Since 1983, the metre has been internationally defined as the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second. This definition can be realised simply and accurately using modern techniques and the speed of light is regarded to be a universal constant, making it ideal as the basis for a length standard.
The definition of the metre can be practically realised in two different ways:
Time of flight – a pulse of light is sent over the length to be measured. The time it takes for the light to travel this distance, in seconds, multiplied by the speed of light (299 792 458 metres per second), gives the length in metres. As the speed of light is very fast, this method is easier to apply over long distances. However, care has to be taken to account for gravitational field effects when measuring astronomical distances.
Interferometry – the technique of interferometry allows a length to be measured in terms of the wavelength of light. By using a light source of known and stable wavelength, lengths up to 100 metres can be directly measured, with accuracies up to 1 part in a few million.
Accurate length measurement and precise definition are needed throughout the modern world. From the tiniest features on a microchip, through standard threads on nuts and bolts, to large, complex sub-assemblies of modern airliners, interchangeability and reproducibility are essential in a global economy where items are sourced from different countries, yet have to fit together perfectly, first time.
Lasers currently used to realise the metre provide very stable optical frequencies (or vacuum wavelengths) by servo-controlling the light that they emit to particular reference absorptions in gases such as iodine, held within small gas cells. However, their accuracies are limited by the motion of the gas molecules in the laser beam at room temperature. By replacing iodine molecules with atoms or ions such as ytterbium or strontium that are held within electromagnetic or optical traps, it is possible to 'laser cool' atoms close to absolute zero (thereby reducing their motion). In this arrangement the optical reference frequencies can be several orders of magnitude more accurate, but other limitations, such as atmospheric conditions or material stability, generally prevent their use in direct length measurement at these improved levels.