The following article originally linked to the BBC website. That material is no longer available but, luckily, much of the pages' static content has been stored by the Internet Archive "Wayback Machine" and the links now refer there. However, the videos are not available.

• Internet Archive

Keeping Time with Atoms

A very precise clock is needed to demonstrate Einstein's relativity theory concerning space and time. Agilent Technologies' 5071A Primary Frequency Standard is widely acknowledged by the world's timekeeping experts as the best commercially available device; an atomic clock that, in laboratory conditions, has stability equivalent to one second in 1.6 million years (2x10^^-14 ) and with typical absolute timekeeping accuracy of about one second in 160 thousand years
(2x10^^-13 ) but conservatively specified at 1x10^{^-12}.

This extraordinary precision makes it ideally suited to the "flying clock" experiment that formed part of the 1999 series of British Royal Institution Christmas Lectures televised by the BBC. The annual lectures aim to introduce science to children in a fun way and their website provides an excellent supporting resource to the series. Using the free RealPlayer™ , you can watch video extracts showing this experiment that illustrates the possibility of time travel.

• BBC OnLine Royal Institution Lectures via the Internet Archive

The Experiment in Summary

Two 5071A standards are used which are time synchronized and whose relative rates of change are known. One is designated as the reference whilst the other is taken to China and back. When the two clocks are reunited it is seen, even after the different drift rates of  each clock is taken into account,  that the traveling clock has lost about 60 nanoseconds compared to the clock that remained in Britain.

Streaming RealVideo from BBC Online
NOTE: Unfortunately, the videos are no longer available from the BBC or the Internet Archive.

• Video 1
• Video 2

Dr John Laverty of the UK National Physical Laboratory wrote an article for the BBC about the experiment.

• Dr Laverty's article via the Internet Archive

He also anticipated its conclusion with a mathematical explanation for the loss of time suffered by the "flying clock". A similar discussion is also published in our Application Note 1289.
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Math explanation

AN1289

Where Else are Atomic Clocks Used?

Atomic clocks play a vital role in modern life, synchronizing the rapid movement of information through communications networks. The more accurate these systems' timing references are, the greater the amount of information that can be transferred without the danger of lost data. That means clear telephone calls, sharp television pictures and fast, error-free Internet connections. They're also the foundation of satellite-based navigation systems, found everywhere these days from yachts to space shuttles and even hikers' rucksacks, where the accuracy of the reference clocks directly relate to the receiver's ability to establish its latitude, longitude and altitude to an accuracy of a few metres.

How Long is a Second?

The principals of a caesium beam frequency standard were first demonstrated by the UK's National Physical Laboratory in 1955 and in 1964 Agilent Technologies (as HP) introduced the first commercially available product. In 1967 the second was internationally agreed as the duration of 9,192,631,770 vibrations of the caesium-133 atom. The International Bureau of the Hour in France maintains International Atomic Time by calculating a weighted average from the performance of several hundred atomic clocks operating in many countries. Agilent caesium clocks contribute 80% of this figure and, consequently, virtually define the length of the world's standard second.

How Does it Work?

Around 1913, the young Danish physicist Niels Bohr, working in England with Ernest Rutherford, developed the original concept of atoms comprising a central nucleus with orbiting electrons, like planets circling the sun. Bohr proposed the revolutionary idea that the electrons did not gain or lose energy in a gradual way like a spring winding down, but did so in lumps by jumping between distinct, allowable orbits. He also realized that the orbital change is accompanied by the release or absorption of a distinct "quantum" of energy and that this corresponds to a particular frequency of electromagnetic radiation.

The caesium atom has a single, outermost valence electron that spins on its axis. This spin produces a magnetic field at the center of the atom called the hyperfine field. The nucleus, which is itself spinning like a magnet, aligns itself in the hyperfine field in a direction that depends on the energy state of the atom. In one energy state the nucleus and the hyperfine field are aligned in the same direction, while in the other state the two are opposed. By arranging for a beam of caesium atoms to pass through a microwave energy field, the atoms can be made to change states either absorbing or emitting energy as the microwave frequency is varied. When the frequency precisely equals the hyperfine resonance of the atom, the greatest number of transitions will occur. This peak activity is detected and used to control the frequency of the microwave field which is, therefore, accurately held at the atom's resonance of about nine billion beats per second. With the addition of digital divider circuitry, an exact one second pulse is derived.