Length

Article By:

Jones, Edwin R. Department of Physics and Astronomy, University of South Carolina, Columbia, South Carolina.

Jennings, Donald A. Time and Frequency Division, National Institute of Standards and Technology, Boulder, Colorado.

Last reviewed:January 2020

DOI:https://doi.org/10.1036/1097-8542.376900

A scalar physical quantity indicating a one-dimensional extension in space. Length is one of the three fundamental physical quantities important in mechanics, the other two being mass and time. It can be measured by comparison with an arbitrary standard; the specific one in common usage is the international meter. The most recent definition of the meter was adopted in 1983 at the meeting of the General Conference on Weights and Measures (Conférence Général des Poids et Mesures, or CPGM), when the meter was redefined in terms of time and the speed of light: “The meter is the length of the path traveled by light in a vacuum during a time interval of 1/299,792,458 of a second.” The effect was to take the speed of light in a vacuum to be exactly 299,792,458 m/s and then to define the meter in terms of that speed and the most accurately known quantity, the second. See also: Light; Time

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