Atomic Clocks

Atomic clock technology continued to improve. Machines which are accurate to one second in 30000 years, such as the HP5071A, are now commercially available and are often used by communications companies to obtain precise frequencies.

These clocks work on the same principle as Caesium I. The latest atomic clocks work in a slightly different way, which gives even greater accuracy. In 1993, the National Institute of Standards and Technology (NIST) in the USA (formerly the NBS) built NIST-7, a caesium atomic clock that uses lasers instead of magnets to separate the atoms before and after they pass through the beam tube. Initially, NIST-7 was accurate to one second in 800000 years but has since been improved to one second in 6 million years. The next generation of atomic clocks will use other methods for controlling the movement of their atoms and will be even more reliable.

It is not immediately obvious why we need clocks of this accuracy. However, each time the accuracy of timekeeping is improved, a new use is found for it. Some of the technologies now being developed will require further improvements.

 

One use of precise timing is in the measurement of distances. The Global Positioning System (GPS) is a constellation of 27 satellites, each carrying three atomic clocks. A GPS unit receives radio signals from at least four of these satellites. By measuring the time differences between the signals from the satellites, it can calculate its position and height to within a few metres. It also gives the time to within a fraction of a second. GPS is widely used for navigation at sea and in the air. Portable receivers are increasingly being used by motorists and ramblers. They are also used by scientists to track animals for research purposes. There are even prototype devices to help blind people find their way around towns.

Atomic timing is widely used in the communications industry. Telephone companies generally transmit calls digitally. Many conversations between the same two cities can be carried down the same wires if computers at both exchanges flick from one conversation to another thousands of times every second.

If the clocks controlling the two exchanges get out of step, the calls will become jumbled up. Atomic clocks ensure that this does not happen. Mobile telephones, digital television and Internet communications use similar technology. Communications companies used to maintain their own atomic clocks. Nowadays, they often take the time from the clocks on GPS satellites.

 

An atomic clock in Cumbria broadcasts radio signals which are received by some clocks and watches. These clocks never need to be set by their owners and adjust themselves automatically for summer time. Transmitters in other countries do similar jobs.

Many industrial and commercial organisations use atomic time indirectly, either from the radio broadcasts in Cumbria or from GPS satellites. Sometimes, they need the precision of atomic clocks but more often it is just because it is cheaper and more convenient than the ordinary clock systems which they used before.

Time can be measured so precisely that the metre is now defined as the distance travelled by light in a vacuum in 1/299792458 of a second. One advantage of this definition is that they can be used anywhere. Previously, it was defined as the distance between marks on metal bars kept only at certain laboratories.

Until recently, all time measurement was based on the length of the day, measured as the time between the Sun crossing the meridian (the North-South line) on one day and on the next. The length of the day measured in this way varies slightly throughout the year because the Earth's orbit around the Sun is not perfectly circular and its axis of rotation is inclined to the plane of its orbit. These variations are well understood and to allow for them, time measurement was based on the mean solar day, the average length of all the days in a year. The second was defined as 1/86400 of the mean solar day.

In astronomical observatories, where the greatest accuracy was needed, time was actually calculated from observations of the stars. The motion of the stars relative to the Earth was known more accurately than clocks kept time and mean solar time avoided errors caused by variations in the speed of the Earth's rotation on its axis.