Scientific community closer to large-scale SI reform

At the end of June, experts from the National Institute of Standards and Technology (NIST, USA) published an article dedicated to the refinement of Planck’s constant. Despite the fact that improving the accuracy of measuring this value does not seem to be something extraordinary, in fact it is one of the last steps towards a planned redefinition of physical quantities. Perhaps due to this and other measurements, already next year, all seven basic values of the SI system will be officially redefined exclusively through fundamental physical constants and properties, and the standard of a kilogram after more than a century of service to science will turn into a museum exhibit.

The need for a general system of measures existed since ancient times, but it especially increased with the beginning of scientific and technical progress, because scientists had to have a common language of measurements in order to share the results of their research. Therefore, in 1795, the metric system was officially adopted in France, and the definitions of the basic quantities were fixed in state documents. In order to give it universality, all its values were tied to natural objects.

For example, the meter in it was described as one forty millionth part of the meridian. Accordingly, it was assumed that the value tied to the size of the Earth would be measurable anywhere on the planet. The same applied to mass: a gram was originally defined as the mass of a cubic centimeter of water at the melting point of ice.

But since the Earth is not perfectly spherical, and the water may contain impurities, the results of measurements of these quantities slightly differed among different researchers in different parts of the planet. Therefore, in the second half of the XIX century, it was decided to redefine the length and mass through the standards, the originals of which were stored at the headquarters of the Bureau of Weights and Measures in the suburbs of Paris, and their exact copies were provided to similar metrology organizations of countries that signed the 1875 metric convention.Standards kilogram consisted of an alloy of platinum and iridium in the ratio of 9 to 1. Scientists decided to make them in the form of a cylinder with equal height and diameter – just over 39 millimeters. Equal dimensions were chosen in order to reduce the surface area and, consequently, the wear of the standard.

## Why did you need a SI system

The metric system was based on mass and length (as well as area and volume derivatives from it) as basic units. But since at the end of the 19th and the beginning of the 20th centuries new scientific fields were rapidly developing, the accepted values began to be missed and scientists began to use modified versions of the metric system. For example, the GHS and MKSA systems are widely used.

In the mid-twentieth century, the development of a new universal system of measures that would meet modern requirements and realities of science began. So in 1960, the International System of Units SI (SI, *Le Système International d’Unités* ) was adopted . Initially, it included six variables that are considered basic: length, mass, time, electric current, thermodynamic temperature and luminous intensity. The amount of substance, measured in moles, was added to the SI in 1971. All other physical quantities are derived, that is, they can be mathematically derived through the basic ones.

Since the adoption of the SI system, some definitions of values have changed, for example, the meter was tied to the speed of light in a vacuum, and the second to the number of hyperfine transitions in the cesium-133 atom. Thus, six of the seven quantities were spared from physical standards and are derived through constant physical properties and phenomena, such as the speed of light or periodic changes in the energy structure of atoms.

Nevertheless, the current definition of a kilogram still reads: A *kilogram is a unit of mass equal to the mass of the international prototype of a kilogram* . Periodically, copies of the standard are checked against the original, and measurements show that the masses of the standards change. This is due to their interaction with the stand on which they are installed, dust that is deposited on them during calibration, and other phenomena. Moreover, it is possible to reliably say only that the masses of the standards gradually “run up”, but not about which of them “lost” or, on the contrary, increased the mass.

## Override values

In order to finally move from standards to physical phenomena, as well as to improve the definition of certain quantities, in 2011, at the General Conference on Measures and Weights, it was decided to redefine four basic quantities: kilogram (mass), amperes (current strength), kelvin (thermodynamic temperature) and mole (amount of substance). It is also planned to change the wording of the definitions of the three remaining quantities, leaving their essence unchanged.

New definitions will be expressed in terms of fundamental physical constants: kilogram in terms of Planck’s constant, amperes in terms of elementary charge, Kelvin in terms of Boltzmann’s constant, and mole in terms of Avogadro’s number. For this, it is necessary to fix these constants and assign values corresponding to the most accurate measurements to them.

In order for the kilogram redefinition to be officially adopted in 2018, several conditions must be met. First, measurements must be performed by at least three independent research groups, one of which must use measurements based on different physical principles from the other two groups. Secondly, the requirements for their accuracy are very high: the uncertainty of measurement of Planck’s constant should not exceed 50 × 10 ^{-9} , and at least for one research group – 20 × 10 ^{-9} .

## How to measure Planck’s constant

Scientists from the National Institute of Standards and Technology of the United States decided to use the Kibble scale to refine Planck’s constant. Their design was developed by Brian Kibble, a specialist in the National Physical Laboratory of Great Britain, back in 1975. In such scales, the pallet on which the standard is set is attached to a coil located between two strong permanent magnets. By applying current to a coil, scientists create a magnetic field that, at a certain current strength, balances the force of gravity on the load. Since the mass of the reference and the magnitude of the current applied to the coil are known, using the watt balance equation, we can calculate the Planck constant with high accuracy.

Recently, physicists have accumulated statistical data on such measurements, as well as slightly reworked the physical model, in particular, revised the effect of the magnetic field of the coil on the installation. On June 30, NIST specialists published an article in which they stated that they were able to measure Planck’s constant to an accuracy of 13 billionths (13 × 10 ^{-9} ), which satisfies the requirement of a resolution on redefinition of values.

The second approach was developed at the National Metrology Institute of Germany. They created almost perfectly smooth silicon spheres with a diameter of about 93.5 millimeters with a roughness not exceeding three tenths of a nanometer and deviations from a spherical shape to several tens of nanometers. It is so small that if such a sphere is scaled to the size of the Earth, deviations from a perfectly even shape will not exceed a few meters. The spheres are made of monocrystal silicon, with a single isotope – ^{28} SI. Silicon was chosen due to the fact that thanks to the advanced semiconductor industry, there are methods for obtaining silicon objects of almost perfect structure. Impurities in such a sphere are so small that its mass differs from the ideal by less than one ten-millionth gram.

Since the sphere can be considered almost perfect in terms of the crystal structure and composition, and its mass is equal to the mass of the standard kilogram, then, by accurately measuring its size, lattice period and packing density of atoms, scientists can find out the number of atoms in it. Based on this, you can get the Avogadro number, and then the Planck constant. At the moment, scientists were able to measure the Avogadro number with an uncertainty of 20 billionths.In addition to these two groups, there are others. For example, physicists from the National Research Institute of Canada also use the Kibble scale and, according to their data, achieved even greater accuracy – 9.1 billionths of a share.

Thus, the results claimed by various groups satisfy the requirements of the 2011 resolution and suggest that the transition from the standard to the new definition will nevertheless take place at the end of 2018.

## Other values

As already mentioned, in addition to the kilogram it is planned to change the definition of three more quantities.

One kelvin will be considered as such a change in temperature, which leads to a change in energy per one degree of freedom *kT* . Thus, the kelvin will be derived through the equality E = kT, where E is the energy, T is the temperature in Kelvin, and k is the Boltzmann constant. From this it follows that the Boltzmann constant must be fixed in accordance with the requirements of the resolution, as well as the Planck constant. This has already been done by several research groups from the USA, China and Germany.

A mole in the current SI system is considered to be such a number of atoms as contained in 0.012 kilograms of the carbon-12 isotope. As is known, the amount of a substance can be expressed as the ratio of the number of atoms to the Avogadro number. According to the new standards, the Avogadro number will be fixed in accordance with the most accurate measurements.

As for the ampere, its today’s definition can be considered one of the most voluminous among the seven basic quantities:

*Amp is a force of unchanging current, which, when passing along two parallel straight conductors of infinite length and a negligible small circular cross-sectional area located in vacuum at a distance of 1 meter one from the other, would cause an interaction force of 2 × 10 ^{−7} newtons.*

If a redefinition occurs, the ampere will be determined through the unit of elementary electric charge, which will also be fixed as a specific value.

Thus, if the data published by various scientific groups are recognized by the participants of the General Conference on Weights and Measures to be held in mid-November 2018, the new definitions are likely to come into effect already in 2019

## Why is it all

One of the obvious reasons for adopting a new SI system is increased requirements for reliability and security – because if a kilogram is determined through standards, then the accuracy of all measurements depends on their safety – after all, standards can be simply stolen or damaged. Fixing fundamental constants means that all scientists will use absolutely identical values. If in applied applications this is not so important, then in theoretical areas the unity of measurements is of great importance.

Most people will not notice any changes, and the old values and measurements will not lose their power. The most notable consequence of this reform will be that the standard of the kilogram, which is likely to remain in the Bureau of Measures and Weights, will in fact turn into a museum piece.