A Wrinkle in Time

The rotation of the Earth is slowing, and atomic clock time is getting further ahead of astronomical time, which is based on the Earth's rotation. Currently, the addition of leap seconds to atomic clock time helps keep the two in sync. For more than 20 years, international forums have debated whether to move to purely atomic-clock-based timekeeping, but nothing has happened thus far to resolve the issue.

Image Credit
The Levine group and Steve Burrows, JILA

Fellow Judah Levine recently presented a discussion of our understanding of time from antiquity to the present day in an insightful paper published in the April 2016 issue of the European Physical Journal H.

Levine recounted that for at least 7000 years, the measurement of time has been linked to the rotation of the Earth, the lunar cycle, the path of the Earth around the Sun, and other observable astronomical phenomena. That the length of a day varies throughout the year didn’t faze the ancients. Our forebears simply adjusted the length of time on their primitive timekeeping devices to accommodate changes in daylight accompanying annual seasonal variations.

The earliest water and sand clocks appeared in Egypt, India, China, and Babylonia before 1500 BC. In general, these devices measured relatively short time intervals that were defined in terms of astronomical periods. The Egyptians also divided the day into 24 hours of 60 minutes each, with each minute comprising 60 seconds––a strategy still in use today.

In contrast to time, ancient civilizations weren’t concerned with absolute frequency. Frequency was the province of musicians rather than astronomers. Musical scales depended more on frequency ratios and differences than on absolute frequency values. In addition, there was no need for maintaining community standards of time or time interval because both of these quantities were defined in terms of readily observable astronomical phenomena.

However, this simple astronomically based time-measurement strategy hasn’t satisfied the requirements for precision timekeeping for more than half a century. Problems arose because of the close connection between time interval and frequency. Any variation in the astronomical standard for time interval introduced a variation in its corresponding frequency. For example, the length of the solar day has been increasing for some time because the Earth’s rotation is slowing down. The long-term steady increase in the length of the day is accompanied by an irregular frequency variation that is hard to predict.

There were attempts to remove this variation by changing the definition of time interval from the apparent solar day to the mean solar day, then to the tropical year, and then to a specific year (1900). However, none of these changes was successful in providing a stable standard for time interval and frequency. Each of these changes did succeed in moving the definitions of time and time interval further away from everyday experience.

Fluctuations in the definition of frequency (caused by variations in the astronomical standard for time interval) began causing major problems in the 1920s when accurate frequencies became important. New radio stations had difficulty maintaining their assigned frequencies, for example. The problem resulted in the separation of the standard of frequency­­––maintained at NBS by various electronic circuits and components––and the definitions of time and time interval, which were defined and observed astronomically.

The invention of the first cesium (Cs) atomic clock at the National Physical Laboratory in the United Kingdom in 1955 offered the world an opportunity to unify the standards of time interval and frequency while removing the astronomically induced variations in them. The definitions of both time interval and frequency would both be tied to counting the number of oscillation cycles in the frequency associated with the hyperfine transition in the ground state of Cs 133.

The link between the astronomically defined second and the Cs transition frequency was measured in a joint experiment between the National Physical Laboratory (NPL) in the United Kingdom and the United States Naval Observatory (USNO). The result of this experiment was that 9192631770 cycles of the hyperfine transition frequency of Cs were equivalent to one astronomical second. The hyperfine frequency of the cesium transition became the standard, and the second became a quantity derived from it. Since ancient times, it had been just the opposite–– astronomical time and time interval had always been the fundamental standards.

However, the value chosen for the number of cycles of the Cs frequency that would correspond to one second was too small. Thus, the length of the day defined in terms of the Cs frequency was shorter than the length of the day defined by astronomy. And, because the Earth is slowing down, it is impossible to define the second based on the hyperfine transition in Cs in a way that will exactly match an astronomically observed second for all time.

Nevertheless, applications that depend on standard frequencies, ranging from telecommunications to financial transactions to power distribution, are fundamental to a modern society. Unless the world community decides to return to 19th century technologies, it’s no longer possible to define frequency as a quantity derived from a varying astronomical time.

To further complicate matters, the Earth’s rotation continues to slow down, and our days are slowly getting longer. The slowing of Earth’s rotation means that atomic clocks are getting further and further ahead of astronomical time. Timekeepers have addressed this problem since 1972 by adding a leap second to atomic clock time––26 times thus far, not counting the 10 seconds that were inserted when the scale was defined. More recently, the interval between leap seconds has gotten longer because the slow-down of the Earth is slowing down.

For more than 20 years, the international timekeeping community and high-ranking national officials have debated whether to keep the leap second and the link between atomic clock time and astronomical time or simply move to atomic clock time for precision timekeeping. If the linkage between atomic time and astronomical time were to be broken by eliminating leap seconds, then atomic time would advance relative to solar time. At present, the rate of advance is about one minute per century.

“We have a controversy to resolve,” said Judah Levine, JILA’s resident timekeeping expert. “The hardest question is: Who is going to resolve the controversy?”

Levine said that the next round of international discussions of what to do with the leap second won’t take place until the next World Radio Conference in 2023 unless the Bureau of Weights and Measures in Paris takes up the question next year. Up until now, however, participants in the controversy have been inclined to “kick the can down the road” to some date in the future rather than agreeing on how to resolve the issue.

“I can’t tell you what’s going to happen,” Levine said. “Time is political, and it always has been. For now leap seconds are going to stay.” In other words, for the time being, official time will continue to be linked to astronomical phenomena as it has been for thousands of years.

In the meantime, researchers at JILA, NIST, and national laboratories around the world are investigating even more accurate frequency standards, including strontium (in the Ye labs at JILA), ytterbium, and calcium. Atomic clock time is only going to get even more precise at the same time astronomical time gets more imprecise with the slowing of Earth’s rotation.

Although it requires the addition of leap seconds to work for modern timekeeping applications, astronomical time has the advantage of being as familiar to people as the sun and the moon, the seasons, and Earth’s journey through the stars. Not surprisingly, astronomers like it. The British like it because it allows them to assert a close connection between Greenwich Mean Time (GMT) and official international time; and, so far national laboratories around the world have adjusted to more than half a century of complex timekeeping requirements imposed by linking precision timekeeping to astronomical time.

Even so, leap seconds cause problems for many users. Real-time applications, such as satellite navigation, use a time scale that does not include leap seconds. Private time scales are likely to proliferate in the future as more real-time applications require a smooth time scale lacking the discontinuities that result from adding leap seconds.

Of course, Levine would like to see international agreement to implement ultraprecise timekeeping based on the next generation of atomic clocks––but he’s not sure he’ll be around long enough to see a global consensus develop around this sensible idea.–– Julie Phillips

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