The references we use as the basis for length measurement have changed considerably throughout history. From body parts such as the forearm, foot and thumb, to physical artefacts and, more recently, to technology based on the wavelength of light from a krypton lamp. A well-known example of an ancient reference is the length of Pharaoh Tutankhamun’s forearm, which is believed to have been used as a local unit of measurement in Egypt. This unit may have been convenient for many practical purposes, but its definition was far from universal – and certainly not very precise.
The aim was to find a definition that could be tied to something stable, unchanging and universally accessible. Eventually, the size of the Earth was used as the basis for defining one metre, making it a unit shared by all of us on the planet. Today, the metre is anchored in something even more fundamental: the speed of light in a vacuum, a constant of nature.
The Norwegian Metrology Service realises the unit of length, the metre, using frequency-stabilised lasers. We use a helium-neon (HeNe) laser with a wavelength of 633 nanometres. The laser is stabilised by locking it to specific absorption lines in iodine vapour, which serve as stable frequency references for the light. When the frequency is known, the wavelength can be calculated using the formula λ = c / f.
The absorption lines are traceable to the units for frequency (hertz) and time (second), which are very precisely defined in the SI system. In this way, a stable frequency can be translated into an exact length – a method that follows one of the recommendations described in the scientific paper “Mise en pratique for the definition of the metre (2019)” from the BIPM (the International Bureau of Weights and Measures in Paris).
Length measurement is used in many fields, including:
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