I. THE RIDDLE OF TIME 1. The Riddle of Time The Nature of Time/What Is Time?/Date, Time Interval, and Synchronization/Ancient 2. Everything Swings Getting Time from Frequency/What Is a Clock?/The Earth-Sun Clock/Meter-Sticks to II. MAN-MADE CLOCKS AND WATCHES 11 3. Early Man-Made Clocks 25 Sand and Water Clocks/Mechanical Clocks/The Pendulum Clock/The Balance-Wheel Clock/ 4. "Q" Is for Quality 31 The Resonance Curve/The Resonance Curve and Decay Time/Accuracy, Stability, and 5. Building Even Better Clocks 39 The Quartz Clock/Atomic Clocks/The Ammonia Resonator/The Cesium Resonator/One 6. The "Correct Time" for the Man in the Street 49 Modern Mechanical Watches/Electric and Electronic Watches/The Quartz-Crystal Watch/ 7. Time Scales III. FINDING AND KEEPING THE TIME The Calendar/The Solar Day/The Stellar or Sidereal Day/Earth Rotation/The Continu- 8. The Clock behind the Clock 59 71 Flying Clocks/Time on a Radio Beam/Accuracy/Coverage/Reliability/Other Considera- 9. The Time Signal on Its Way Choosing a Frequency/Very Low Frequencies/Low Frequencies/Medium Frequencies/ 79 THE USES OF TIME 10. Standard Time Standard Time Zones and Daylight-Saving Time/Time as a Standard/Is a Second Really a Second?/Who Cares about the Time? 11. Time, The Great Organizer Electric Power/Modern Communication Systems/Transportation/Navigation by Radio Beacons/Navigation by Satellite/Some Common and Some Far-out Uses of Time and Frequency Technology 89 99 V TIME, SCIENCE, AND TECHNOLOGY 12. Time and Mathematics 113 Taking Apart and Putting Together/Slicing up the Past and the Future-Calculus/Conditions and Rules/Getting at the Truth with Differential Calculus/Newton's Law of Gravitation/What's Inside the Differentiating Machine?-An Aside 13. Time and Physics 123 Time is Relative/Time Has Direction/Time Measurement Is Limited/Atomic and Gravitational Clocks/The Struggle to Preserve Symmetry/The Direction of Time and Time Symmetries-An Aside 14. Time and Astronomy 135 Measuring the Age of the Universe/The Expanding Universe/Time Equals Distance/Big 15. Clockwork and Feedback Open-Loop Systems/Closed-Loop Systems/The Response Time/System Magnification or 16. Time as Information Three Kinds of Time Information Revisited/Geological Time/Interchanging Time and 17. The Future of Time Using Time to Increase Space/Time and Frequency Information-Wholesale and Retail/ Time Dissemination/Clocks in the Future/The Atom's Inner Metronome/Time Scales of the Future/The Question of Labeling-A Second is a Second is a Second/Time through the Ages/What Is Time, Really?/Particles Faster than Light-An Aside 143 151 159 This is a book for laymen. It offers an introduction to time, timekeeping, and the uses of time information, especially in the scientific and technical areas. It is impossible to consider time and timekeeping without including historical and philosophical aspects of the subject, but we have merely dabbled in these. We hope historians and philosophers will forgive our shallow coverage of their important contributions to man's understanding of time, and that scientists will be forbearing toward our simplified account of scientific thought on time in the interest of presenting a reasonably complete view in a limited number of pages. Time is an essential component in most disciplines of science ranging from astronomy to nuclear physics. It is also a practical necessity in managing our everyday lives, in such obvious ways as getting to work on time, and in countless ways that most persons have never realized, as we shall see. Because of the many associations of time, we have introduced a certain uniformity of language and definition which the specialist will realize is somewhat foreign to his particular field. This compromise seemed necessary in a book directed to the general reader. Today the United States and some parts of the rest of the world are in the process of converting to the metric system of measurement, which we use in this book. We have also used the American definitions of billion and trillion; thus a billion means 1000 million, and a trillion means 1000 billion. Several sections in this book-the "asides" printed over a light blue background-are included for the reader who wishes to explore a little more fully a particular subject area. These may be safely ignored, however, by the reader who wishes to move on to the next major topic, since understanding the book does not depend upon reading these more "in-depth" sections. This book could not have been written without the help and support of a number of interested persons. James A. Barnes, Chief of the Time and Frequency Division of the Natural Bureau of Standards, first conceived the idea of writing a book of this kind. He has contributed materially to its contents and has steadfastly supported the authors in their writing endeavors. George Kamas, also of the Time and Frequency Division, played the role of devil's advocate, and for this reason many muddy passages have been cast out or rewritten. Critical and constructive comments from many others also helped to extend and clarify many of the concepts presented. Among these are Roger E. Beehler, Jo Emery, Helmut Hellwig, Sandra Howe, Howland Fowler, Stephen Jarvis, Robert Mahler, David Russell, and Collier Smith-all members of the National Bureau of Standards staff. Thanks go also to John Hall and William Klepczynski of the United States Naval Observatory, and Neil Ashby, Professor of Physics at the University of Colorado. Finally, we thank Joanne Dugan, who has diligently and good naturedly prepared the manuscript in the face of a parade of changes and rewrites. DEDICATION The authors dedicate this book to the many who have contributed to man's understanding of the concept of time, and especially to Andrew James Jespersen, father of one of the authors, who as a railroad man for almost 40 years-understands better than most the need for accurate time, and who contributed substantially to one of the chapters. vii |