**Measurement**, the simple process of comparing an unknown quantity with a standard quantity, is the essential component of all scientific endeavours. We are currently about to enter a new epoch of metrology, one which will permit the breath-taking progress of the last hundred years to continue unimpeded into the next century and beyond.

**The dawn of this new age** has been heralded this week by the publication of an apparently innocuous paper in the journal *Metrologia*. The paper is entitled:

*Data and Analysis for the CODATA 2017 Special Fundamental Constants Adjustment*

and its authors, Peter Mohr, David Newell, Barry Taylor and Eite Tiesinga constitute the *Committee on Data for Science and Technology*, commonly referred to as CODATA. In this article I will try to explain the relevance of CODATA’s paper to developments in the science of metrology.

**The Past**

**The way human beings began** to make sense of their environment was by measuring it. We can imagine that our agrarian ancestors might have wondered whether crops were taller or heavier this year than last. Or whether plants grew better in one field rather than another. And they would have answered these questions by creating standard weights and measuring rods.

**But to effectively communicate** their findings, the standard units of measurement would need to be shared. First between villages, and then towns, and then counties and kingdoms. Eventually entire empires would share a system of measurement.

**First units of weight and length were shared**. Then, as time became more critical for scientific and technical endeavours, units of time were added to systems of the measurement. And these three quantities: mass, length and time, are shared by all systems of units.

**These quantities formed the so-called ‘base units’** of a system of measurement. Many other quantities could be described in terms of these ‘base units’. For example, speeds would be described in multiples of [*the base unit of length*] divided by [*the base unit of time*]. They might be [*feet*] per [*second*] in one system, or [*metres*] per [*second*] in another.

**Over the last few hundred years**, the consistent improvement in measurement techniques has enabled measurements with reduced uncertainty. And since no measurement can ever have a lower uncertainty that the standard quantity in that system of units, there has been a persistent drive to have the most stable, most accurately-known standards, so that they do not form a barrier to improved measurements.

**The Present**

**Presently, all scientific and technical measurements** on Earth are made using the *International System of Units*, the SI. The naming of this system – as an explicitly international system – represented a profound change in conception. It is not an ‘imperial’ system or an ‘English’ system, but a shared enterprise administered by the *International Bureau of Weights and Measures* (BIPM), a laboratory located in diplomatically-protected land in Sèvres, near Paris, France. Its operation is internationally funded by the dozens of nations who have signed the international treaty known as the *Convention of the Metre*.

**In essence, the SI is humanity’s standard way** of giving quantitative descriptions of the world around us. It is really an annex to all human languages, allowing all nationalities and cultures to communicate unambiguously in the realms of science and engineering.

**Founded in 1960,** the SI was based upon the system of measurement using the metre as the unit of length, the kilogram as the unit of mass, and the second as the unit of time. It also included three more base units.

**The kelvin and degree Celsius** were adopted as units of temperature, and the ampere was adopted as the unit of electric current. The candela was defined as the unit of luminous efficacy – or how bright lights of different colours appear to human beings. And then in 1971 the often qualitative science of chemistry was included in the fold with the introduction of the mole as a unit of amount of substance, a recognition of the increasing importance of analytical measurements.

**The SI is administered by committees of international experts** that seek to make sure that the system evolves to meet humanity’s changing needs. Typically these changes are minor and technical, but in 1984 an important conceptual change was made.

**Since the foundation of the SI**, the ability to measure time had improved more rapidly than the ability to measure length. It was realised that if the metre was defined differently, then length measurements could be improved.

**The change proposed** was to define what we mean by ‘one metre’ in terms of the distance travelled by light, in a vacuum, in a fixed time. Based on Einstein’s insights, the speed of light in a vacuum, *c*, is thought to be a universal constant, but at the time it had to be measured in terms metres and seconds i.e. human-scale measurement standards. This proposal defined a metre in terms of a natural constant – something we believe is truly constant.

**The re-definition went well**, and set metrologists thinking about whether the change could be adopted more widely.

**The Future**

**Typically every four years**, CODATA examine the latest measurements of natural constants, and propose the latest best estimate of the values of a range of natural constants.

**This is a strange**. We believe that the natural constants are *really* constant, not having changed measurably since the first few seconds of our universe’s existence. Whereas our human standards are at most a few decades old, and (as with all human standards) are subject to slow changes. Surely, it would make more sense, to base our measurement standards on these fundamental constants of the natural world? This insight is at the heart of the changes which are about to take place. The CODATA publication this week is the latest in a series of planned steps that will bring about this change on 20^{th} May 2019.

**After years of work by hundreds of scientists**, the values of the natural constants recommended by the CODATA committee will be fixed – and will form the basis for the new definitions of the seven SI base units.

**What will happen on 20th May 2019?**

**On the 20 ^{th} May 2019**, revised definitions of four of the base units of the SI will come into force. More than 10 years of careful measurements by scientists world-wide will ensure that the new definitions are, as closely as possible, equivalent to the old definitions.

**The change is equivalent to removing the foundations** underneath a structure and then inserting new foundations which should leave the structure supported in exactly the same way. However the new foundations – being based on natural constants rather than human artefacts – should be much more stable than the old foundations.

**If the past is any guide to the future**, then in the coming decades and centuries, we can anticipate that measurement technology will improve dramatically. However we cannot anticipate exactly how and where these improvements will take place. By building the SI on foundations based on the natural constants, we are ensuring that the definitions of the unit quantities of the SI will place no restriction whatever on these future possible improvements.

**The kilogram**

**The kilogram is the SI unit of mass**. It is currently defined as the mass of the *International Prototype of the Kilogram* (IPK), a cylinder of platinum-iridium alloy held in a safe at the BIPM. Almost every weighing in the world is, indirectly, a comparison against the mass of this artefact.

**On 20 ^{th} May 2019, this will change.** Instead, the kilogram will be defined in terms of a combination of fundamental constants including the Planck constant,

*h*, and the speed of light,

*c*. Although more abstract than the current definition, the new definition is thought to be at least one million times more stable.

**The new definition will enable** a new kind of weighing technology called a Kibble balance. Instead of balancing the weight of a mass against another object whose mass is known by comparison with the IPK, the weight will be balanced by a force which is calculable in terms of electrical power, and which can be expressed as a multiple of the fundamental constants *e*, *h* and *c*.

**The ampere**

**The ampere is the SI unit of electrical current**. It is presently defined in terms of the current which, if it flowed in two infinitely thin, infinitely long, parallel wires would (in vacuum) produce a specified force between the wires. This definition, arcane even by metrologists’ standards, was intended to allow the measurement of the ampere in terms of the force between carefully constructed coils of wire. Sadly, it was out of date shortly after it was implemented.

**On 20 ^{th} May 2019, this will change**. Instead, the ampere will be defined in terms of a particular number of electrons per second, each with an exactly specified electrical charge

*e*, flowing past a point on a wire. This definition finally corresponds to the way electric current is described in textbooks.

**The new definition will give impetus** to techniques which create known electrical currents by using electrical devices which can output an exactly countable number of electrons per second. At the moment these devices are limited to approximately 1 billion (a thousand million) electrons per second, but in future this is likely to increase substantially.

**The kelvin**

**The kelvin is the SI unit of temperature**. It is currently defined as the temperature of the ‘triple point of water’. This temperature – at which liquid water, solid water (ice) and water vapour (but no air) co-exist in equilibrium – is defined to be 273.16 kelvin *exactly*. Glass cells re-creating this conjunction are located in every temperature calibration lab in the world, and every temperature measurement is a comparison of how much hotter a temperature is than the temperature at one position within a ‘triple point of water cell’.

**On 20 ^{th} May 2019, this will change**. Instead, the kelvin will be defined in terms of a particular amount of energy per molecule as specified by the Boltzmann constant,

*k*

_{B}. This definition finally corresponds to the way thermal energy is described in textbooks.

**The requirement to compare** every measurement of temperature with the temperature of the triple point of water adds uncertainty to measurements at extremely low temperatures (below about 20 K) and at high temperatures (above about 1300 K). The new definition will immediately allow small improvements in these measurement ranges, and further improvements are expected to follow.

**The definition of the degree Celsius** (°C) in terms the kelvin will remain unchanged.

**The mole**

**The mole is the SI unit of ‘amount of substance’.** It is currently defined as the amount of substance which contains the same number of ‘elementary entities’ as there are atoms in 12 grams of carbon-12. The change in the definition of the kilogram required a re-think of this definition.

**On 20 ^{th} May 2019, it will change**. The mole will be defined as the amount of substance which contains a particular, exactly specified, number of elementary entities. This number – known as the Avogadro number,

*N*

_{A}– is currently estimated experimentally, but in future it will have fixed value.

**The specification of an exact number of entities** effectively links the masses of microscopic entities such as atoms and molecules to the new definition of the kilogram.

**The ‘New’ SI**

**On 20 ^{th} May 2019 four of the seven base units** of the SI will be re-defined. But what of the other three?

**The second is already defined** in terms of the natural frequency of microwaves emitted by atoms of a particular caesium isotope. The metre is defined in terms of the second and the speed of light in vacuum – a natural constant. And the candela is defined in terms of *K*_{cd}, the only natural constant in the SI that relates to human beings. So from 20^{th} May 2019 the entire SI will be defined in terms of natural constants.

**The SI is not perfect.** And it will not be perfect even after the redefinitions come into force. This is because it is a system devised by human beings, for human beings. But by incorporating natural constants into the definitions of all its base units, the SI has taken a profound step towards being a system of measurement which will enable ever greater precision in metrology.

**And who knows what features of the Universe** these improved measurements will reveal.