Being "on time" in the world of science does not exactly have the same meaning as "being on time" in everyday life. In fact, there are physicists whom have constructed clocks so accurate that, were those clocks first turned on at the time of the big bang, the time they report today would still not be out by more than a second.
Dr. Hermann Uys, a quantum physicist from the CSIR and Stellenbosch University, agreed to give us a look into the weird and wonderful world of keeping time with atoms.
Let's start with the basics: What is time?
Without getting too philosophical, time characterises the order in which events take place. To measure time, one basically needs two things: something that 'ticks', by which I mean that it produces a repeated signal, and you need something that counts those ticks. Then you can say "this event happened between tick 8 and 9", or "let's do that between ticks 12 and 13". Your device characterises the order in which events took place relative to the ticks. And there you have it! A time-keeping device.
One obvious 'ticker' is the earth itself, which goes through day-night cycles. You can create an extremely simple time keeping device by making a mark on a stick once a day when the sun is at its highest in the sky. Then you can make separate marks on the stick to indicate when certain events had taken place relative to the timing marks. I would imagine that this is what the first calendars looked like thousands of years ago.
However, checking the height of the sun in the sky by eye is not very precise. On any given day you are probably off by half an hour give or take. To make very precise time keeping devices you need to improve in two ways. Firstly, the ticks must follow each other very quickly so that you can order events that happened in very quick succession. Secondly, those ticks must follow each other very regularly, i.e. with exactly the same delay between successive ticks.
A grandfather clock, based on a swinging pendulum, is better than the earth as clock in that it ticks much faster than once a day, but its ticks are also not perfectly regular. So after some days or weeks the time it reports will have drifted compared to when the sun is highest in the sky.
Over the years many technologies have improved our ability to build better time keeping devices, and the most successful of these are the atomic clocks.
How do we measure time with atoms?
Remember I said that to measure time accurately we need something that ticks both quickly and very regularly? It turns out that the right kinds of atoms fulfil both those requirements. Specifically, caesium-133 atoms are used world-wide to define what is called Atomic Time. When electrons in the ground energy state of those atoms have oscillated 9,192,631,770 times, then we say a second has passed. This is the internationally accepted definition of the second. Caesium atoms are very fast tickers, but that is not all. Physicists believe that every caesium atom in the universe ticks in exactly the same way. So caesium atoms are also very regular tickers.
What is Atomic Time?
International Atomic Time (TAI – for the French Temps Atomique International), is the time based on about 400 of these atomic clocks in many different laboratories around the world. By the way, South Africa also has atomic clocks that give our national time. They are hosted by the National Metrological Society of South Africa (NMISA) in Pretoria. Dial 1026 on your phone and you'll hear a voice indicating the time provided by these clocks every 10 seconds.
But there is a slight problem with using Atomic Time to run our daily lives. The trouble is that the earth's rotation is not as regular as Atomic Time due to various physical effects. So had we used only Atomic Time then midnight would not always be at exactly 24:00. Instead we use what is called Coordinated Universal Time (UTC) to run our daily lives. UTC compares the time determined by the earth's rotation, also called astronomical time or Universal Time 1 (UT1), to Atomic Time. Whenever there is a difference of 0.9 seconds between them, a leap second is added to UT1 so that the earth's rotation remains well synchronised with our time standard.
How accurate are these clocks?
One way to test how accurate the clocks are is to build two of exactly the same clocks, and then to see how long it takes before the time those two clocks report has drifted apart by one second. It turns out that for the best caesium atomic clocks this will take about 300 million years!
Future inventions – are there efforts to make atomic clocks even MORE accurate? Why would that be important to do?
Many laboratories around the world are working very hard to build even better atomic clocks. There is a sort of friendly competition between these laboratories to be the leader in this race. Two technologies in particular are usually neck and neck at the front. They are what we call trapped ions and optical lattice clocks. These clocks are so accurate that they will not lose nor gain a second over many billions of years, if one could run them that long.
In our laboratory at Stellenbosch University we are now constructing an ytterbium ion trap, which we could ultimately use for pursuing improved atomic clocks. We also do a lot of theoretical physics research to better understand what causes the small inaccuracies in atomic clocks and how to overcome those problems.
Many technologies rely on precise timing, such as the internet and the telecommunications industry. And did you know that every GPS satellite has more than one atomic clock on board?
So all of modern navigation relies heavily on precise time keeping. Today the best laboratory clocks are so accurate they will literally tick slightly faster if you lift them just a few centimetres off the ground. This is due to the slightly lower gravitational field as one moves a little farther away from the Earth.
This is an effect that can only be understood through Einstein's theory of general relativity. Anyhow, they would also tick detectably slower if they were positioned above some area with very dense rock or mineral deposits directly below them as compared to say very porous rock. One can therefore imagine doing geo-exploration for minerals using precise enough clocks.
Far from being just interesting laboratory experiments, accurate atomic clocks are very useful!
On the photo, Physicists at Stellenbosch University are constructing an ytterbium ion trap, which could ultimately be used for pursuing improved atomic clocks. The process also involves a lot of theoretical physics research to better understand what causes the small inaccuracies in atomic clocks and how to overcome those problems. Pictured here, the heart of the experiment, the ion trap where single ionized atoms are captured. Picture: Stefan Els
The original article was published in the popular science magazine Quest as part of a series of articles on "time".
