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metre

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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 $\Delta\nu_{\rm{Cs}}$.

This definition is valid from 2019-05-20
Unit metre
Symbol m
Quantity length
Defining Constant speed of light
Defining Resolution
CGPM Resolution 1 (2018)
Unit Type SI base unit
Defining Equation $$1\;{\rm{m}} = \left(\frac{c}{299\:792\:458}\right) {\rm{s}} = \frac{{9\:192\:631\:770}}{299\:792\:458} \frac{c}{\Delta\nu_{\rm{Cs}}} \approx {30.663\:319} \frac{c}{\Delta\nu_{\rm{Cs}}}$$
Notes:
  1. 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 $\Delta\nu_{\rm{Cs}}$: \[$$1\;{\rm{m}} = (\frac{c}{299\:792\:458})\ {\rm{s}} = \frac{{9\:192\:631\:770}}{299\:792\:458} \frac{c}{\Delta\nu_{\rm{Cs}}} ≈ {{30.663\:319}} \frac{c}{\Delta\nu_{\rm{Cs}}}$$\]
  2. 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.
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