This new book by Stephen Hawking and Leonard Mlodinow has generated some publicity and so I thought I’d check it out. The first part of my review will explain the basic questions that are being addressed by the book, the second will describe the physics behind the solutions that the authors propose, the third part will provide some of the more basic physics background that lies behind those ideas, and the last part will discuss the religious implications of the book, which have received the most attention, and some of my own reactions.
I should warn readers that cosmology and general relativity are not my fields of study, although I am a theoretical physicist and thus familiar with the basic theories of modern physics. So my knowledge of the book’s subject matter is likely to be not that much greater than that of an informed layperson. If you want a really authoritative reaction, you will need to ask your friendly neighborhood cosmologist or read reviews by them such as the one by Sean Carroll in the Wall Street Journal.
The book seeks to address three questions: Why is there something rather than nothing? Why do we exist? Why this particular set of laws and not some other? These are, of course, big questions that have long been the province of philosophers and theologians. But modern science has wrestled such questions away from them and made them into empirical questions to be addressed the same way that science addresses any questions about the physical world, making purely philosophical and theological speculations about them superfluous. Needless to say, philosophers and theologians are not happy about this development and are trying to assert that they still have a contribution to make and it is this that largely constitutes the modern science-religion debate.
To begin, we live in a universe that has three space dimensions and one time dimension, which we think of as distinct from the space dimensions. We are comfortable with the idea that there is no ‘beginning’ to space but with the conventional big bang theory there is the sense that there is a beginning to time, which naturally raises the question of what existed before that time or what triggered the start of the universe.
One answer could well be that the universe began as a quantum fluctuation and that there was no such thing as time before the universe began. The laws of science came into being with the universe and there is no mystery of why they happened to be such as to produce life like ours because if they hadn’t been, we would not be here to ponder such questions. The laws had to take some form and the very fact of our existence means that that laws happened to be such as to produce us. Such as answer is sufficient for many people.
But the authors seek answers that go beyond that, hence the book.
At present, our understanding of the physical world is spanned by theories of gravity, quantum mechanics, electromagnetism, and the weak and strong nuclear forces, each successfully working in a specific domain of application. There has been some success in straddling the boundaries of the domains, especially those areas in which quantum mechanics, electromagnetism and the strong and weak nuclear forces overlap.
Gravity has been the tough nut, the outlier, resisting strongly all attempts at combining it with other theories, and its unification with quantum mechanics has been the major challenge. Gravity is important in dealing with massive objects like planets, stars, and galaxies, while quantum mechanics deals with the very small. We use the theories of gravity to explain the large-scale structure of the universe and quantum mechanics to explain the sub-atomic world. For most things, the two domains do not overlap. But the unification of gravity and quantum mechanics becomes important in dealing with cosmological questions because when we speak of the beginning of the universe, we are talking about the entire universe being compressed into a tiny region of space and so we need a theory that combines the two domains if we are to make sense of that early state.
The main difficulty that has stumped scientists for so long is that space and time are not distinct but are intertwined due to the warping of space by gravity. At low speeds and in the presence of weak gravitational fields, the mixing is so slight as to be not noticeable which is why we perceive them as independent. The highly successful theory of quantum mechanics was developed for use in space that is ‘flat’, i.e., not warped by gravitational effects. But when we are dealing with the origins of the universe at very early times, the density of matter is extremely high. Consequently the gravitational fields are so large and the warping of space so great that the laws of physics, which were developed for use in flat spaces, appear to break down, depriving us of the only tools we have to study the world. As a result, we could not say what happened at times very close to zero or before. This has been a big barrier to progress.
The search for a quantum theory of gravity was the search for a theory that would work even under conditions of the extreme curvature of space that constituted the beginning of our universe. The original hope of Einstein and his successors in the search for such a unified theory was that it would be simple and elegant. But many have failed in this search and that goal has proved to be frustratingly elusive.
This book outlines a solution to this problem that is currently in vogue among cosmologists. It is based on what is known as M-theory and the ‘no boundary’ condition. The book lays this out in chapter 5, which is the heart of the book. (No one seems to know who coined the name M-theory or even what M stands for. I suspect that it was tossed out casually at a physics conference and became adopted by word of mouth.)
Next: M-theory and the no boundary condition.