**Welcome to the Interregnum.**

**At midnight on the 30th 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.

**So…**

**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 world-wide 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 July 1st 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*, a high-powered diplomatic meeting, will approve them. And on May 20th 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 20th 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 difference what so ever.

**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.

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