National Physical Laboratory

Welcome to the interregnum

As of 1 July 2017, the worldwide effort to redefine the International System of measurement units in terms of fundamental constants has entered a new phase. Michael de Podesta, temperature scientist at the National Physical Laboratory (NPL), explains all on his Protons for Breakfast blog.

SI logo with constants

At midnight on 30 June 2017, the world stepped over the threshold into a new domain of metrology. It is now too late to ever measure the Boltzmann constant or the Planck constant.

What do you mean?

Measuring is the process of comparing one thing – the thing you are trying to measure – with a standard, or combination of standards. So when we measure a speed, we are comparing the speed of an object with the speed of 'one metre per one second'.

  • The Boltzmann constant tells us (amongst other things) the amount of energy that a gas molecule possesses at a particular temperature
  • The Planck constant tells us (amongst other things) the quantum mechanical wavelength of a particle travelling with a steady speed

To measure these constants we need to make comparisons against our measurement standards of metres, seconds, kilograms and kelvins.


But actually we think that quantities such as the Planck constant are really more constant than any human-conceived standard. That's why we call them 'constants'! And so it seems a bit 'cart-before-horse' to compare these 'truly-constant' quantities to our inevitably-imperfect 'human standards'.

Over the last few decades it has become apparent that it would make much more sense if we reversed the direction of comparison. In this new conception of measurement standards, we would base the length of a metre, the mass of kilogram, etc on these truly constant quantities.

And that is what we are doing. Over the last decade or so, metrologists worldwide have made intense efforts to make the most accurate measurements of these constants in terms of the current definitions of units embodied in the International System of Measurement, the SI.

On 1 July 2017, we entered a transition period - an interregnum - in which scientists will analyse these results. The analysis is complicated and so for practical reasons, even if new and improved measurements were made, they would not be considered.

If the results are satisfactory the General Conference on Weights and Measures will approve them. And on 20 May 2019 the world will switch to a new system of measurement. This will be a system of measurement which is scaled to constants of nature that we see around us.

And afterwards?

The value of seven 'natural constants' including the Boltzmann constant and the Planck constant will be fixed.

So previously people placed known masses onto special 'Kibble balances' and made an estimate of the Planck constant. By 'known masses' we mean masses that had been compared (directly or indirectly) with the mass of the International Prototype of the Kilogram.

After 20 May 2019, people carrying out the same experiment will already know the value of the Planck constant: we will build our system of measurement on that value. And so the results of the same experiment will result in an estimate for the mass of object on the Kibble balance.

What difference will it make?

At the point of the switch-over it will make no noticeable difference. Which begs the question: "Why are you doing this?" The reason is that these unit definitions form the foundations for measurements in every branch of every science. And the foundations of every complex structure – be it a building or the system of units – needs occasional maintenance.

Such work is often expensive and afterwards there is nothing to show except confidence that the structure will not subside or crack. And that is the aim of this change. The advances in measurement science over the last century have been staggering. And key developments would have been inconceivable even a few decades before they were made.

Similarly we anticipate that over future centuries, measurement science will continue to improve, presumably in ways that we cannot yet conceive. By building the most stable foundations of which we can conceive, we are making sure that – to the very best of our ability – scientific advances will not be hindered by drifts or inconsistency in the system of units used to report the results of experiments.

Find out more about the redefinition of the kilogram

Find out more about the SI measurement units

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Last Updated: 8 Aug 2017
Created: 6 Jul 2017


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