SPLITTING THE SECOND

SPLITTING THE SECOND

The Story of Atomic Time

Tony Jones

INSTITUTE OF PHYSICS PUBLISHING

BRISTOL AND PHILADELPHIA

c© IOP Publishing Ltd 2000

All rights reserved. No part of this publication may be reproduced,stored in a retrieval system or transmitted in any form or by any means,electronic, mechanical, photocopying, recording or otherwise, withoutthe prior permission of the publisher. Multiple copying is permittedin accordance with the terms of licences issued by the CopyrightLicensing Agency under the terms of its agreement with the Committeeof Vice-Chancellors and Principals.

British Library Cataloguing-in-Publication Data

A catalogue record for this book is available from the British Library.

ISBN 0 7503 0640 8 pbk

Library of Congress Cataloging-in-Publication Data are available

Publisher: Nicki DennisProduction Editor: Simon LaurensonProduction Control: Sarah PlentyCover Design: Victoria Le BillonMarketing Executive: Colin Fenton

Published by Institute of Physics Publishing, wholly owned by TheInstitute of Physics, London

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Contents

Foreword vii

Preface ix

1 Astronomers’ Time 1

2 Physicists’ Time 25

3 Atomic Time 53

4 World Time 69

5 The Leap Second 95

6 Time Transfer 115

7 Uses of Accurate Time 141

8 The Future of Time 161

Appendix Timekeeping Organisations 183

Glossary of Abbreviations 189

We wish to acknowledge the following for permission to reproduce fig-ures. Science Museum, London (Figure 1.6). Bureau International desPoids et Mesures (Figure 1.7). National Physical Laboratory c© CrownCopyright 2000. Reproduced by permission of the Controller of HMSO(Figures 1.8, 2.3, 2.12, 4.6, 4.9, 5.7, 6.5, 6.6, 7.1, 7.4, 7.7, 8.2, 8.4, 8.6).National Institute of Standards and Technology (Figures 2.10, 4.2, 8.5).United States Naval Observatory (Figure 3.2). Physikalisch-TechnischeBundesanstalt (Figure 4.1). Bureau National de Metrologie, LaboratoirePrimaire du Temps et des Frequences (Figure 4.5). National MaritimeMuseum (Figures 5.1, 6.4). National Aeronautics and Space Adminis-tration (Figure 5.2). Alan Pedlar and Tom Muxlow, Jodrell Bank Obser-vatory, University of Manchester (Figure 7.5). Figure 8.1 courtesy of theLong Now Foundation.

Foreword

Just fifty years ago, the global time standard was still based on the ro-tation of the earth on its axis. It was the oldest physical standard in useand also the most accurate. However, in 1955, the National PhysicalLaboratory developed a new and more accurate time standard, usingcaesium atoms to set the rate of the clock. Since then, through the effortsof many exceptional individuals and institutions around the world, theatomic clock has transformed the way we measure and use time.

The caesium atom now underpins the very definition of time. Theatomic clocks themselves have improved by a factor of nearly a mil-lion, with the latest generation using laser-cooled atoms to extract suchtremendous accuracy. At this level, Einstein’s theory of relativity hasbecome just an everyday engineering tool for comparing the time ofatomic clocks. And yet in spite of this extraordinary progress, thoseat the cutting edge are seeking to exploit alternative atoms to push backthe frontiers of time measurement even further.

However, the story told in this excellent book is not just one ofscientists breaking through arbitrary boundaries. It is one which affectsall our lives. Ultimately we set the time on our watches to a standardmaintained by atomic clocks. Telephone networks, electricity grids andsatellite navigation systems make full use of the accuracy offered bythis technology, and there are countless other examples linking the mostadvanced and the most mundane of human activities to the beat of thecaesium atom.

In spite of its wide spread influence, the story of atomic timekeepingis one that is largely unknown outside a small community of specialists.Splitting the Second: The Story of Atomic Time brings up-to-date thetraditional account of how we measure and use time. I hope the readerwill enjoy this fascinating story.

John LavertyHead of Time MetrologyNational Physical LaboratoryJune 2000

vii

Preface

On the wall in my study I have a radio-controlled clock. It is essen-tially a common-or-garden quartz-crystal clock connected to a tiny radioreceiver. Every two hours it tunes in to the rhythmic pulses from aradio station controlled by the atomic clocks at the National PhysicalLaboratory and corrects itself to Coordinated Universal Time (which—you will soon discover—is commonly, though incorrectly, called Green-wich Mean Time). It adjusts automatically to the beginning and end ofsummer time and it can even cope with leap seconds, though not in themost elegant fashion. It means we no longer need to wait for radio timesignals or to phone the Speaking Clock to get accurate time. It is nice tohave a clock guaranteed to remain correct to a tiny fraction of a second,though it is a bit excessive for domestic purposes.

The fact that such clocks and the accuracy they bring are now com-monplace is a sign of the upheaval in timekeeping that took place duringthe twentieth century. It could even be called a revolution. When thecentury began, timekeeping was firmly in the hands of astronomers,where it had rested for millennia. By the century’s end timekeeping wascontrolled by physicists, and astronomers were relegated to a supportingbut not insignificant role. If we were to place dates on the revolutionwe could say it began in 1955, with the operation of the world’s firstsuccessful atomic clock, and was all but complete by 1967 when theatomic second finally ousted the astronomical second as the internationalunit of time.

The start of a new century seems an opportune moment to tell thisstory, coinciding as it does with the centenary of the National Physi-cal Laboratory. NPL played a central role in that revolution, as youwill see, and by a kind of right of conquest is now the official supplierof time to the United Kingdom. Indeed this book owes its origins toFiona Williams, of NPL, who saw the need for it and has generouslysupported the project over the past year. I am also grateful to the NPLscientists who have given freely of their time, knowledge and experience,

ix

x PREFACE

especially John Laverty, James “Mac” Steele, Peter Whibberley and PaulTaylor, and the staff of other institutions who have supplied me withbackground material and illustrations and answered many queries. Imust also thank the staff of the NPL library for their hospitality, TerryChristien for drawing the diagrams and Margaret O’Gorman, Robin Reesand Nicki Dennis at Institute of Physics Publishing who brought thebook to fruition.

Tony JonesMay 2000