The current understanding of the physical structure of the universe is bipolar. There is Einstein's theory of relativity, which explains the macroscopic behavior of the universe to many places to the right of the decimal point. At the other end of the size spectrum, there is the quantum theory of fields, which explains the observed behavior of fundamental particles to many places to the right of the decimal point. Although one should always be very reluctant to state such a position, the resolution of this bipolar state into a unified one may be the last, great discovery of physics.
The purpose of this book is to present a debate between Stephen Hawking and Roger Penrose concerning the possibility of the issue being resolved, and in what manner. It is a series of six short lectures, three from each man and ends with a brief debate between them. These lectures are not for the general audience, as each lecturer assumes a fundamental understanding of general relativity and quantum theory. Nevertheless, there is a great deal of explanation, including diagrams, in the lectures. Therefore, it is possible to understand the material if you have a basic understanding of the two main topics. Without that, don't bother opening the book.
Of course, the issue is not resolved, as that must wait for a later date. It is interesting that Hawking tends to emphasize the points of difference, while Penrose goes to some length to describe how similar their positions are. Penrose continues with the position of Albert Einstein, in that he argues that quantum mechanics is not a final theory, but only the "gross" appearance of much subtler events. Hawking believes otherwise, arguing that the probabilistic features of quantum mechanics is the way nature does things, and there is no underlying mechanism yet to be discovered that will remove them.
The arguments are strong, yet unconvincing. Not due to their lack of power, but because they are made by two equally strong and forceful personalities. When two such powers collide, there is rarely resolution. Nevertheless, the debate sheds a great deal of light on the current state of thinking in physics, and points out some ways in which it may be resolved.