The biggest revamp of scientific units since the French Revolution offers "the best experience in metrology"

At the biggest overhaul of the international system of units since 1875, countries voted to redefine four basic measurement units: the ampere, the kilogram, the kelvin and the mole.

At a meeting on November 16, 60 delegates from governments around the world voted unanimously to change the definition of these units – which will come into effect on May 20, 2019.

The vote at the Intergovernmental General Conference on Weights and Measures in Versailles, France, was greeted with ovation and champagne.

"It's big," says Zeina Kubarych, a metrologist at the US National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland. "It's the best experience in metrology."

In the new International System of Units (called simply SI), all measurements will be described using fundamental constants of nature and will be derived from experiments, separating the last links between the SI. and physical objects or arbitrary references.

The movement allows units to be generated from their definition anywhere in the world and to remain immutably stable. The vote is the culmination of several decades of work and a triumph for metrologists. This means that the old palm-sized metal cylinder, The Big K, which resides in a vault in Sèvres, near Paris and which sets the kilogram since 1889, will lose its special status.

"Since probably Mesopotamia, 5,000 years ago, each weighing was, in principle, a mbad of one side and a local standard weight of the other," said Terry Quinn, former head of the International Bureau of Weights and Measures (BIPM). "From May 20, this will no longer be the case. I think it's extraordinary.

But these important changes do not mean that the expanded IS system is now perfect, says David Newell, a physicist at NIST. Metrologists will then work to improve the definition of the second and possibly add more units to the system. "It's like the end of the Harry Potter series. Good triumphs, but everything is destroyed, "says Newell. In the SI, "there is still a lot of damage that still needs to be cleaned up," he adds.

Use constants

Measurements must always be performed against a benchmark, and standard references are used to ensure that units are comparable and consistent worldwide – from measuring in milligrams of drugs to the timing of global positioning systems. For the mbad, this means that each measure must be able to be traced up to the kilogram in France.

The idea of ​​basing all units on the constants of nature – immutable and not belonging to any country – exists since the late 19th century. But it took scientists nearly 150 years to measure values ​​with enough precision to do so.

Metrologists working on electricity have tweaked experiments that count the flow of individual electrons, allowing them to use the charge of a single particle to determine the ampere, thus replacing a definition based on a hypothetical experiment involving two infinitely long threads. This quantum redefinition aligns the SI with the electrical community, which has been using such standards since 1990 because of their greater accuracy.

The Kelvin will soon be defined by the Boltzmann constant, which connects energy and temperature, rather than referring to conditions at a specific water temperature, called triple point.

Meanwhile, the mole – a measure of the substance that has long been equivalent to the number of atoms in 0.012 kg of carbon-12 – will soon be equal to Avogadro's number.

Heavy material

In the case of the kilogram, the redefinition consisted of measuring the Planck constant, a number that defines the size of energy packets at the quantum scale, with exquisite precision. One of the methods, known as the kibble balance, calculates the Planck constant by comparing a known mbad with an electromagnetic force (see "Kilogram: the kibble balance"). Another counts the atoms in two spheres of silicon-28 to get a value for the Avogadro number, which is converted to Planck's constant.

The teams applying the two different methods only reached precise values ​​and were close enough in 2015.

"The fact that they accept a few parts out of 10 million is absolutely extraordinary, because these are definitions based on completely different areas of physics," says Quinn.

The constants will be set to their agreed values ​​- like the speed of light when it became the basis of the meter definition in 1983. To derive the units, metrologists will reverse the previously used experiments to get the constants – for example, in using a fixed value of the Planck constant in a hook scale to determine the electromagnetic force and weigh it against an unknown mbad.

This value was set not only in the SI system, but also physically on NIST metrologists, a number of those who have recently tattooed Planck's constant on their forearms.

As physical artifacts may be lost or damaged, the change makes the mbad definition more reliable. Although, by definition, The Big K always weighed exactly 1 kilogram, its mbad has changed slightly compared to the copy (see "The unstable kilogram"). It is impossible to say if Big K loses or gains atoms, but future studies should reveal it.

The system has the advantage of being able to use any experience – once international comparisons have proven it – can be used to determine the unit, says Estefanía de Mirandés, a metrologist at BIPM. This makes not only the system more democratic, but also the definition of definitions for the future, so that they can be used in the context of more recent and accurate experiments in the future. she said, potentially liberating new technologies.

Already, this makes it possible to measure very large and very small mbades with a precision far superior to that of today, she says.

The new kilogram

Mbad metrologists can not penetrate directly into this new world. Although the agreement between the experiments was sufficient for the redefinition, it is not perfect. Until slight disagreements between the best mbad measures in the world are resolved, the BIPM will act as an arbitrator. He will ask each group to measure the same objects and to propose average mbad values ​​that the world can compare to. Settlement of disputes could take 10 years, says de Mirandés.

Even at this stage, the precious physical weights held by national measurement laboratories around the world will not be thrown away. If few laboratories have the necessary equipment to make a kilogram from scratch, the weights play the role of transferring mbad standards from national laboratories such as NIST to industries that need it, such as 39, aerospace and biotechnology. "Babies are always my babies and no one touches them," says Kubarych, who oversees the copies in the United States.

NIST and the UK and German national laboratories are all working on cheaper Kibble table scales, so that smaller companies and laboratories can one day be able to make mbades themselves.

Just a second

The redefinition has resorted to metrologists since at least 2005. The next objective is the second, currently described in relation to the frequency of the microwave light absorbed and emitted by the cesium-133 atoms. These atoms are now surpbaded by the "optical clocks" Which use different atoms that interact with higher frequency visible light and seem to be able to conserve time with fewer errors: just one second above the age of the Universe.

To update the definition of the second in 2026, as many metrologists want, the community will have to develop methods to compare optical clocks around the world and decide which atom (s) to use as a standard. "It will be a political football," said Newell.

Another problem that metrologists might try to solve is to find a gentler way to include dimensionless quantities – such as the radian, the ratio of the length of an arc to its radius – in the SI system. "In some communities, there is enormous pressure to do that," says de Mirandés.

or the BIPM, founded in 1875 to house the standards of the kilogram and the physical meter, the IS revolution is bitter-sweet. At the meeting, the speakers happily joked that it was no longer necessary to go to Paris. The BIPM now hopes to play a role in making comparisons between achievements around the world, to ensure their accuracy, said de Mirandés. "It's the end of a period, but also the beginning of a new one."