The Measurement of Time: Time, Frequency and the Atomic Clock

The Measurement of Time: Time, Frequency and the Atomic Clock , Claude Audoin and Bernard Guinot (translated from French by Stephen Lyle) Cambridge U. Press, New York, 2001. $110.00, $39.95 paper (335 pp.). ISBN 0-521-80080-3, ISBN 0-521-00397-0 paper

Several essays, philosophical or scientific, exist on the concept of time. As its title indicates, this book by Claude Audoin and Bernard Guinot covers a practical aspect of the concept of time: its measurement. Among physical quantities, time can be measured with greatest accuracy: In fact, accuracies of a few parts in 10¹⁵ are routinely reached in the laboratory. In the International System of units (SI), time is a quantity whose representative unit (the second) is defined in terms of an atomic property: the hyperfine frequency of the cesium atom in its ground state.

A book about time measurement is a challenge, and because of their experience, the authors meet this challenge exceptionally well. Audoin formerly directed the Laboratoire de l’Horloge Atomique of CNRS and Guinot previously directed the Bureau International de l’Heure. Both authors, highly respected physicists, have been active in the field for more than 40 years.

The book contains historical background that describes how time measurements evolved from an astronomical to an atomic basis; it also describes how time scales were redefined as the understanding of underlying physical concepts evolved. The authors explain atomic and astronomical time in enough detail and interlace enough basic physics to make the information extremely useful to readers who know some basic physics but are otherwise unfamiliar with the accurate measurement of time.

The authors also have set the field of time measurement in its proper, general-relativistic context. They show that the basic conclusions of general relativity are required to define time scales properly and are also required in navigation by satellite—one of the most successful applications of time measurement. Neil Ashby’s article (Physics Today, May 2002, page 41 ) highlights the authors’ theme. Unfortunately, many will find the general-relativistic framework difficult to comprehend. Toward remedying this problem, the authors have attempted to simplify the concepts. Chapter 3, although requiring serious effort to read, provides insight into the necessity of using general relativity. The authors, realizing the level of difficulty of the content, advise the daunted readers “… to move directly to the summary at the end of this chapter” (p. 17). The summary indeed helps readers to understand the parts of the book concerned specifically with time and time-scale definitions.

In chapter 5, which deals with the state of the art in frequency stability, the authors offer a good introduction to basic concepts and define a helpful set of parameters to characterize instruments and their measurements.

Any book about time measurement needs a description of the atomic clock—the key to the accuracy of modern measurements. In chapter 6, the authors not only describe these devices conceptually, but also discuss the inherent limitations of atomic-clock accuracy, the performance of current devices, and likely near-future improvements. A rather clear explanation of laser cooling reinforces the discussion in the same chapter 6.

A book of this nature would not be complete without providing readers with an understanding of the applications of accurate time measurement. In chapter 9, the authors list applications in sufficient detail to awaken a reader’s curiosity, and provide a guide to readings in these applications. The chapter helps explain the need for greater frequency stability and accuracy, and hence the need for research in that direction.

Another book, P. Kartaschoff’s Frequency and Time (Academic Press, 1978), covers the same field but is oriented mostly to engineering. Audoin and Guinot are essentially concerned with physical concepts basic to time measurement. Furthermore, Audoin and Guinot describe the present state of the art in the field. Both texts are fine complements of each other.

In all, The Measurement of Time is pleasant to read, and provides the basic information required for understanding the concepts of time measurement. It should guide the engineer involved in system design in which timing is critical, and should help the graduate student gain insight into the whole field. Finally, the text should be useful to anyone who wants to quickly obtain a general idea of time measurement. It is a good reference to have on one’s personal bookshelf.