Atomic Clocks

Atomic Clocks

With advances in science and technology, tradi­tional mechanical clocks have been supplemented with , which give far more accurate measurements. Further progress in atomic clocks will enhance practical uses ranging from medical research applications to outer space travel.


In 1945, Isidor Rabi, professor of at Columbia University, proposed the idea that an exceptionally accurate form of timekeeping could be procured from a process known as Atomic Beam Magnetic (ABMR), a process developed by Rabi during the mid to late 1930s. In essence, ABMR is composed of a harmonized magnetic field with an oscillating magnetic field being applied to it at right angles. This forces a transition between the nuclei of atoms with vary­ing states of magnetic quantum numbers. The transition occurs only each time the frequency of the oscillating magnetic field possesses parallel and precise qualities. Given the closed environment in which this process occurs, the interval between one nucleus and the next obtaining the specific characteristic values is nearly flawlessly precise, thus making ABMR an excellent platform from which to launch a method of logging time. Mechanical clocks also use oscillating motion, with tuning forks, pendulums, and balance wheels. But the rate of movement for an ammonia atom in this type of motion is much higher—23,870 vibra­tions per second, as opposed to the average of 5 to 10 vibrations per second on a mechanical watch— with the elevated oscillations correlating to a greater degree of accuracy.


Given the allure of a method of time recording and reporting more true than astronomical time (itself accurate to within only 3 minutes, 55.9095 seconds per 24-hour cycle), the United States National Bureau of Standards (NBS, now referred to as the National Institute of Standards and Technology, NIST) unveiled the world’s first atomic in 1949, with nuclei of ammonia molecules providing the movement, or vibrations, as a result of the interaction between constant and oscillating magnetic fields. The mean accuracy of ammonia-based atomic clocks, as dispersed throughout the globe, is approximately 1 second every 3,000 years (with even the best mechanical watches being off a handful of seconds each day). By necessity, the device—and all future atomic catalysts used for the gauging of time—is connected to a time recording device; without the registering device being affixed, the near-perfect oscillation cycle would have no way of presenting itself to the outside world, thus rendering it useless as a of time.


In 1955, the National Physical Laboratory (NPL) based in Teddington, United Kingdom, constructed the first cesium beam clock, utilizing atoms of cesium rather than ammonia. The clock’s very pre­cise frequency of 9,192,631,770 vibrations per second has been correlated to a margin of error equaling plus or minus 2 seconds every 3 million years. Throughout the following years, cesium-based clocks began to undergo various modifications and advancements, and during the 13th General Conference on Weights and Measures it was deter­mined the SI second—or cesium-based 9,192,631,770 vibrations per second—would define 1/60 of 1 min­ute. Because of the acute accuracy of such a mea­surement, there is little reason to question the motives of this conference when, in 1967, it was determined world time would forever be based on an atomic and not an astronomical basis.

Despite the nearly flawless regularity of cesium vibrations and the subsequent recording of said vibrations, slight discrepancies still exist from one clock—most of which are housed in various research institutions throughout the world—to the next. As such, all “official” clocks (commer­cially produced cesium-based clocks are available, but their accuracy is at best dismal when com­pared to that of the laboratory-based models) are recorded and then averaged to produce the global standard, International Atomic Time (TAI, an acronym for the French: Temps Atomique International).

Points of Interest

Usage for so accurate an instrument is vast; time signals broadcast via shortwave radio stations and artificial satellite make it possible for “true time” to be had anywhere upon the globe. This highly accurate and standardized time format is then used in a variety of applications, from synchronization of the Internet to space shuttle launches, to the opening and closing of stock markets, and every­thing in between.

Contrary to the belief held by many, atomic clocks are in no way radioactive. The clocks are not a result of, related to, or bound in any way by atomic decay, the precursor to radioactivity. Without a working relationship with the catalyst to radio­activity, the chance of accidental radiation is nil. Instead of using the potentially hazardous energy or decay of an atom, the clocks rely on an oscillating part and “springs,” not dissimilar to commonplace mechanical clocks. In lieu of the conventional metal coil spring and balance wheel used in mechanical movements, an atomic clock uses gravity, the mass of the nucleus, and an “electrostatic spring” caused by the positive charge of the nucleus and negative charge of its surrounding electron assemblage.

Of distinct interest is the direction that the advancement of atomic clocks has taken during the past decade. As of 1999, NIST began using the world’s most precise and stable cesium clock, it being accurate to within 1 second over 20 million years. This instrument is composed of a 3-feet- long vertical tube that is encased within a larger unit. Lasers are incorporated to cool the cesium atoms so that ideal conditions are maintained, helping to ensure greater accuracy in the replica­tion of vibrations per second, thus accuracy. Simultaneously, other lasers are incorporated to “toss” the cooled ball of atoms, thus creating a fountain effect within the 3-foot tube, allowing greater opportunity for the observation of oscillat­ing atoms, much more so than previously allowed within enclosed housing units.


The future of atomic clocks seems full of promise when considering ever-diminishing margins of error for replicability of motion, that is, identical units of time. First with ammonia and now cesium, research­ers have begun construction of clocks based on hydrogen, beryllium, and mercury atoms, with good indications that these prototypes, once refined into fully operable units, could be up to 1,000 times more accurate than the atomic clocks of today.

Daniel J. Michalek

See also Attosecond and Nanosecond; Clocks, Mechanical; Dating Techniques; Decay, Radioactive; Time, Measurements of

Further Readings

Audoin, C., & Guinot, B. (2001). The measurement of time: Time, frequency and the atomic clock. Cambridge, UK: Cambridge University Press.

Jones, T. (2000). Atomic timekeeping (Vol. 1). Oxford, UK: Taylor & Francis.

What do you think?

Doomsday Clock

Doomsday Clock

Biological Clocks

Biological Clocks