INSIDE AN ATOMIC CLOCK

Measuring time using the Earth's rotation was always an imprecise business, so now we do it using the 'tick' of an atom

- ALEX CLONEY

Atomic clocks provide timing that's so precise they would lose or gain just one second in 300 million years. It's safe to say you could set your watch by them. Knowing the time with such precision is vital to the modern way of life. Atomic clocks are at the heart of the Global Positioning System (GPS). They are also used in controlling air traffic, as well as playing a part in banking, the internet, email and the distribution of electricity.

It's been over half a century since physicist Louis Essen demonstrated the first caesium atomic clock at the National Physical Laboratory (NPL) in the UK, setting in motion the shift to atomic timekeeping. Prior to this, a second was defined as 1/86,400 of the time it took our planet to rotate, and therefore there should be 86,400 seconds each day. But using something as prone to fluctuation as the planet's rotation proved unreliable. A new standard was needed that would remain constant in any conditions. Essen showed that, at an atomic level, nature already has this precision.