Table Of ContentJames Jespersen and Jane Fitz-Randolph
Understanding Time and Frequency
US. Department of Commerce Technology Administration
National Institute of Standards and Technology
Monograph 155, 1999 Edition
' From
Sundials'
TO
Atomic Clocks
Understanding
Time and
Frequency
i James Jespersen and
Jan e Fit z- Ran d o I p h
Illustrated by John Robb
and Dar Miner
March 1999
U.S. DEPARTMENT OF COMMERCE
William M. Daley, Secretary
TECHNOLOGY ADMINISTRATION
Gary R. Bachula, Acting Under Secretary for Technology
NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY
Raymond G. Kammer, Director
National Institute of Standards and Technology, Monograph 155,1999 Edition
Natl. Inst. Stand. Technol. Monogr. 155,1999 Ed., 304 pages (Mar. 1999)
For sale by the Superintendent of Documents, US.G overnment Printing Office,
Washington, DC 20402.
Contents
1. THE RIDDLE OF TIME
1. The Riddle of Time 3
The Nature of Timemat Is Time?/Date, Time Interval, and Synchroniza-
tiodAncient Clock Watchers/Clocks in Naturemeeping Track of the Sun and
Mooflhinking Big and Thinking Small-An Aside on Numbers
2. Everything Swings 15
Getting Time from Frequencymat Is a Clock?PThe Earth-Sun Clock/Meter
Sticks to Measure Timemat Is a Standard?/How Time Tells Us Where in
the World We AreBuilding a Clock that Wouldn’t Get Seasick
II. HAND-BUILT CLOCKS AND WATCHES
3. Early Clocks 33
Sand and Water ClocksNechanical Clocksmhe Pendulum ClocWThe Bal-
ance-Wheel ClocWurther Refinementsmhe Search for Even Better Clocks
4. “Q” Is for Quality 41
The Resonance Curvefinergy Build-up and the Resonance Curve-An
Aside on Q/ The Resonance Curve and the Decay Time/Accuracy, Stability,
and Q/High Q and Accuracy/High Q and Stabilitymaiting to Find the
Timepushing Q to the Limit
5. Building Even Better Clocks 51
The Quartz ClocWAtomic Clocksmhe Ammonia Resonatormhe Cesium
ResonatodOne Second in 10 000 000 YeardAtomic Definition of the Sec-
ond/The Rubidium ResonatorPThe Hydrogen Maser/Can We Always Build a
Better Clock?
6. A Short History of the Atom 67
Thermodynamics and the Industrial Revolution/Count Rumford‘s Can-
nodsaturn’s Rings and the AtondBringing Atoms to a HalUAtoms Collide
7. Cooling the Atom 77
Pure LighUShooting at Atoms/Optical MolassesPTrapping Atomsh’enning
TrapsPaul Trapsmeal Cool ClockdCapturing Neutral AtomdAtomic Foun-
tains/Quantum Mechanics and the Single Atom
8. The Time for Everybody 89
The First WatchesNodern Mechanical Watchesfilectric and Electronic
Watchesmhe Quartz-Crystal Watch/How Much Does “The Time” Cost?
111. FINDING AND KEEPING THE TIME
9. Time Scales 101
The CalendarPThe Solar Daymhe Stellar or Sidereal DayEarth Rota-
tion/The Continuing Search for More Uniform Time: Ephemeris Timemow
Long Is a Second?/”Rubber”S econdsPThe New UTC System and the Leap
Second/The Length of the Year/The Keepers of Timemorld Time Scales/
Bureau International de Poids et Mesures
10. The Clock behind the Clock 117
Flying ClocksPTime on a Radio Beadime in the Sky/Accuracy/Cover-
age/Reliability/Other Considerations/Other Radio Schemes
11. The Time Signal on Its Way 129
Choosing a FrequencyNery Low FrequenciesLow FrequencieslMedium
Frequenciesmigh FrequenciesNery High FrequencieslFrequencies above
300 megahertz/Noise-Additive and MultiplicativePThree Kinds of Time
Signals
IV. THE USES OF TIME
12. Standard Time 141
Standard Time Zones and Daylight-Saving Timemime as a Standarms a
Second Really a Second?/Who Cares about the Time?
13. Time, The Great Organizer 153
Electric PowerfTransportatiodNavigationb y Radio BeaconsiNavigation by
Satellitemhe Global Positioning SystedSome Common and Some Far-out
Uses of Time and Frequency Technology
14. Ticks and Bits 169
Divide and Conquer/Sending Messages the Old Fashioned Wax One Bit at
a Time/Automated TelegraphyIFrequency Division Multiplexing/Simulta-
neous Time and Frequency MultiplexinglDon’t Put All Your Messages in one
BasketKeeping the Clocks in Step
V. TIME, SCIENCE, AND TECHNOLOGY
15. Time and Mathematics 183
A New Directiod‘aking Apart and Putting TogetherISlicing up the Past and
the Future-Calculus/Conditions and RuledGetting at the Truth with
Differential CalculusAVewton’s Law of GravitatiodWhat’s Inside the Dif-
ferentiating Machine?-An Aside
16. Time and Physics 197
Time Is Relativemime Has Directiodl’ime Measurement Is LimitedlAtomic
and Gravitational Clocksmhe Struggle to Preserve Symmetrymhe Direction
of Time and Time Symmetries-An Aside
17. Time and Astronomy 21 3
Measuring the Age of the UniverseA’he Expanding Universe-Time Equals
Distance/Big Bang or Steady State?/Stellar Clocks/White DwarfsNeutron
StarsBlack Holes-Time Comes to a Stopmime, Distance, and Radio Stars
18. Until the End of Time 225
ParadoxesiTime Is Not Absolute/General Theory of Relativity/A Bang or a
Whimper?
19. Time’s Direction, Free Will, and All That 233
Time’s Direction and InformatiodDisorganization and Informatioflhase
Space for the UniverseBlack Holes and EntropyA’he Problem of Free
WilVCleopatra’s Nose/Computing the FutureiThe Brain Problem
20. Clockwork and Feedback 251
Open-Loop SystemJClosed-Loop Systemsmhe Response Time/System
Magnification or GaidRecognizing the SignaVFourier’s“ Tinker Toys”/Find-
ing the SignaVChoosing a Control System
21. Time as Information 261
Three Kinds of Time Information Revisitemime Information-Short and
Long/Geological Timehterchanging Time and Location Informatioflime
as Stored Informatioflhe Quality of Frequency and Time Information
22. How Many Seconds in a Meter? 271
Measurements and UnitdRelativity and Turning Time into SpaceNature’s
Constants and the Number of Base UnitsLength Standardshleasuring
Volts with FrequencyIStudent Redux
23. The Future of Time 285
Using Time to Increase Spacemime and Frequency Information-whole-
sale and RetaiYTime DisseminatiorJClocks in the Futuremhe Atom’s Inner
MetronomeParticles Faster than Light--An Asidemime Scales of the Fu-
turemhe Question of Labeling-A Second is a Second is a Second/Time
through the Ages/What Is Time-Really?
PREFACE
It has been two decades since the first edition of this book appeared. Over that time
launching artificial satellites has become routine, computers have become household
items, digital messages routinely travel between cities and nations by communication
satellites and optical fibers, astronomers have identified black holes, and researchers
have learned to manipulate individual and small groups of atoms. These changes have
had a profound impact on the arts of time keeping and distribution and on our
understanding of the nature of time and space.
In this new edition I have attempted to deal with these matters, and many others,
by introducing six new chapters and by making numerous changes and additions to the
chapters from the first edition.
In the beginning I imagined this book would be of most use and interest to a general
audience. In that regard it offered an eclectic, too eclectic I sometimes thought,
introduction to time, timekeeping, and the uses of time, especially in the scientific and
technical areas. But as I soon discovered, many of my colleagues referred to the book,
on occasion, to brush up on an item here and there. Perhaps this is not surprising. The
business of generating, maintaining and applying time and frequency technology is a
vast enterprise. Although this second edition does not pretend to give an in-depth,
textbook presentation, I hope it still maintains scientific integrity while continuing to
be comprehensible to the general reader.
Finally, in 1988, the National Bureau of Standards (NBS) was renamed the National
Institute of Standards and Technology (NIST). Where historically appropriate I refer
to NBS, otherwise the current designation, NIST, is used.
James Jespersen
March 1999
ACKNOWLEDGEMENTS
As I said in the preface, time and frequency is a vast subject which no single person
can be expert in. Therefore this book could not have been written without the help and
support of many people. The first edition benefited greatly from the encouragement and
suggestions of James A. Barnes who first conceived the idea of writing this book. That
first edition also came under the scrutiny of George Kamas who played the role of devil’s
advocate. Critical and constructive comments came from others who helped to extend
and clarify many of the concepts. Among those were Roger E. Beehler, Jo Emery, Helmut
Hellwig, Sandra Howe, Howland Fowler, Stephen Jarvis, Robert Mahler, David Russell,
Collier Smith, John Hall, William Klepczynski, and Neil Ashby Finally, Joanne Dugan,
diligently and good naturedly prepared the manuscript in the face of a parade of changes
and rewrites.
Like the first edition, the second edition owes much to a host of people. First, Don
Sullivan, Chief of the Time and Frequency Division of the National Institute of Stand-
ards and Technology, like his predecessor Jim Barnes, provided continuing support and
much beneficial criticism. Without his efforts this second edition would have not
materialized. Matt Young, with his ever present eagle eye, found more ways to improve
the book than I ever could have imagined. Barbara Jameson brought coherence to many
a disjointed thought while Edie DeWeese corrected many a miscue.
I also thank the following people for their useful and critical comments on individual
chapters: Fred Walls, Mike Lombardi, Dave Wineland, Marc Weiss, Chris Monroe, John
Bollinger, and Jun Liang.
Much of the success of the first edition was due to the novelty and ingenuity of the
art work. Fortunately, for me, Dar Miner has been able to continue that tradition.
Last, but not least, hurrahs to Gwen Bennett who not only prepared the manuscript,
but deciphered my practically undecipherable scribbling.
DE D I CAT10 N
The authors dedicate this book to the many who have contributed to humanity’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.
This is a book for laymen. It offers an introduction to time, timekeeping, and the
uses of time information, especially in 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 humanity’s under-
standing 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 astron-
omy 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. Additionally, in numbers with more than four
digits, every third digit is marked with a space rather than a comma to avoid confusion
in countries where the comma is a decimal marker.
Several sections in this book-the “asides” printed over a grey 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-depths ections.
