When I was a child of 8, I heard a story that will stay with me for the rest of my life. I remember my school teachers telling us about a great scientist who had just died. They talked about him with great reverence, calling him one of the greatest scientists in all history. They said that very few people could understand his ideas, but that his discoveries changed the entire world and everything around us.

But what most intrigued me about this man was that he died before he could complete his greatest discovery. They said he spent years on this theory, but he died with his unfinished papers still sitting on his desk. I was fascinated by the story. To a child, this was a great mystery. What was his unfinished work? What problem could possibly be that difficult and that important that such a great scientist would dedicate years of his life in its pursuit? Curious, I decided to learn all I could about Albert Einstein and his unfinished theory. Some of the happiest moments of my childhood were spent quietly reading every book I could find about this great man and his theories. When I exhausted the books in our local library, I began to scour libraries and bookstores across the city and state eagerly searching for more clues. I soon learned that this story was far more exciting than any murder mystery and more important than anything I could ever imagine. I decided that I would try to get t o the root of this mystery, even if I had to become a theoretical physicist to do it.

Gradually, I began to appreciate the magnitude of his unfinished quest. I learned that Einstein had three great theories. His first two theories, the special and the general theory of relativity, led to the development of the atomic bomb and the present-day theory of black holes and the Big Bang. These two theories by themselves earned him the reputation as the greatest scientist since Isaac Newton. However, Einstein was not satisfied. The third theory, which he called the Unified Field Theory, was to have been his crowning achievement. It was to be the Theory of the Universe, the Holy Grail of physics, the theory which finally unified all physical laws into one simple framework. It was to be the ultimate goal of all physics, the theory to end all theories.

Sadly, it consumed Einstein for the last 30 years of his life; he spent many lonely years in a frustrating pursuit of the greatest theory of all time. But he wasn't alone; I also learned that some of the greatest minds of the twentieth century, such Werner Heisenberg and Wolfgang Pauli, also struggled with this problem and ultimately gave up.

Given the fruitless search that has stumped the world's Nobel Prize winners for half a century, most physicists agree that the Theory of Everything must be a radical departure from everything that has been tried before. For example, Niels Bohr, founder of the modern atomic theory, once listened to Pauli's explanation of his version of the unified field theory. Bohr finally stood up and said, "We are all agreed that your theory is absolutely crazy. But what divides us is whether your theory is crazy enough."

Today, however, after decades of false starts and frustrating dead ends, many of the world's leading physicists think that they have finally found the theory "crazy enough" to be the Unified Field Theory. Scores of physicists in the world's major research laboratories now believe we have at last found the Theory of Everything.

The theory which has generated so much excitement is called the superstring theory. Nearly every science publication in the world has featured major stories on the superstring theory, interviewing some of its pioneers, such as John Schwarz, Michael Green, and Yoichiro Nambu. (Discover magazine even featured it twice on its cover.) My book, Beyond Einstein: the Cosmic Search for the Theory of the Universe, was the first attempt to explain this fabulous theory to the lay audience.

Naturally, any theory which claims to have solved the most intimate secrets of the universe will be the center of intense controversy. Even Nobel Prize winners have engaged in heated discussions about the validity of the superstring theory. In fact, we are witnessing the liveliest debate in theoretical physics in decades over this theory.

To understand the power of the superstring theory and why it is heralded as the theory of the universe (and to understand the delicious controversy that it has stirred up), it is necessary to understand that there are four forces which control everything in the known universe, and that the superstring theory gives us the first (and only) description which can unite all four forces into a single framework.