VOL. 14 July ISSUE YEAR 2013

Off the Beaten Track

in Vol. 14 - July Issue - Year 2013
Right on Time!
Louis Essen (right) with the first caesium atomic clock

Louis Essen (right) with the first caesium atomic clock

“Wait, wait, you haven’t seen the best part yet!” He was standing squarely in front of me. I felt intimidated by his stance, very similar to that of a professional wrestler ready to engage his opponent. He moved in even closer and occupied what little empty space was left between us. Now his jaw was only a few inches from my nose. His tone became conspiratorial and the volume of his voice dropped. His face beamed in anticipation of the shocked look that he expected to see on my face when he dropped the bombshell. “It’s radio-controlled! Radio-controlled! Do you realize what that means?” I had no idea what that meant, but I didn’t have the courage to say so. Futilely I searched his face for a clue to the answer that I was expected to give. His voice droned on and on, as he repeatedly pointed at his left wrist with his right index finger. With great effort I tried to stay focused on what he was saying. I made a mental note to myself to never, ever again ask this particular friend of mine about his new wristwatch. What a mistake I had made!


Up to some years ago, our concept of time was based solely on the movement of the planets and the stars. For as long as we can remember, this mechanism has been an invaluable aid in determining various moments of the day and night, and an essential tool for navigation. A day was defined as a complete rotation of the Earth on its axis; a year as the time it took the Earth to make a complete orbit around the Sun. A second was defined as 1/86400 of the mean solar day, averaged over a year.

However, by the beginning of the twentieth century these natural standards were beginning to prove inaccurate. It had been observed that the speed of rotation of the Earth was slowing down and that the Earth was wobbling on its axis, meaning that the length of the solar day was not constant. As a consequence, the accuracy of the second was negatively affected.

A new standard had to be found. Working on ideas first suggested by Lord Kelvin in 1879, researchers in the United States developed the world’s first atomic clock in 1949. This apparatus measured the vibrations of the ammonia molecule and, although far less accurate than present-day quartz clocks, served to prove the concept and demonstrated that a clock based on atomic structure could be far more accurate than any system based on celestial observations.

The first accurate atomic clock was built in 1955 by Louis Essen, a British scientist working at the National Physical Laboratory in Teddington. This clock measured the frequency of electromagnetic radiation released by caesium-133 atoms, to this day the standard upon which most atomic clocks around the world work. Continuous improvements to the various components have led to atomic clocks that are now about 100,000 times more accurate than Essen’s first clock. A typical modern atomic clock has an accuracy of about one second in twenty million years and the current “top-of-the-line” model, the NPL-CsF2 clock operated by the National Physical Laboratory in the U.K., has an accuracy of one second in 138 million years!

Despite strong opposition by the astronomical community, in 1967 the caesium atomic clock became the international time standard. At present there are some 260 atomic clocks managed by government, research and military establishments around the world, not to mention about 100 atomic clocks in satellites orbiting the Earth as part of the Global Positioning System (GPS).

The accuracy of atomic clocks is essential for aircraft and ship navigational systems, mobile phone and railway networks and in many branches of scientific research. Two fields of application, which are well known to the public, are the GPS system and the time signal radio transmitters. Thanks to miniaturized built-in receivers, radio-controlled clocks and watches capture radio signals from transmitters connected to atomic clocks located around the world. Currently there are two radio transmitters in Japan, one in China, two in Europe and one in the United States, each with a range of about 1,500 – 2,000 km. Anyone wearing a radio-controlled wristwatch is certain of always having the time determined with the precision of an atomic clock and doesn’t even need to worry about switching between daylight-savings time and standard time, for the radio signal takes care of it automatically.

Development is under way on the next generation of atomic clocks, called trapped ion and quantum logic clocks. These will be at least one thousand times more accurate than the best caesium clocks, with one experimental model reaching an accuracy of about one second in two billion years!

By Giovanni Gregorat, Contributing Editor MFN

Author: Giovanni Gregorat