The twentieth century gave us two great theories of physics. The general theory of relativity describes the behavior of very large things, and quantum theory the behavior of very small things. In this landmark book, John Gribbin—one of the best-known science writers of the past thirty years—presents his own version of the Holy Grail of physics, the search that has been going on for decades to find a unified “Theory of Everything” that combines these ideas into one mathematical package, a single equation that could be printed on a T-shirt, containing the answer to life, the Universe, and everything. With his inimitable mixture of science, history, and biography, Gribbin shows how—despite skepticism among many physicists—these two great theories are very compatible, and point to a deep truth about the nature of our existence. The answer lies, intriguingly, with the age of the universe: 13.8 billion years.