Of time and the atom…

AUG 01, 1965

The current international standard for the physical measurement of time is based on the use of atomic‐frequency control devices that are now being studied intensively in the search for an ultimate basis for the precise definition of time. The author is assistant chief of the Physics Division of the NBS Radio Standards Laboratory, Boulder, Colo., where the atomic‐frequency standard is being actively investigated.

DOI: 10.1063/1.3047591

George E. Hudson

In 1952, Harold Lyons of the National Bureau of Standards put into brief operation an atomic clock of the ammonia‐absorption type, and thus reflected the intense interest in devising atomic standards for measuring time intervals. This interest has grown rather than diminished. As progress continued with the advent of cesium‐beam frequency controls—first with L. Essen and J. V. L. Parry at the National Physical Laboratory in England, and later with the models investigated at the Boulder Laboratories of the NBS, and by Bonanomi in Neuchatel, Switzerland, by Kalra in Canada, and by McCoubrey and Holloway of the National Company in the United States—the standard for time measurement of highest possible accuracy was removed in effect from the astronomic realm to the atomic. In collaborating with Essen, W. Markowitz of the US Naval Obsevatory determined in 1958 the frequency characteristic of the cesium standard in terms of the accepted international unit of time, the ephemeris second. Since the international adoption of a temporary atomic standard to realize the unit of time, the figure they gave with a considerable uncertainty, 9 192 631 770 Hz, must be regarded not as a measured value, but rather as an exactly defined one, accurate to any number of significant figures. The uncertainty of 2 or 3 parts in $10^{9}$ that originally attached to the measurement was evidently due to the uncertainties and inconvenience in the methods used to realize the ephemeris second; that is, the limitation lay in the astronomical observations, not in the atomic device.

References

1. H. Lyons, Ann. N.Y. Acad. Sci. 55, 831–871 (1952).https://doi.org/ANYAA9
2. L. Essen and J. V. L. Parry, Phil. Trans. Roy. Soc. (London) A250, 45 (1957).
3. R. E. Beehler, R. C. Mockler, and J. M. Richardson, private communication on cesium beam atomic time and frequency standards.
4. R. C. Mockler, R. E. Beehler, and C. S. Snider, IRE Trans. Instr. 9, 2, 120 (1960).https://doi.org/IRINAG
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8. Nat. Bur. Std. Tech. News Bull. 48, 209 (1964).
9. D. H. Andrews, “LF‐VLF Frequency and Time Services of the National Bureau of Standards” (to be published).
10. W. Markowitz, Explanatory Supplement to the Ephemeris (distributed by the British Information Services, New York, 1961) Chap. 3, p. 66.
11. J. A. Barnes, D. H. Andrews, and D. W. Allan, private communication on the NBS—a time scale—its generation and dissemination.
12. J. Bonanomi, P. Kartaschoff, J. Newman, J. A. Barnes, and W. R. Atkinson, Proc. IEEE 52, 439 (1964); https://doi.org/IEEPAD
see also: J. Newman, L. Fey, and W. R. Atkinson, Proc. IEEE 51, 498 (1963).https://doi.org/IEEPAD
13. US Naval Observatory Weekly Bulletin, “Preliminary Emission Times for Signals from NBA, GBI, WWV, CHU, and other Co‐ordinated Stations.”
14. D. Brouwer, Astron. J. 57, 125 (1952).https://doi.org/ANJOAA
15. International Radio Consultation Committee, Documents of the Xth Plenary Assembly (International Telecommunications Union, Geneva, 1963) Vol. III, p. 193.
16. The US Federal Register, December 19, 1964, p. 18095.
17. H. Jeffreys, The Earth, (Cambridge University Press, New York, 1959) 4th ed., pp. 246–250.
18. G. Gamow, Biography of the Earth, (Mentor Press, New York, 1959) p. 187.

More about the Authors

George E. Hudson. NBS Radio Standards Laboratory, Boulder, Colo..