Stephen Hawking’s worldwide bestseller, A Brief History of Time, has been a landmark volume in scientific writing. Its author’s engaging voice is one reason, and the compelling subjects he addresses is another: the nature of space and time, the role of God in creation, the history and future of the universe. But it is also true that in the years since its publication, readers have repeatedly told Professor Hawking of their great difficulty in understanding some of the book’s most important concepts.

This is the origin of and the reason for A Briefer History of Time: its author’s wish to make its content more accessible to readers—as well as to bring it up-to-date with the latest scientific observations and findings.

Although this book is literally somewhat “briefer,” it actually expands on the great subjects of the original. Purely technical concepts, such as the mathematics of chaotic boundary conditions, are gone. Conversely, subjects of wide interest that were difficult to follow because they were interspersed throughout the book have now been given entire chapters of their own, including relativity, curved space, and quantum theory.

This reorganization has allowed the authors to expand areas of special interest and recent progress, from the latest developments in string theory to exciting developments in the search for a complete unified theory of all the forces of physics. Like prior editions of the book—but even more so—A Briefer History of Time will guide nonscientists everywhere in the ongoing search for the tantalizing secrets at the heart of time and space....

Einstein said that the most incomprehensible thing about the universe is that it is comprehensible. But was he right? Can the quantum theory of fields and Einstein's general theory of relativity, the two most accurate and successful theories in all of physics, be united in a single quantum theory of gravity? Can quantum and cosmos ever be combined? On this issue, two of the world's most famous physicists--Stephen Hawking (A Brief History of Time) and Roger Penrose (The Emperor's New Mind and Shadows of the Mind)--disagree. Here they explain their positions in a work based on six lectures with a final debate, all originally presented at the Isaac Newton Institute for Mathematical Sciences at the University of Cambridge.

How could quantum gravity, a theory that could explain the earlier moments of the big bang and the physics of the enigmatic objects known as black holes, be constructed? Why does our patch of the universe look just as Einstein predicted, with no hint of quantum effects in sight? What strange quantum processes can cause black holes to evaporate, and what happens to all the information that they swallow? Why does time go forward, not backward?

In this book, the two opponents touch on all these questions. Penrose, like Einstein, refuses to believe that quantum mechanics is a final theory. Hawking thinks otherwise, and argues that general relativity simply cannot account for how the universe began. Only a quantum theory of gravity, coupled with the no-boundary hypothesis, can ever hope to explain adequately what little we can observe about our universe. Penrose, playing the realist to Hawking's positivist, thinks that the universe is unbounded and will expand forever. The universe can be understood, he argues, in terms of the geometry of light cones, the compression and distortion of spacetime, and by the use of twistor theory. With the final debate, the reader will come to realize how much Hawking and Penrose diverge in their opinions of the ultimate quest to combine quantum mechanics and relativity, and how differently they have tried to comprehend the incomprehensible....