metre — from formula anatomyThe metre (m) is the SI base unit of length, defined as the distance light travels in a vacuum in exactly 1/299,792,458 of a second. It is the foundation for measuring size, distance, and position in physics, from the scale of atoms to galaxies. Its definition has evolved from a physical artifact to a constant of nature, ensuring universal reproducibility.
Originally defined in 1793 as one ten-millionth of the distance from the North Pole to the Equator, it was later standardized as a platinum-iridium bar in 1889. In 1983, the current definition based on the speed of light was adopted by the General Conference on Weights and Measures.
The metre is realized in practice by using the speed of light, which is fixed by definition. To measure a length, one can compare it to the wavelength of a laser or use interferometry, counting interference fringes. This makes the metre accessible in any laboratory with the right equipment, independent of a physical prototype.
It is used in all scientific measurements of length, from semiconductor manufacturing to satellite navigation, and in everyday tools like rulers and tape measures.
The metre was once defined by a metal bar kept in Paris, but that bar was replaced because it was slightly too short due to manufacturing error.
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