AT THE END OF THE 19TH CENTURY, Sigmund Riefler, based in Munich, developed a radical new design of regulator—a highly accurate timekeeper that served as a standard for controlling others. Housed in a partial vacuum to minimize the effects of barometric pressure and equipped with a pendulum largely unaffected by temperature variations, Riefler's regulators attained an accuracy of a tenth of a second a day and were thus adopted by nearly every astronomical observatory.
Further progress came several decades later, when English railroad engineer William H. Shortt designed a so-called free pendulum clock that reputedly kept time to within about a second a year. Shortt's system incorporated two pendulum clocks, one a “master” (housed in an evacuated tank) and the other a “slave” (which contained the time dials). Every 30 seconds the slave clock gave an electromagnetic impulse to, and was in turn regulated by, the master clock pendulum, which was thus nearly free from mechanical disturbances.
Although Shortt clocks began to displace Rieflers as observatory regulators during the 1920s, their superiority was short-lived. In 1928 Warren A. Marrison, an engineer at Bell Laboratories, then in New York City, discovered an extremely uniform and reliable frequency source that was as revolutionary for timekeeping as the pendulum had been 272 years earlier. Developed originally for use in radio broadcasting, the quartz crystal vibrates at a highly regular rate when excited by an electric current [see left illustration in box on page 54]. The first quartz clocks installed at the Royal Observatory in 1939 varied by only two thousandths of a second a day. By the end of World War II, this accuracy had improved to the equivalent of a second every 30 years.
Quartz-crystal technology did not remain the premier frequency standard for long either, however. By 1948 Harold Lyons and his associates at the National Bureau of Standards in Washington, D.C., had based the first atomic clock on a far more precise and stable source of timekeeping: an atom's natural resonant frequency, the periodic oscillation between two of its energy states [see right illustration in box on page 54]. Subsequent experiments in both the U.S. and England in the 1950s led to the development of the cesium-beam atomic clock. Today the averaged times of cesium clocks in various parts of the world provide the standard frequency for Coordinated Universal Time, which has an accuracy of better than one nanosecond a day.
Up to the mid-20th century, the sidereal day, the period of the earth's rotation on its axis in relation to the stars, was used to determine standard time. This practice had been retained even though it had been suspected since the late 18th century that our planet's axial rotation was not entirely constant. The rise of cesium clocks capable of measuring discrepancies in the earth's spin, however, meant that a change was necessary. A new definition of the second, based on the resonant frequency of the cesium atom, was adopted as the new standard unit of time in 1967.
The precise measurement of time is of such fundamental importance to science and technology that the search for ever greater accuracy continues. The performance of atomic clocks had been improving by a factor of at least 10 per decade for about 50 years. But over the past decade improvements in atomic clock accuracy have dramatically accelerated. Recent advances in laser science—particularly the Nobel Prize–winning development of femtosecond laser frequency combs—and atomic physics have enabled the development of many new types of optical atomic clocks, some based on transitions in single ions in electromagnetic traps and some based on collections of cold neutral atoms held in lattices formed by laser light. Several of these atomic clocks are already stable to within a few hundred femtoseconds per day and continue to rapidly improve.
At this level of performance, formerly negligible effects become important and measurable. For example, the best atomic clocks can now measure changes in gravity over the distance of a stair step, tiny magnetic fields generated by heart and brain activity, and other quantities such as temperature and acceleration. Companies are now manufacturing “chip-scale” atomic clocks the size of a quarter. In addition to keeping time with increasing accuracy, new generations of atomic clocks will be used as exquisite sensors for myriad applications and will become ever smaller and more portable.
Although our ability to measure time will surely improve in the future, nothing will change the fact that it is the one thing of which we will never have enough.