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Sunday, December 14, 2003

Relativity Demystified

I just finished reading Simply Einstein: Relativity Demystified by Richard Wolfson, professor of physics at Middlebury College. This small book (240 pages) is written for the non-scientist and does a great job of explaining both Einstein's theory of special relativity and his later theory of general relativity.

I really like the way the author tells the story in the first 5 chapters of scientific discovery from a historical perspective starting with Aristotle's conception of the natural state of motion (at rest), on to Galileo's and Newton's views and the idea of Galilean relativity (physics works the same as long as your in a uniform state of motion), and finally to a brief overview of electricity, magnestism and their synthesis by James Clerk Maxwell around 1880.

He then starts his exploration of Einstein's special relativity with a discussion of the Michelson-Morley experiment and how it showed how the speed of light is the same for all observers (the basis of relativity from which all else follows) although Michelson and Morley didn't believe their own results. His explanation of special relativity then consumes the next 8 chapters and is presented in a conversational tone that plays on your natural questions (which I'm sure are driven from his teaching experience) and assumptions, many of which are of course wrong in a world described by relativity. One of the points the author touches on is the same one I made in a previous post, namely that the way we think about nature is based on our everyday experience confined to an area of weak gravity and moving at a relatively very slow speed (what Bacon called "Idols of the Tribe") and it is that experience which makes it impossible to intuitively understand relativity. We can understand it intellectually and mathematically but can never know it "in our bones".

The only math in the entire book is an explanation of the time dilation equation which explains how time is relative for different observers travelling at different speeds (the famous "twins paradox"). An appendix covers the details but the math is no more difficult than 10th grade geometry. Although time dilation is the most well known result of special relativity he also explains the relativity of distance and the order of some classes of events for two different observers. For me, those sections were by far the most challenging and interesting in the book.

One of the great features of this book is that the author constantly reminds the reader of correct relativistic terminology and thinking. For example, it is relativistically incorrect to say that in the case of the twins paradox that "moving clocks run slow". They only run slow from the perspective of an observer in a different frame of reference.

The last three chapters touch on Einstein's general relativity and how it describes the nature of gravity as the curvature of spacetime. Since the mathematics involved in general relativity (non-Euclidian geometry for starters) are complex, he does not attempt to describe any of it but merely describes the axoims of general relativity; that spacetime is curved and that curvature determines the natural state of motion of objects, and that the curvature is gravity, the nature of which is determined by the amount of mass+energy present. He has a great analogy of a basketball sitting on a taught rubber sheet and how marbles or other objects rolled on the sheet mimics in 2 dimensions what is happening in 4 dimensional spacetime in general relativity.

The book has plenty of diagrams and the chapters are short enough to read in a single sitting, which is important for a book like this where you need to digest a complete argument before moving on.

Wolfson also has some lectures available from the Teaching Company which follow the same lines as the book. However, I enjoyed the book more since you really need the diagrams to better understand the concepts. I've also attempted to read other short books on relativity but none are as clear and incorporate the historical perspective, which I always enjoy. A highly recommended book.

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