Cesium: The element that redefined time
Reporter: Aviva Lev-Ari, PhD, RN
Until about 175 years ago, it was the sun that defined time. Wherever you were, high noon was high noon, and on a clear day a quick glance up into the sky or down at a sundial told you everything you needed to know.
It was not until the 1930s that the physicist Louis Essen developed the first quartz ring clock, the most accurate timepiece of its day, and a precursor of the cesium clock.
Quartz clocks exploit the fact that quartz crystals vibrate at a very high frequency if the right electrical charge is applied to them. This is known as a resonant frequency, everything on earth has one.
It is hitting the resonant frequency of a champagne glass that – allegedly – allows a soprano to shatter it when she hits her top note. It also explains why a suspension bridge at Broughton in Lancashire collapsed in 1831. Troops marching over it inadvertently hit its “resonant frequency”, setting up such a strong vibration the bolts sheared. Ever since, troops have been warned to “break step” when crossing suspension bridges.
To understand how this phenomenon helps you to measure time, think of the pendulum of a grandfather clock. The clock mechanism counts a second each time it swings.
Quartz plays the same role as a pendulum, just a lot quicker: it vibrates at a resonant frequency many thousands of times a second.
And that’s where cesium comes in. It has a far higher resonant frequency even than quartz – 9,192,631,770 Hz, to be precise. This is one reason Essen used the element to make the first of the next generation of clocks – the “atomic” clocks.
Essen’s quartz creation erred just one second in three years. His first atomic clock created at NPL in 1955 was accurate to one second in 1.4 million years. The cesium fountain at NPL today is accurate to one second in every 158 million years. That means it would only be a second out if it had started keeping time back in the peak of the Jurassic Period when diplodocus were lumbering around and pterodactyls wheeling in the sky.
But modern technology means these days even more staggeringly accurate clocks are possible. That’s because cesium was always a compromise element when it came to timekeeping. Louis Essen chose cesium, because the frequency of its transition was at the limit of what the technology of his day could measure. We have today new ways of measuring time.
The frequency of the transition of strontium, for example, is 444,779,044,095,486.71 Hz. A strontium clock developed in the US would only have lost a second since the earth began: it is accurate to a second in five billion years. The scientists at NPL reckon optical clocks that keep time to within one second in 14 billion years are on the horizon – that’s longer than the universe has been around.
Now, if such insane levels of accuracy seem pointless, then think again. Without the caesium clock, for example, satellite navigation would be impossible. GPS satellites carry synchronized cesium clocks that enable them collectively to triangulate your position and work out where on earth you are.
Source: www.bbc.com
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