{"pid":"https://si-digital-framework.org/SI/units/metre","unitId":"metre","label":"metre","symbol":"m","startValidity":"2019-05-20","unitType":"SI base unit","unitTypeId":"https://si-digital-framework.org/SI#SIBaseUnit","unitDefinitionLabel":"metre2018","unitDefinitionId":"https://si-digital-framework.org/SI#metre2018","definition":"The metre, symbol $\\rm{m}$, is the SI unit of length. It is defined by taking the fixed numerical value of the speed of light in vacuum, $c$, to be $299\\:792\\:458$ when expressed in the unit ${\\rm{m}}\\ {{\\rm{s}}^{-1}}$, where the second is defined in terms of the caesium frequency $\\Delta\\nu_{\\rm{Cs}}$.","definingResolution":"CGPM26-Res1","eventName":"26th meeting of the CGPM","eventDate":"2018-11-16","resolutionLink":"https://doi.org/10.59161/cgpm2018res1e","unitDefinitionStatus":"current","previousVersionDate":"2019-05-19","hasPreviousDefinition":true,"previousDefinition":"https://si-digital-framework.org/SI#metre1983","resNbr":"1","resolutionBody":"CGPM","resolutionType":"Resolution","equation":"1\\;{\\rm{m}} = \\left(\\frac{c}{299\\:792\\:458}\\right) {\\rm{s}} = \\frac{{9\\:192\\:631\\:770}}{299\\:792\\:458} \\frac{c}{\\Delta\\nu_{\\rm{Cs}}} \\approx {30.663\\:319} \\frac{c}{\\Delta\\nu_{\\rm{Cs}}}","cstLabel":"speed of light","cstHidden":"SpeedOfLight","constant":"speed of light","constantPid":"https://si-digital-framework.org/constants/SpeedOfLight","prefixRestriction":false,"unitQuantities":[{"code":"LENG","label":"length"}],"notes":["This definition implies the exact relation ${c} = {299\\:792\\:458}\\ {{\\rm{m}}\\ {{\\rm{s}}^{-1}}}$. Inverting this relation gives an exact expression for the metre in terms of the defining constants $c$ and $\\Delta\\nu_{\\rm{Cs}}$: \\[$$1\\;{\\rm{m}} = (\\frac{c}{299\\:792\\:458})\\ {\\rm{s}} = \\frac{{9\\:192\\:631\\:770}}{299\\:792\\:458} \\frac{c}{\\Delta\\nu_{\\rm{Cs}}} ≈ {{30.663\\:319}} \\frac{c}{\\Delta\\nu_{\\rm{Cs}}}$$\\]","The effect of this definition is that one metre is the length of the path travelled by light in vacuum during a time interval with duration of ${1/299\\:792\\:458}$ of a second."],"notesIds":["https://si-digital-framework.org/SI#metre2018note1","https://si-digital-framework.org/SI#metre2018note2"]}