by predicting that experimental tests will be available shortly after the theory is formulated. It is this kind of bold claim, together with the author's lively style and recollections of 25 years in the quantum-gravity business, that make his book so readable.

Smolin charts the three different routes that theorists are taking to find a single law governing the history of the universe, including the big bang. Such a theory has to reconcile Einstein's views of gravity with the rules of quantum mechanics, which govern the elementary particles that were created in the big bang.

The most well trodden of these paths originates from quantum mechanics and has led to a superstring theory. According to this idea, tiny loops of strings replace the elementary particles that form the building blocks of matter and carry the forces in nature. In much the same way that different vibrations of guitar strings produce distinct notes, vibrations of these strings correspond to the particles. Crucially for quantum gravity, one of these particles carries the gravitational force. Although there is no unique superstring theory, theorists have developed a successor known as M-theory, which looks a promising candidate for a theory of quantum gravity.

Smolin was one of the first people to walk the second route, known as loop quantum gravity, which is based on the idea that space is made of discrete chunks. He describes this period as the most exhilarating of his life. During his time researching quantum gravity, Smolin has had his feet in both camps and he understands the bigger theoretical picture, which, he complains, many people who work on quantum gravity do not.

He also respects those few brave souls who discard both relativity and quantum mechanics as starting points for quantum gravity and instead formulate their own approach. He believes that this route will give essential insights into the nature of space and time.

Three Roads to Quantum Gravity is full of challenging concepts, but Smolin has a knack of making these big ideas appealing. In a final bold statement, he predicts that quantum gravity will be taught in schools by the end of the century. I look forward to seeing what TV advertisers will make of that. Anyone for a ten-dimensional burger with superstring fries?

Valerie Jamieson is features editor, Physics World , and a former particle physicist.