How the metre is defined: the speed of light since 1983, reworded in the 2019 SI
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Since 1983 the metre has been defined through the speed of light in vacuum, which makes c exactly 299 792 458 m/s. The 2018 CGPM reworded the definition to fix c explicitly as a defining constant; the revised SI took effect on 20 May 2019.
Current definition
The International Bureau of Weights and Measures (BIPM) defines the metre (symbol m) by fixing the numerical value of the speed of light in vacuum, c, at exactly 299 792 458 when expressed in m/s, with the second defined via the caesium hyperfine frequency. Equivalently, one metre is the distance light travels in vacuum in 1/299 792 458 of a second.
How we got here
- 1889: the 1st CGPM sanctioned an international prototype bar of platinum-iridium as the metre.
- 1960: the 11th CGPM replaced the bar with a wavelength of a specific krypton-86 emission line.
- 1983: Resolution 1 of the 17th CGPM defined the metre as the length of the path travelled by light in vacuum during 1/299 792 458 of a second, and abrogated the krypton definition.
- 2018/2019: Resolution 1 of the 26th CGPM (November 2018) kept the concept but rephrased all base-unit definitions in terms of fixed defining constants. The revision entered into force on 20 May 2019.
Practical consequences
Because c is fixed by definition, it has no measurement uncertainty in SI units. Measuring "the speed of light" more precisely is now, in effect, a better realization of the metre (or the second). The 2019 revision changed the kilogram, ampere, kelvin and mole substantively; the metre, second and candela were already tied to constants, so the metre's size did not change.
Limits and caveats
A definition is not a measurement method: laboratories realize the metre with techniques described in BIPM mises en pratique, each with its own uncertainty. Everyday length standards (gauge blocks, tapes) are traceable to these realizations, not directly to light. Dates for intermediate history (1889, 1960) are summarized from the BIPM history page. The wording of the 1983 definition repeated here is the CGPM's own and is not covered by this record's license.