Splitting the Second: The Story of Atomic Time

The story of humanity's efforts to keep time with the required accuracy has not excited a high degree of popular interest. Perhaps this disinterest stems from the perception that accurate time is always available, as electricity is always there at the flip of a switch. The source of such ubiquitous phenomena as time and electricity just has not generated all that much interest. Horologists have traditionally been concerned with the design and fabrication of mechanical clocks, and philosophers deal with the nature of time. Physicists, astronomers, and engineers continue to provide and report technical developments applicable to timekeeping. But popular descriptions of timekeeping and the efforts to improve it have been rare.

In Splitting the Second, Tony Jones takes a step toward filling that gap, providing a very readable, popular account of the development of modern atomic timekeeping, the revolution in horology marked by the change in the basis for timekeeping from Earth's rotation to energy-level transitions within atoms. Jones begins with a short review of what he calls "Astronomers' Time." He adopts this terminology to indicate that, until relatively recently, the time of day was based solely on astronomical observations designed to refine the precision with which Earth's rotation can be measured. The contrast is with "Physicists' Time," derived from the frequency of atomic energy level transitions. To Jones, the transition from the one kind of time to the other truly represents a fundamental change in the human experience.

Splitting the Second goes on to give a comprehensive review, at a popular level, of the state of modern timekeeping. The author details the cooperation of the national timing laboratories to set an international standard time scale, including descriptions of the clocks used at these laboratories and the means by which the times provided by the clocks are combined to create the basis for all modern civil timekeeping. Jones describes the use of leap seconds, the one-second adjustments to Coordinated Universal Time (UTC), the world's standard time, to reconcile "Physicists' Time" with "Astronomers' Time." He identifies the reconciliation of the two time scales as "the problem with atomic timekeeping" and provides a clear description of this rather confusing issue.

The author's British background has apparently contributed a decidedly British perspective to the story. There is no denying the major contribution of UK scientists to modern timekeeping. British scientists were, in fact, largely responsible for developing the first operational atomic clock. However, contributions to the science of timekeeping from other national laboratories do not receive the attention they have earned.

There are some minor factual problems and some descriptions that might be considered oversimplifications. The global positioning system (GPS), while mentioned, is not properly credited for its importance in the worldwide distribution of accurate time. GPS has now become the dominant means to obtain time, even in ways of which the ultimate user is unaware. Its success and easy availability in providing time even threaten the development of new, more expensive clocks.

The author briefly describes the general concepts of one-way and two-way time transfer. In the former, the user can synchronize a clock using a "broadcast" time signal, without any interaction with the signal provider. In the latter, the user must interact with the provider. Jones details the use of common-view time transfer in the formulation of the Coordinated Universal Time. However, he has left out the use of two-way satellite time transfer, a technique that makes an important contribution to the formation of UTC.

In summary, Splitting the Second: The Story of Atomic Time is an interesting, readable account of the state of modern timekeeping. It is not comprehensive and is not the definitive history of this stage of the human attempt to provide accurate timekeeping. It is, however, a good introduction and well worth reading by those interested in this rather poorly documented area of modern science and technology.