How do atomic clocks work?

How was atomic time discovered? And how do atomic clocks work?
30 June 2015

Interview with 

Ray Essen, writer and historian of science


The Atomic Clock at The National Physical Labratory


Throughout human history, timekeeping has been based on the rotation of the Earth. Atomic ClockBut the Earth's orbit is irregular, its rotation is gradually slowing, so the length of a day is steadily lengthening. 60 years ago, this posed a pretty big problem in the world of timekeeping: the length of a second kept on changing. It was imperative to find an accurate way of keeping time, and the solution was the "atomic clock". In 1955, Englishman Louis Essen won that race. Graihagh Jackson spoke to his relative, and biographer, Ray Essen...

Ray - Louis Essen was a quiet and unassuming physicist who worked on the measurement of time for 40 years. He was the man who's most remembered who was building the first atomic clock. One national newspaper called him a rebel at one stage...

Graihagh - A rebel?

Ray - That's right, yes. He was not always conventional. One of his first successes was to see that there was an error in the speed of light, which during second world war was essential for any distance measurement particularly that used in radar. It was only in the Korean war did the Americans adopt Essen's value of the speed of light and so, corrects their own equipment and gave it much more accurate radar.

Graihagh - A remarkable man then in many ways, for not only his radio navigation system but also for his atomic clock. Tell me a bit more about the atomic clock. How did he come to build it so-to-speak?

Ray - The idea for the atomic clock came initially from America, just after the second world war. Essen had been doing similar work in the UK and so, a number of scientists invited him to America in 1949 to look at the work they've been doing. They demonstrated a prototype atomic clock. But quite frankly, it wasn't accurate. Nevertheless, Essen was really excited by the prospect. So, we had a sort of race developing where the Americans on the one hand had a lot of money that they were throwing at it. Essen meanwhile, he wasn't very well-funded at all in his research. But nevertheless, he came up with the idea.

Graihagh - Whilst in America, they were using ammonia, Louis was using caesium. What's interesting about caesium atoms is that if you blast them with microwaves, they resonate at a very precise rate - 9.2 billion times a second. Louis Essen's atomic clock essentially was able to strip off individual atoms by heating the caesium up to a hundred degrees, firing them down an airless tube, and blasting them with microwaves, and then detect this bouncing. One, two, three, four, five... 9.2 billion. And every time it counted to that, it went tick, a second had passed. Unlike the pendulum, or the quartz clocks at that time, the rate at which caesium atoms resonate is constant, whereas pendulums and Quartz clocks are affected by changes in temperature, gravity, deeps in air pressure. So, Louis' clock was much more accurate by a factor of 10, losing or adding a second every 300 years as opposed to every week. But how did Louis do this, especially when the odds were stacked against him?

Ray - Many of the best physicists in America came from academic backgrounds and they were interested in purely devising something that could prove or disprove Einstein's relativity theory, that they weren't in the early days, interested in clock making, just something with a regular frequency. It was Essen that saw the practical application of this and the need to redefine the second of time.

Graihagh - And his definition of the second still survives today?

Ray - It does. It took him 2 years to build the atomic clock and then after that, the next 12 years to get acceptance of a new definition of the second based not on astronomical cycles, but based on the heartbeat of the caesium atom.

Graihagh - You must feel very proud.

Ray - Yes. He was a quiet, unassuming man. He rarely talked about his work, but deep down, he's changed the lives of many of us in a way that would be unimaginable back in the 1950s.


Add a comment