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Countdown to the SI redefinition – metre

Following the recent decision, taken by measurement scientists from around the world, to redefine the International System of Measurement (SI) units; on the 20th of each month we will be looking at one of the seven SI base units. You'll be able to find out where it's used in everyday life, how it's defined now, and the changes that will come into force on 20 May 2019.

 20 November 2018 — metre (m) 20 December 2018 — candela (cd) 20 January 2019 — ampere (A) 20 February 2019 — kelvin (K) 20 March 2019 — second (s) 20 April 2019 — mole (mol) 20 May 2019 — kilogram (kg)

Unit of the month: metre

"You've never heard of the Millennium Falcon? … It's the ship that made the Kessel run in less than 12 parsecs!" Han Solo's description of the Millennium Falcon in Star Wars is impressive, but something's not quite right. Do you know why?

The unit he uses to illustrate the prowess of the Falcon – a parsec – isn't actually a measure of time, but length! It probably won't surprise anyone that Han Solo isn't very precise when it comes to the physics of his ship, but in fact he isn't too far from the truth. This is because we use time to define length.

What does this mean? Well, in the case of Han Solo, one parsec is about 3.26 light-years, and a light-year is the distance light travels in one year. Back down on Earth, we have the same method for defining length. In the International System of Units (SI), the base unit of length is the metre, and it can be understood as:

A metre is the distance travelled by light in 1/299 792 458
of a second.

The reason we use the distance travelled by light in a certain amount of time is because light is the fastest thing in the universe (that we know of) and it always travels at exactly the same speed in a vacuum. This means that if you measure how far light has travelled in a vacuum in 1/299 792 458 of a second in France, Canada, Brazil or India, you will always get exactly the same answer no matter where you are!

On 20 May 2019, the official definition of the metre will change to:

The metre 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 m s−1,
where the second is defined in terms of the caesium frequency ∆ν.

We'll be returning to the definition of the second on 20 March 2019.

So, what's the difference? Actually, there's no big change coming for the metre. Although the word order has been modified, the physical concepts remain the same.

Here's something to think about: a photon emitted from the Sun today (20 November 2018) will have travelled approximately 4 700 000 000 000 km (just short of 5 trillion km, 0.15 parsecs or just over one percent of the Kessel run) by 20 May 2019. So, if it wasn't absorbed by anything en route, it would already be over a tenth of the way to our nearest neighbouring star, Alpha Centauri. It would certainly be much too far away to witness the new definitions of the SI units coming into force on Earth. Nevertheless, its own journey would be very exciting too. And no matter where in the universe, based on the SI definition, the metre will have the exact same length there – so we can always work out just how far away that photon is.

Find out more about Redefining the SI units

Find out more about Dimensional measurement at NPL

Last Updated: 15 Dec 2018
Created: 19 Nov 2018

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