# The metre

The metre (m) defines length (roa).

The metre is defined using a fundamental constant of nature – the speed of light (c), which is given a fixed numerical value of 299,792,458 m s-1, so a metre can be defined as the distance covered by light in a vacuum in exactly 1/299,792,458 of a second.

The definition also relies on another fundamental constant – the frequency of a 133Cs atom (represented as ΔνCs), which has a fixed value of 9 192 631 770 Hz (or s-1).

Discussion point: The metre has gone from being defined by a physical artefact, to waves of light from a krypton atom, to a fraction of the speed of light. Do you think that this definition will last – or will we redefine it again?

## Transcript

PETER SAUNDERS

The metre used to be defined in terms of the length of a metal bar – it’s in a vault in Paris – that was based on the measurement of the circumference of the Earth. But the problem with having an artefact standard like that is it becomes infinitely valuable. And if for example, it drifts in length or if somebody damages it, then by definition, it is still a metre so everything else in the universe must change size. And it’s also having an artefact standard that everyone has to go back and compare with limits technological advances.

And so this was recognised quite early on with the metre, and so in 1960, they redefined the definition of the metre to be a certain number of wavelengths of light that’s emitted by a krypton atom. And defining it that way improved the accuracy of length measurements by about a factor of 25. But even that wasn’t enough, and so soon after, they redefined it again.

And so we know from Einstein’s theory of relativity that the speed of light is a constant, no matter how you observe it or where you observe it, and so it’s a really good foundation for a definition of a unit because it is unchanging in the universe. And so in 1983, the definition of the metre was defined by setting the speed of light to be this exact number here [c = 299,792,458 m s-1]. So previously, people used to be able to measure the speed of light, now this is it exactly, and it’s never going to change. And so the metre now is defined as the distance that light travels in a very short, small fraction of a second, where that fraction of a second is one divided by this number [299,792,458]. And so having the metre defined that way actually improved the accuracy by another factor of about 40. But it means now that anyone can use any method they like to determine what a metre is as long as they can relate, have the apparatus and can relate their measurements back to the speed of light.

And essentially we’ve now taken the metre out of the vault – both literally and figuratively – so you know, and the metre now defines, well the speed of light defines not just the metre but the kilometre or the nanometre. Any scale that you want to measure, you can measure by relating your measurements back to the speed of light.

### Acknowledgements

This video clip is from a recording of a presentation by the Measurement Standards Laboratory of New Zealand (MSL) in celebration of the redefinition of the International System of Units (SI), which happened on 20 May 2019. The presentation by Peter Saunders and Farzana Masouleh of MSL was filmed at Unleash Space, Faculty of Engineering, Auckland University.

Filming and editing by Jonathon Potton of Chillbox Creative. MSL produced these videos to share the story of metrology development.

**Published:**15 August 2019