Countdown to the SI redefinition

metr_bck_3Throughout history, measurement has been a fundamental part of human advancement. The oldest systems of weights and measures discovered date back over 4000 years. Early systems were tied to physical objects, like hands, feet, stones and seeds, and were used, as we still do now, for agriculture, construction, and trade. Yet, with new inventions and global trade more ever more accurate and unified systems were needed. In Europe, it wasn’t until the 19th Century that a universally agreed measurement system began to be adopted and the International System of Units (SI units) was born.

Now, after years of hard work and scientific progress, we are ready once again to update and improve the SI units. The redefinition of the International System of Units enacted on the 16 November 2018 during the General Conference for Weights and Measures will mean that the SI units will no longer be based on any physical objects, but instead derived through fundamental properties of nature. Creating a system centred on stable and universal natural laws will ensure the long-term stability and reliability of measurements, and act as a springboard for the future of science and innovation.

The redefinition of the SI units will come into force on the 20th of May 2019, the anniversary of the signing of the Metre Convention in 1875, an international treaty for the international cooperation in metrology.  To celebrate, we’ll be counting down each of the SI units – the metre, second, kilogram, kelvin, mole, candela, and ampere. Join us on the 20th of every month to find out where units are commonly used, how they’re defined, and the changes that will take place!


“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 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.

metre facts

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 1/299792458 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 1/299792458 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 next year 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, so join us again then to find out more.

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

The National Measurement Laboratory turns 30!

In 1988, Government Chemist Alex Williams, seeing the need for improved quality of analytical measurements, initiated and launched the Valid Analytical Measurement (VAM) programme to develop a chemical measurement infrastructure in the UK.

This programme would go on to evolve into the National Measurement Laboratory for chemical and bio-measurement. The UK was one of the pioneers within the global measurement community to recognise the need to address the new and developing challenges of measurement across chemistry and biology.

An article from the early VAM bulletins (1989).

That means 2018 marks the NML’s 30th birthday and kicks off our ‘Year of Measurement’. It is an opportunity to celebrate the importance of measurement science (‘metrology’) as we enjoy our 30th birthday and join the upcoming Festival of Measurement, which launches in September and lasts through May 2019.

In our thirty year history of performing measurements to support the UK, we’ve experienced a lot of growth, seen big changes in the challenges we’ve been set and made some major breakthroughs. We’ve asked (and answered) a lot of questions, like ‘What are the best methods for the detecting the adulteration of honey’ or ‘Is the computer a friend or foe?’ (The answer is ‘friend’…or ‘both’ if you’ve invested heavily in encyclopaedias.)

We’ve already outlined in a recent blog post how important accurate measurement is, affecting everything from food and drink to medicine. Accurate and precise measurement is the foundation of public health and safety. But it’s also just as important to the economy.  In 2009, it was estimated that £622 billion of the UK’s total trade relied on measurement in some way, meaning that measurement plays a role in nearly every aspect of our lives.

Our Chief Scientific Officer, Derek Craston, agrees that good measurement is crucial to economies. ““In my role, I am fortunate to be able to see the major benefits that chemical and biological measurements make to the prosperity of companies and the lives of individuals across areas as broad as clinical diagnosis, drug development, environmental protection and food security. Indeed, in a global economy, with complex supply chains and regulatory frameworks, it is hard to see how many markets could function without it.”

We’re proud of the work we’ve done as the National Measurement Laboratory, where our work supports manufacture and trade, protects consumers and enhances quality of life. And over the next few months, we plan to share stories and case studies from our thirty years at the forefront of measurement with you, as well as look forward to the next thirty years.