Tempus Fugit

by David Bradley

From the sundial and hourglass to the digital watch and atomic clock, we have been preoccupied by time. It is ironic to think that time does not actually exist in Einstein's theory of relativity, but nevertheless, explained Dr Felicitas Arias of the Bureau International des Poids et Mesures the measurement of time is a continuous activity, an international and restless enterprise hidden in time laboratories spread over the planet.

The BIPM is, she added, in charge of coordinating the activities for international time keeping and to make use of our capacity to produce a remarkably stable and accurate conventional timescale. Commercial atomic clocks beat time in national laboratories with a stability of one part in a 100,000 billion over a five-day averaging time. This compelling accuracy ensures remote clocks can be compared with great precision. By definition the unit of the international time scale, the SI second, is even more accurate at one part in a million billion as obtained from a caesium fountain atomic clock. Physicists are now striving to increase this accuracy by an order of magnitude - to one part in 10 million billion. New techniques for time and frequency transfer are currently being developed to accomplish this goal.

Femtosecond comb technology, as discussed by Ted Haensch in "Combing the constants" might soon allow scientists to define the second using an optical clock rather than the caesium fountain. Challenges remain but as time goes by science can catch a glimpse of the ever smaller split second.

Back to part one... Weights and Measures