Metre (source code)
= Metre
{title2=$\mathrm m$}
{wiki}
The <metre>, symbol $\mathrm m$, is the SI base <unit of measurement> for <length>. It is defined by fixing the <speed of light> in <vacuum> to $299\,792\,458\,\mathrm{m\,s^{-1}}$, with the <second> defined independently. Thus light in <vacuum> travels one <metre> in $1/299\,792\,458$ of a <second>.
The exact defining constant does not make every measured <length> exact. Realizing a <length standard> can involve optical <interferometry>, <frequency> references, alignment and corrections for propagation through air; their imperfections contribute to <measurement uncertainty>. The definition specifies the reference, whereas an experiment supplies an imperfect realization.<The International System of Units (SI), ninth edition, SI Brochure>[section 2.3.1: definition of the metre]{ref}
Back to article page
