String theory is seen as a way of reconciling Einstein's theory of relativity with quantum mechanics. According to the theory, the fundamental particles and their interactions are generated by the vibrations of tiny loops of string. Progress in the field has been rapid in recent years. Recent breakthroughs have included the demonstration that five previously distinct string theories are in fact all variations of the same theory. And although string theory is a notoriously difficult subject to explain, the book has received lavish praise from several quarters. "Everyone who is curious about the horizons of theoretical physics will greatly enjoy this book, " writes Ed Witten, one of the world's leading string theorists, on the blurb on the back of the book.

Greene spent two years writing the book, after being approached by the New York literary agent John Brockman. "I mainly worked during the evenings so that my writing wouldn't encroach on my research work, " explains Greene. However, he admits he was not sure if the book would succeed. "I tried to keep it informal by writing as if I was talking to someone about string theory, but when you're writing you just never know what the reaction will be."

Since the book was published Greene has appeared on several television talk shows and has been interviewed by the press and radio. "I have enjoyed the media attention, although it takes lots of energy and concentration." However, he says he has not felt any animosity from his colleagues towards the book's success. "They have generally been supportive. String theorists have very much been working behind closed doors and they are glad that their work is being exposed more widely."

That's a view echoed by his namesake Michael Green from Cambridge University, one of the inventors of string theory. "I know Brian quite well. He's one of the leading researchers on the somewhat more mathematical side of string theory, " says Green. "He's a natural person to have written the book because he is well known for his popular lectures and he knows how to explain the subject with great clarity to non-experts."

But Michael Green is a little mystified why the book has sold so well so quickly. "I'm very gratified that string theory is proving so popular with the public, but the problem is that there's no real punch-line yet because it's very much work in progress. In fact, I'm rather jealous since I'd have liked to have written this book myself!"