**It's About Time: Understanding Einstein's Relativity**

*It’s About Time: Understanding Einstein’s Relativity* began as a series of notes and lectures Mermin, a physicist at Cornell University, wrote for his nonscientist students during the 1990s. The notes were continually organized and revised as students raised new questions.

According to the author, only a basic knowledge of high-school algebra and plane geometry is needed to comprehend the theory. That said, he also warns that although the mathematical level in this book is elementary, it cannot be read like a novel.

The book begins fairly easy, with Mermin’s explanation of events from two different frames of reference. To make it simpler, he gives each point of reference a name: Alice and Bob. The key to understanding relativity is to be able to change points of reference: “Take a situation which you don’t fully understand. Find a new frame of reference in which you do understand it. Then translate your understanding in the new frame back into the language of the old one.” (6) To be able to do this, visualization is crucial, yet not so easy to achieve. To illustrate this, the author includes diagrams and simple equations. Since his examples of moving frames of reference are at low speeds everybody can relate to, the concept isn’t difficult to grasp at first. It is later, when the idea of the absolute constancy of the speed of light is introduced, that the theory becomes a real challenge to grasp.

As the book progresses, it becomes more difficult, with increasingly complex ideas and diagrams. Concepts of colliding objects, combining velocities, simultaneous events, and synchronized clocks are explained. Particularly fascinating are the chapters on moving clocks and sticks: Do moving clocks actually run slowly? Do moving sticks actually shrink? The reader will be surprised to find out that, yes, they actually do. The answer, of course, would depend on whether or not the clock or the stick is moving toward you or away from you. Mermin also explains the Doppler effect, the difference between the general and the special theory of relativity, and how both the objects that move at the speed of light and close to the speed of light behave in strange ways. In his chapter on space-time, he uses the idea of equilocs and equitemps, which he developed.

Is this a book for the average lay reader or the beginner nonscientist? No.

It is misleading to claim that the theory of relativity is easy to understand when even some physics and math teachers struggle with it.

Most of the concepts in the book are a challenge to grasp for the nonscientist, especially those who have already forgotten their high-school algebra and geometry, and who didn’t take math in college. This is a book to read, re-read, study and ponder on. Beginners will have questions as they move through the chapters, questions that can only be answered by a teacher in a classroom environment. In this sense, this would make a fine course book. Mermin also intended the book for a secondary audience: undergraduate physics majors, graduate students, and even professional practitioners of relativity. Hence, toward the end of chapters, the author delves further into the subject and becomes more technical and complex.

The problem with the book is that its intended audience – beginner, nonscientists and professionals (this last one even if secondary)- is too broad. How can one combine both readerships in the same book when dealing with such a difficult subject? The beginner will finish reading, only having grasped a small portion of it, while the professional will be left wanting more complex mathematics.

Besides the profundity of the topic itself, often the passages are unnecessarily verbose and unclear, confusing the reader. Consider this example: “A particularly important collection of events, for an object small enough, on a length scale of interest, to be considered to occupy just a single point of space at any moment of time, is the set of all events at which the object is present.” (107). Given the rather steep learning curve of the book’s primary audience, the text should have been simplified by an editor. For the reader, the theory is hard enough to grasp also to have to deal with ambiguous sentences.

Beginners who are interested in learning about the theory of relativity should first read *Relativity Simply Explained*, by Martin Gardner, before moving on to this book. Even then, Mermin’s book will be challenging outside of a classroom setting. It’s About Time should have been aimed at teachers of beginners and not at beginners alone.

*An award-winning author, reviewer and freelance writer living in Belgium*.