It’s About Time: Understanding Einstein’s Relativity , N. David Mermin , Princeton U. Press, Princeton, NJ, 2005. $29.95 (192 pp.). ISBN 0-691-12201-6

I first came across relativity theory in an example featured in an instruction book for an early electronic calculator. I was 11 years old and was amazed to read that an astronaut could travel for 50 years (as measured on Earth) near the speed of light and return to Earth with his clocks showing that only 25 years had elapsed since his journey began. I wanted to know more—and I wish I’d had David Mermin’s new book then!

*It’s About Time: Understanding Einstein’s Relativity * is based on Mermin’s lectures to non-science majors at Cornell University, which the author has been giving intermittently for the past 30 years. This is his second book on relativity theory, the first being *Space and Time in Special Relativity * (McGraw-Hill, 1968). Both books share the goal of introducing special relativity to nonexperts; in broad terms, both cover similar topics. But since his first book, Mermin’s ideas about how relativity should be taught have developed considerably. Given the lectures on which the new book is based, it’s no surprise that *It’s About Time* contains only simple algebra; yet there’s nothing superficial about the level of understanding that it aims to impart.

The book begins with an unusually clear discussion of the principle of relativity in Newtonian mechanics. The reader needs very little prior knowledge. For example, in chapter 1, Newton’s first law is stated, and Mermin spells out the distinction between speed and velocity. Reference frames are introduced and used to predict what will happen in simple collisions. The way in which velocities transform between frames is considered carefully, so that when the constancy of the speed of light appears for the first time in chapter 3, its strangeness is immediately apparent. Later chapters cover the relativistic velocity-addition law, simultaneity, and measurements of time and distance by different observers. The book ends with an introduction to spacetime diagrams, a derivation of *E* = *mc* ^{2} in which the algebra gets a little harder, and a brief look at time in general relativity.

Topics covered are standard for a book of this sort. What makes Mermin’s book special, however, is the high quality of exposition at every level. He offers such delightful minor touches as using “Alice” and “Bob” to replace the reference frames *S* and *S*’. His use of the foot as the unit of length—redefined so that the speed of light is exactly 1 ft/ns—is amusing, especially when combined with centimeters. The book has plenty of clever and—to me, at least—original arguments that get the important ideas across in a very clear way. For those who wish to teach the subject themselves, one example in particular, which involves two trains with opposite velocities, is well worth copying. What makes the book as a whole so enjoyable to read is the steady pace at which the subject unfolds. The author spends as much time on each idea as he considers necessary instead of treating each in proportion to the number of questions anticipated on an end-of-the-year examination. As a result, the book has excellent sections on reference frames and collisions in nonrelativistic mechanics, and its long introduction to spacetime diagrams is the best I’ve ever seen. Nowhere is the book too intense, and the learning curve for readers has a fairly constant slope.

The book contains no four vectors and no Lorentz transformation; it mentions the Michelson–Morley experiment only in passing. Despite its elementary level, I’d recommend *It’s About Time* to physicists. They won’t learn any new physics, but they can watch Mermin, a master teacher, at work—and instructors will almost certainly include some of the ideas in their own teaching. Although Mermin’s book doesn’t have enough material to make it suitable as a standalone text for an undergraduate course, any undergraduate can read it with profit, either before or after learning about special relativity.

What about younger people? I teach physics to pupils aged 13 to 18, and I shall certainly be ordering some copies for a few of them to read. The book’s high English-to-math ratio means that even some of the 13-year-olds could get something out of it. Those 16 and older should be able to learn a lot from the book, and I’m sure they’d enjoy it. The physics I teach at school rarely involves chains of reasoning with more than two steps. I hope *It’s About Time* will show some of my students what real physics is like.