Why cesium is used for atomic clocks

It was a momentous decision. Until then, mankind had always defined time even Railway Time by reference to the movement of the sun relative to the earth. No more. The sun was dethroned, and caesium took its place - though one wonders how long it will be before strontium or ytterbium knocks caesium off its perch. The switch to atomic time was for good reason.

The rotation of the earth, it turned out, was not such a reliable measure of time. No day or year is exactly the same length. First off, the earth is very gradually slowing down and thus the average day is getting infinitesimally longer. Then you have to add in the idiosyncrasies of oceanic tides, tectonic drift and the convection of the earth's mantle, all of which cause minuscule wobbles.

This is a big issue for Felicitas Arias, whose job is to keep time for the entire world. When UTC was first adopted in the s, long before the advent of GPS, it posed a potential problem for sailors, who still relied on clocks to work out their longitude on the high seas. In the late 17th Century, clock and watch design was part of national security. Navigation and mapping were both essential for the successful conduct of war - and England was involved in a sequence of wars against the French and the Dutch in this period.

Lisa Jardine: How the world's first smartwatch was built. They still exploited a system that the super-accurate clocks of another British pioneer of timekeeping, John Harrison, had first made possible in They compared the position of the sun or the stars at their particular location, with the time on a clock taken from another fixed location, typically Greenwich. Every four minutes' difference represented a single degree east or west.

But in order for this technique to continue working, they insisted that UTC remain synchronised with the earth's wobbly rotation. And that means every now and then an extra "leap second" is inserted. And it is Ms Arias' job to decide when. Then they came back every two or two-and-a-half years.

But every time a leap second needs to be inserted, all the atomic clocks across the world need to be changed. Most of us wouldn't notice a second or two every couple of years, but computers do. They might momentarily shut down, which would, apparently, make them vulnerable to cyber-attack.

Or they could get out of sync, leading to electronic train crashes. It hasn't happened yet but Ms Arias believes the consequences could be disastrous. The prospect of City traders losing out on million-dollar deals may not fill you with horror, but she's worried that as power stations, mobile phone networks and satellite navigation systems are increasingly synched to caesium time they could fail too. That's why there is a move now to get rid of leap seconds completely and go over to unadulterated atomic time.

Most sailors already use satellite navigation, so it might not be such a serious problem for them today as it was in the past. But severing the link between time and the motion of the celestial bodies completely would have some significant consequences. Because the earth's spin is slowing, the time on your watch would gradually diverge from the rising and setting of the sun.

She's surprisingly relaxed about the prospect. She points out that most of us are already out of sync with solar time. Due to the earth's elliptical orbit, the sun can be as much as 16 minutes out of line with mean solar time.

Add the distortion of time zones, which average time across huge regions, and the difference is far greater. China, which is almost 5,km wide, has a single time zone spanning 1h40 of solar time. The decision of some countries to adjust the clocks twice a year as a "daylight saving" measure exaggerates the issue yet further. Nevertheless many nations resist the move to end leap seconds. The UK, for example, whose Greenwich Meridian was for centuries the benchmark for global timekeeping, insists that leap seconds are not a serious inconvenience.

And maybe that is a reasonable position given just how slippery the whole concept of time can be. In scientists sent three caesium clocks around the world on commercial airliners. When they returned they were compared with caesium clocks that had stayed at home in the United States Naval Observatory. The jet-setting clocks were found to be slower by exactly the amount Einstein's theories of relativity predicted.

Thereby proving his hypothesis that the speed at which time passes depends on where you are in the universe and how fast you are moving. Today, the clocks aboard GPS satellites have to be adjusted to take account of precisely this effect. And the new generation of insanely accurate optical clocks may make use of relativity to map the gravitation field of the earth, by measuring the minute differences in recorded time caused by the effect of gravity.

But stop and consider the irony here. Caesium clocks have proved that an absolute measure of time is - impossible. Image source, Science Photo Library. Caesium centre: a relay station in Colorado where atomic time signals are transmitted across the US.

This clock in Bristol still has two minute hands 10 minutes apart. Image source, Getty Images. Mobile Newsletter chat avatar. Mobile Newsletter chat subscribe.

Prev NEXT. Physical Science. Nuclear Science. Some Definitions. Read More. Cite This! Print Citation. Try Our Crossword Puzzle! What Is the Missing Number? Try Our Sudoku Puzzles! More Awesome Stuff. Improve this question. Pinki Pinki 1 1 gold badge 5 5 silver badges 3 3 bronze badges.

Rubidium has also been used in commercially available atomic clocks. I know that many people are also working on Yb atomic clocks because the element offers even better time-stability. Show 1 more comment. Active Oldest Votes. Improve this answer. Floris Floris k 12 12 gold badges silver badges bronze badges. I do see that I was using it in the "now more current" sense which is the first definition that shows up if you Google 'what does "beg the question" mean'.

I think my meaning is well-understood. See also english. Still - I have edited my answer - don't want poor language to get in the way of clarity. Saying "The second was defined that way" does more or less presuppose the answer to "Why is the second defined using caesium clocks", which is what begging the question means.

That's not exactly a round number. Add a comment. But why was cesium chosen? Various factors: At reasonable temperatures, cesium has a high vapor pressure, making resonance effects relatively easy to observe. Large hyperfine transition, creating better Q of the resulting resonator. Mostly AHMs are considered to be more stable in frequency than caesium in the industry. So why do atomic clocks only use caesium? They don't The modern second is defined in terms of the Cs hyper-fine transition so, of course, no other clock can be as accurate as caesium, purely by definition.

Secondary representations of the SI second So if the second is defined by caesium, why did I say that not all clocks use it? Emilio Pisanty k 28 28 gold badges silver badges bronze badges. CharlieB CharlieB 4 4 silver badges 10 10 bronze badges. Thus, using any other atom is irrelevant even if calculate some correction time factor. There are two parts of the answer, "because there is a standard which we all need to follow or we're not quite sure we're talking about the same thing", and then "we chose caesium as the standard because The original post as posed is well answered by the first and your answer - Pinki would have to elaborate on how much detail they are after.

Show 2 more comments. It's also the SI standard. We define the second by Caesium. Specifically: The second is the duration of 9 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium atom. DigiDuncan DigiDuncan 1 1 1 bronze badge.