Standards and Units

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Reproducibility of experimental results is central to the scientific method. To facilitate this we need standards, and to get convenient measures of the standards we need systems of units.

The standards are defined by operational definitions.

The quantities length, time and mass are central to the physical sciences. (See also physical unit)


The SI unit of length is the metre. One metre is defined as the distance light travels in a vacuum in 1/299792458 second. This standard was adopted in 1983, when the speed of light in vacuum was defined to be precisely 299792458 m/s.


The SI unit of time is the second. One second is defined as the time required for 9192631770 cycles of a hyperfine transition in cesium 133. This definition was adopted in 1967.


The SI unit of mass is the kilogram. One kilogram is defined to be the mass of a specific cylinder of platinum-iridium alloy, kept at the International Bureau of Weights and Measures (near Paris).

The most widely used system of units in science is the SI units (Système international).