An Instant in Time How the Atomic Clock Revolutionized Time-Keeping

book reviews An instant in time How the atomic clock revolutionized time-keeping. Splitting the Second: The Story of NPL Atomic Time by Tony Jones Institute of Physics: 2000. 199 pp. £14.99, $19.99 Daniel Kleppner By the mid-1950s astronomical timekeeping had achieved unprecedented accuracy, based on a deepened understanding of planetary and terrestrial dynamics and the development of the quartz crystal clock. In 1952 the International Astronomical Union introduced a new timescale — ephemeris time, based on the motions of the planets — and in 1956 it redefined the second in terms of the length of a solar year. But the 1950s also saw the start of a revolution in timekeeping that in a single decade wrested the second from the astronomers and made ephemeris time obsolete. The revolution was precipitated by the creation of the atomic clock, a device so unclock-like in appearance and operation that it was every bit as unpalatable to astronomers in the 1950s as John Harrison’s Clocking on: Parry (left) and Essen with their caesium atomic frequency standard. mechanical chronometer was in the eighteenth century. Splitting the Second and cars to measuring the drift of continents. dards. The system is cumbersome but the recounts the events of this revolution, the Accurate time is useless unless it can be timescale is accurate and reliable. subsequent progress in atomic clocks and the disseminated, and Splitting the Second Jones recounts the historical events that many applications of timekeeping, few of describes time dissemination in some detail. underlie contemporary timekeeping and which were dreamed of when atomic clocks The echoes of history can still be heard, for provides vignettes of the human factors were created. instance, in the continued use of the term involved in scientific and technological deci- The possibility of an atomic clock was Greenwich Mean Time long after it became sions. He makes a valiant effort to explain the first suggested in 1945 by the Nobel technically obsolete. The conflict between science. His explanation of an atomic clock prizewinning physicist Isidor I. Rabi in a physicists’ and astronomers’ time can be conveys the flavour of the physics and is throwaway remark during a speech to the detected in the international agreement for probably as good as one can do in describing American Physical Society. A number of US time dissemination. The time interval that is quantum phenomena in everyday language. groups picked up on his suggestion, but the broadcast — the second — is physicists’ Some details, such as the operation of a first working atomic clock was created in the time, controlled by an atomic frequency maser, are slightly confused, and some United Kingdom. In 1955, Louis Essen and standard. But to satisfy the needs of astrono- choices seem idiosyncratic. The name of John Parry operated a caesium atomic beam my, the timescale is adjusted to keep step Alfred Kastler, for instance, whose invention clock at the National Physical Laboratory in with the slightly irregular motion of the of optical pumping has played a continuing Teddington. The device was actually an Earth by introducing a leap second whenever role in the development of atomic clocks, atomic frequency standard, but the term the astronomical and physical times diverge appears only in a list of Nobel prizewinners. atomic clock has become generally accepted. by more than 0.9 seconds. The book concludes with speculations The Essen–Parry clock achieved an accuracy In one respect — reliability — atomic on the future of timekeeping. These are not of one part in 10 billion (1 in 1010). Refine- clocks cannot compete with astronomical far-fetched, for the field is advancing rapid- ments over the decades have increased the timekeeping. To physicists, who generally ly. For instance, a method for measuring accuracy by a factor of close to 10,000. care about measuring a frequency accurately optical frequency, published too late to be Recently, the fountain clock, a new type of rather than knowing the time itself, this is not described in the book, has opened the way clock that exploits laser-cooled atoms, of deep concern. But if one particular atomic to clocks that may one day achieve an accu- achieved an accuracy of close to 1 in 1015. clock, presumably the best in the world, were racy in the range of 1018. Anyone who is An obvious question is why anyone selected as the primary standard, sooner or intrigued by fantastic precision or curious would want such high accuracy. The book later it would need to stop for repairs and the about how we really know the time, or sim- addresses this question by demonstrating timescale would be broken. Consequently, ply likes tales of science and technology, will that as timekeeping improved, new applica- primary atomic time standards are main- enjoy Splitting the Second. I tions steadily arose. The most spectacular of tained not at one but at seven laboratories Daniel Kleppner is in the Department of Physics, these is the Global Positioning System, which scattered around the world. These constantly Massachusetts Institute of Technology, is finding fresh applications almost daily, compare their clocks both among themselves 77 Massachusetts Avenue, Cambridge, from aiding the navigation of ambulances and with more than 200 secondary stan- Massachusetts 02139, USA. NATURE | VOL 410 | 26 APRIL 2001 | www.nature.com © 2001 Macmillan Magazines Ltd 1027.

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