The year 2018 can be called crucial in metrology, because this is the time of a real technological revolution in the international system of units of physical quantities of SI. It is a revision of the definitions of the main physical quantities. Will a kilogram of potatoes in a supermarket now weigh in a new way? With potatoes will continue. Another thing will change.
What came before the SI system
Common standards in measures and weights were needed in ancient times. But the general measurement rules became especially necessary with the advent of scientific and technological progress. Scientists needed to speak a common language: one foot - how many centimeters? And what is a centimeter in France when it does not coincide with Italian?
France can be called an honorary veteran and winner of historical metrological battles. It was in France in 1791 that the system of measurements and their units was officially approved, and the definitions of the main physical quantities were described and endorsed as state documents.
The French were the first to realize that physical quantities should be tied to natural objects. For example, one meter was described as 1/40000000 of the length of the meridian from north to south to the equator. He was thus attached to the size of the earth.
One gram was also tied to natural phenomena: it was defined as the mass of water in a cubic centimeter at a temperature close to zero (melting ice).
But, as it turned out, the Earth is not at all an ideal ball, and the water in a cube can have very different properties, if there are impurities in it. Therefore, the sizes of these quantities at different points on the planet were slightly different from each other.
At the beginning of the 19th century, the Germans entered the business, headed by the mathematician Karl Gauss. He proposed updating the centimeter-gram-second measure system, and since then metric units have gone into the world, science and have been recognized by the international community, an international system of units of physical quantities has been formed.
They decided to replace the length of the meridian and the mass of the cube of water with the standards stored in the Bureau of Weights and Measures in Paris, with the distribution of copies to countries participating in the metric convention.
A kilogram, for example, looked like a cylinder made of an alloy of platinum and iridium, which in the end also did not become an ideal solution.
The international system of units of physical quantities SI was established in 1960. At first, it included six basic quantities: meters and length, kilograms and mass, time in seconds, current in amperes, thermodynamic temperature in kelvins and light intensity in candelas. Ten years later, another one was added to them - the amount of matter, measured in moles.
It is important to know that all other units of measurement of the physical quantities of the international system are considered to be derived from the basic ones, that is, they can be mathematically calculated using the basic quantities of the SI system.
Away from the standards
Physical standards were not the most reliable measurement system. The kilogram standard itself and its copies by country are periodically checked against each other. Reconciliations show changes in the masses of these standards, which occurs for various reasons: dust during calibration, interaction with the stand, or something else. Scientists have noticed these unpleasant nuances for a long time. The time has come to revise the parameters of units of physical quantities of the international system in metrology.
Therefore, some definitions of quantities gradually changed: scientists tried to get away from physical standards, which somehow changed their parameters over time. The best way is to derive quantities through constant properties, such as the speed of light or changes in the structure of atoms.
On the eve of the revolution in the SI system
Fundamental technological changes in the international system of units of physical quantities are carried out through a vote of members of the International Bureau of Weights and Measures at the annual conference. If the decision is positive, the changes take effect in a few months.
All this is extremely important for scientists in whose research and experiments the extreme accuracy of measurements and formulations is needed.
The new standards of the sample in 2018 will help to achieve the highest level of accuracy in any measurement, anywhere, time and scale. And all this without any loss in accuracy.
Redefinition of values in the SI system
It concerns four of the seven existing basic physical quantities. It was decided to redefine the following quantities with units:
- kilogram (mass) using the Planck constant in terms of units;
- Ampere (current) with measurement of charge;
- kelvin (thermodynamic temperature) with an expression of unity using the Boltzmann constant;
- mole through Avogadro constant (amount of substance).
In relation to the remaining three quantities, the definition wording will be changed, but their essence will remain unchanged:
- meter (length);
- second (time);
- candela (luminous intensity).
Amp changes
What amperes represent as a unit of physical quantities in the international SI system today was proposed back in 1946. The definition was tied to the strength of the current between two conductors in a vacuum at a distance of one meter with a clarification of all the nuances of this structure. The inaccuracy and cumbersomeness of measurement are the two main characteristics of this definition from today's point of view.
In the new definition, an ampere is an electric current equal to the flow of a fixed number of electric charges per second. The unit is expressed in electron charges.
To determine the updated ampere, you need only one tool - the so-called single-electron pump, which is able to move electrons.
New mole and 99.9998% pure silicon
The old definition of mole is associated with the amount of substance equal to the number of atoms in a carbon isotope with a mass of 0.012 kg.
In the new version, this is the amount of substance that is contained in a precisely defined number of specified structural units. These units are expressed using the Avogadro constant.
With the number of Avogadro, too, a lot of worries. To calculate it, it was decided to create a sphere of silicon-28. This silicon isotope is distinguished by its crystal lattice, which is ideally accurate. Therefore, it is possible to accurately calculate the number of atoms in it using a laser system that measures the diameter of a sphere.
One can, of course, argue that there is no fundamental difference between the silicon-28 sphere and the current alloy of platinum and iridium. Both that and another substance loses atoms in time. Loses, right. But silicon-28 loses them at a predictable speed, so adjustments will be constantly made to the standard.
The purest silicon-28 for the sphere was received recently in the USA. Its purity is 99.9998%.
Now kelvin
Kelvin is one of the units of physical quantities in the international system and is used to measure the level of thermodynamic temperature. “In the old way” it is equal to 1/273.16 of the temperature of the triple point of water. The triple point of water is an extremely interesting component. This is the level of temperature and pressure at which water is immediately in three states - “steam, ice and water”.
The definition of "limped on both legs" for the following reason: the value of kelvin depends primarily on the composition of water with a theoretically known ratio of isotopes. But in practice, it was impossible to obtain water with such characteristics.
The new kelvin will be defined as follows: one kelvin is equal to the change in thermal energy by 1.4 × 10 −23 j. Units are expressed using the Boltzmann constant. Now the temperature level can be measured by fixing the speed of sound in the gas sphere.
A kilogram without a standard
We already know that in Paris there is a standard made of platinum with iridium, which in one way or another has changed its weight during its use in metrology and the system of units of physical quantities.
The new definition of kilogram is as follows: one kilogram is expressed in terms of the Planck constant divided by 6.63 × 10 −34 m 2 · s −1 .
Mass measurement can now be done on "watt" scales. Let this name not mislead you, it’s not the usual scales, but the energy that is enough to lift an object lying on the other side of the scales.
Changes in the principles of constructing units of physical quantities and their system as a whole are needed, first of all, in the theoretical fields of science. The main factors in the updated system are now natural constants.
This is the logical conclusion to the many years of activity of an international group of serious scientists whose efforts have long been aimed at finding ideal measurements and definitions of units based on the laws of fundamental physics.