| This author is one of the most briliant, the most optimistic, and the most enthusiastic writer in all of physics, and in this book, his competence as a physicist and his deep fascination with the physical world is brought out dramatically. He is clearly a man who is feeling a powerful sense of exhiliration of the discoveries now taking place in all areas of knowledge. His foundation and his theme in the book is a simple geometric principle, namely that the boundary of a boundary is zero. He then guides the reader, assumed to have a rudimentary knowledge of mathematics, in a splendid presentation of the power of this principle in gravitational physics. The first chapter is an overview of the history behind the subject, via the work of people who contributed to our current understanding of gravity. And then, with a masterfull use of diagrams he gives the reader a taste of the simplicity of the equivalence principle and the need to tack on an additional dimension (time) to the 3-dimensional space of everyday experience. The Pound-Rebka experiment is discussed as one that illustrates the idea of the spacetime interval, and the role of time dilation is discussed via the possibility of practical space travel. And such enthusiasm in his dialog: "the universe will grow ever more exciting", he says, and looking at the developments now taking place in today's science, he is indeed correct. Chapter 4 gives a fascinating overview of what the author calls the boomerang, which illustrates the action of curvature on nearby test masses. This thought experiment involves the motion of a spacecraft through an imaginary tunnel through the Earth. The author analyzes the motion from the standpoint of Newtonian physics and general relativity. Curvature as the "grammar of gravity" is the topic of the next chapter, with illustrations of the paths of ants on spaces of zero, positive, and negative curvature. A very intuitive treatment of parallel transport around a closed path on a curved surface is given. The tides are discussed as a natural manifestation of the gravitational influence of the Moon on Earth. Must difficult for a layman to understand is how spacetime acts on masive objects, but the author explains it brilliantly in the next chapter, taught via the concept of "momenergy". This entity is a 4-vector, and the author uses it to show how its creation in a spacetime region can be written as the sum of 8 terms, reflecting the fact that the "boundary" of a four-dimensional block in spacetime consists of eight three-dimensional cubes. That the contents of these cubes sum to zero is the famous "boundary of a boundary is zero", which is discussed in the next chapter. This chapter is one of the best explanations ever given (at this level) of the physics behind spacetime curvature and massive objects. The actual mathematical quantification of curvature is detailed in chapters 8 and 9, using elementary mathematics. The author discusses nicely the famous Scharwzschild geometry. Concepts of a more concrete nature are discussed in chapter 10, wherein the author discusses the famous Pound-Rebka experiment and planetary motion. This is followed by a discussion of the elusive gravitational waves in chapter 11. Again with a clever use of illustrations, the author explains the transverse property of gravitational waves, and compares gravitational waves with electromagnetic waves. The role of the quadrupole moment in the creation of gravitational waves is brought out briliantly by the author. He discusses briefly various attempts to detect gravitational waves. Black holes are the topic of chapter 12, wherein the famous Penrose process for extracting energy from a black hole is discussed, and the "no-hair" theorem for black holes. A neat symbolic representation of the Bekenstein number of a black hole is given. The role of the Hawking process, connection quantum processes with the physics of black holes is briefly discussed. The author ends the book with a look at the expansion of the universe, the missing mass problem, and another very interesting topic that has gained much attention recently: the concept of gravitomagnetism. This is a "weak-field" prediction of general relativity, and predicts that the rotation of the Earth should influence the motion of orbiting satellites. This topic is currently bringing together ideas such as the quantum Zeno effect, Mach's principle, and the notorious "frame dragging" effect in general relativity. Experiments do measure it are currently in play and in the proposal stage, namely the LAGEOS and LAGEOS II experiments, which measure the gravitomagnetic orbital perturbation, which is known as the Lense-Thirring effect. |
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