Consider the following: the purpose of a pendulum in a clock is to keep track of the seconds. In other words, by decreasing the period of the clock, one automatically increases its accuracy. According to Thomas Udem of the Max Planck Institute for Quantum Optics in Garching, Germany, to better the accuracy of a clock one needs to “increase the rate at which it ‘ticks'”. That is equivalent to losing or gaining no more than one second in the age of the universe.Understanding how the old definition of the second proved to be inadequate in carrying out accurate measurements gives us motivation to find a better mechanism to measure time.

This can also be achieved for atoms trapped by intersecting light beams.Īt NPL, optical clocks are being developed which target accuracies of around 100 times higher than the best current microwave frequency atomic clocks. In this way, certain optical absorptions in the ion can exhibit a very pure frequency.

Ions are charged atoms which can be trapped almost indefinitely by electromagnetic fields, and cooled by laser beam close to absolute zero. The Global Positioning System (GPS) satellites broadcast timing signals from onboard atomic clocks, which enable land vehicles, shipping and aircraft to know their location within a few metres.Ĭlocks for the 21st century based on single cold trapped ions or collections of atoms are being developed. Time measurement has become a basic part of everyday life and accuracies of the nearest minute or a few seconds are usually good enough for most human activities, but highly accurate timing plays a vital role in many other aspects of the modern world. By measuring the frequency of the electromagnetic radiation, like counting the number of pendulum swings, we can measure the passage of time. This energy is absorbed or released in the form of electromagnetic radiation, the frequency of which depends on the difference in energy between the two levels. The orbits correspond to energy levels, and electrons can only move between levels when they absorb or release just the right amount of energy. The atom can be pictured as a mini solar system, with the heavy nucleus at the centre surrounded by electrons in a variety of different orbits. However, the quartz and atomic clocks invented during the 1930s and 1950s were even better timekeepers, and showed that the Earth does not rotate steadily but 'wobbles'. Modern atomic clocks can be as much as a million times more accurate than this, and underpin satellite technology, like GPS or the internet.įor thousands of years the Earth's rotation was our most stable timekeeper. This clock was so accurate that it would only gain or lose one second in three hundred years. NPL developed the first operational caesium-beam atomic clock in 1955. However this period of rotation is not regular enough to serve as a definition for modern-day applications.Ītomic clocks, which keep time using transition energies in atoms, revolutionised timekeeping. These methods of timekeeping were based on the daily rotation of the Earth around its own axis. As well as enabling us to tell the time of the day, accurate timekeeping is key to satellite navigation systems, underpins the functioning of the internet and facilitates timestamping for transactions in financial trading.Īncient civilisations used sundials and obelisks to tell the time, which was not very precise, and was restricted in cloudy weather or at night.

Caesium 133 clock