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National Physical Laboratory


Countdown to the SI Redefinition: Meter

Published: Thursday, December 13, 2018 - 13:02

(NPL: Middlesex, UK) -- 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 the unit is used in everyday life, how it’s defined now, and the changes that will come into force on May 20, 2019.

20 November 2018

meter (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: Meter

“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 meter, and it can be understood as: the distance traveled by light in 1/299,792,458 of a second.

The reason we use the distance traveled 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 traveled 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 May 20, 2019, the official definition of the meter will change to:
The meter 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 March 20, 2019.

So, what’s the difference? Actually, there’s no big change coming for the meter. 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 (Nov. 20, 2018), will have traveled approximately 4,700,000,000,000 km (just short of 5 trillion km, 0.15 parsecs, or just over 1 percent of the Kessel run) by May 20, 2019. So, if the photon wasn’t absorbed by anything en route, it would already be more than a tenth of the way to our nearest neighboring 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 meter 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.


About The Author

National Physical Laboratory’s picture

National Physical Laboratory

Founded in 1900, the National Physical Laboratory (NPL) is a world-leading center for the development and exploitation of measurement science, technology, related standards, and best practices in a diverse range of technical areas and market sectors. As the United Kingdom’s National Measurement Institute, NPL capabilities underpin the U.K. National Measurement System (NMS), ensuring consistency and traceability of measurements in support of U.K. and overseas customer interests.