The Handbook of Artificial Intelligence ed. by Avron Barr, Paul R. Cohen, Edward A. Feigenbaum (review)

Relativity Visualized. Lewis Epstein. Insight Press, San Francisco, 1983. 200 pp., illus. Paper, $12.95. ISBN: 0-935218-03-3. Physicists try to understand the world by creating theories to describe how things work. To grasp these theories it helps to have a concrete picture of how the theory applies to the physical system of interest. For example, Newton’s theory of gravity, applied to a baseball, can be visualized by imagining the force pulling the ball towards the earth as a large arrow pointing down from the ball. Going even further, we imagine the baseball’s velocity as two more arrows, one horizontal and one vertical; gravity makes the vertical arrow grow longer with time. Twentiethcentury physics has given us physical theories that are difficult to visualize: quantum theory and the theory of relativity. These theories defy our everyday experience, and consequently they are usually difficult to understand. Inthis book, Lewis Epstein has given us a way to visualize the theory of relativity. Epstein’s visualization-he calls it a mythforms the focus of Relativity Visualized. Chapters 1-4 describe the “Special Theory of Relativity” in a fairly standard way, chapter 5 describes the myth, and in chapters 6-12 Epstein applies his ideas to “General Relativity”. The book is written in a chatty style with a good deal of humor, which will probably appeal to the general reader. Occasionally I felt his colloquial expressions were confusing rather than amusing, and sometimes they were downright irritating. His use of “the aliens are faked out” on page 53 is an example of this. The development of the basic tenets of the theory in chapters 1-4 is quite well done, with clear examples to make the ideas accessible to a nonscientist. The myth is introduced with the idea of “enabling the inquiring mind to feel at home in a mysterious universe” (page 77). It is a unique and appealing idea, and it works. We are asked to believe that everything always travels at the speed of light. When an object is “at rest”, it is actually traveling at the speed of light through time; as speed through space increases, speed through time decreases. By imagining the arrow that describesthe velocity as rotating in a space-timediagram, the results of the first 4 chapters can indeed be easily visualized. The myth has some problems, however. Dr. Epstein never clearly points out that his space-time diagram is no/ the standard space-time diagram. Maybe this doesn’t matter to a reader who has no professional interest in the subject, but if any readers go on to study relativity seriously, they will be very confused. The myth seems to blur the distinction between proper time and coordinate time, in particular the fact that proper time is different for each different reference frame seems to get lost when Epstein talks about the twin paradox, especially in figure 5-11. His description of the resolution of the paradox occurs in a figure caption and is very weak. His attempt to explain the reasons behind the myth fails dismally. His use of the word “atom” to describe something that is not an atom in the usual sense of the word merely adds to the confusion. The last part of the book deals with general relativity and is less successful. Chapter 6, “The Big Bang”, is misleading. The blurred distinction between proper time and coordinate time coupled with an attempt to circumvent a proper treatment of expansion and general relativistic effects lead Epstein to make statements that are incorrect. Chapter 8 includes some nice discussion of mass and energy (“a wound-up springweighs more than an unwound spring”), but is marred by the derivation ofB = mczon pages 125-6, which is based on the presumption that light has mass and is quite wrong. A whole chapter is devoted to the idea that slow time makes gravity, before we are told that that is only half of the effect! In summary, Lewis Epstein has given us an interesting visualization of some results of relativity theory which should help the general reader. To the eye of a professional physicist, his development of the theme is somewhat sloppy and...