Metrologists, who study the measurement of magnitudes, have proposed redefining the units of measurement of mass (kilogram), electric current (ampere), quantity of substance (mol) and temperature (kelvin) based on the fixed value of constants of nature, so that they can not vary more.

When the IPK was created, with the idea of homologating the weight of a liter of liquid water, international reference copies were also created, in theory identical. However, when trying to calibrate new weights, it has been observed that the masses of the different kilo patterns, including the original, vary among themselves by values of at least 50 micrograms (millionths of a gram). The material can absorb atoms from the environment, and can lose them with cleaning. In science, this discrepancy is “intolerable,” says Phillips, especially given that the kilogram is used to define three other basic units of the International System – the candle, the ampere and the mol- and 17 derived units, such as July and the newton.

William Phillips, Nobel Prize in Physics in 1997, says it is a scandal “that the unit of mass is still a physical object”.Phillips works at the National Institute of Standards and Technology (NIST), in the US, one of the metrology centers that participates in the revision of the International System of Units. The mission of NIST was to find a new definition of the kilo that is not only invariable, but also “democratic”, that is, that is within the reach of any laboratory that wants to calibrate a pattern. “To this day, the only way to know the true value of the kilo is to go to the International Bureau of Weights and Measures in France, which has only taken the IPK out of its protective hood a handful of times in two centuries.

Inspiration has finally come from the metro, another basic unit that in 1983 passed legally from being “the length of a platinum bar in Paris” to being “the distance traveled by light in 1 / 299,792,458 seconds”. This way of fixing the units is not intuitive, since it goes on to first define the exact value of a constant of nature, to which an arbitrary numerical value is imposed based on the characteristics of the physical object from which the science wants to be undone. With the meter, the scientists took the approved prototype – the platinum bar – to study its relationship with a natural constant: the speed of light in a vacuum. By knowing exactly what fraction of a second the light takes to travel the length of the bar, they officially set the speed of light at 299,792,458 meters per second. To immortalize the kilo it is also necessary to set the numerical value of a natural constant. Chemists have chosen the number of Avogadro – which relates the number of atoms or molecules to the mass of a sample – and physicists, the Planck constant – which relates the energy of a photon to the frequency of its wave. More than competing, the two methods are complementary, since the consensus has been to reach a level of precision that allows using fixed figures of both constants to obtain the same numerical value of the kilo. In addition, the Avogadro constant, which has been defined by measuring the amount of atoms in a perfect sphere of silicon, will also be used to redefine the mol.

Metrologists have developed methods to fix the Boltzmann constant, which will define the kelvin, and the elementary charge constant, which will give definition to the ampere. The other basic units – the second, the meter and the candle – are already defined by physical constants. In November of this year, the General Conference of Weights and Measures will meet in Versailles to vote on the proposed changes to the International System.

These new definitions, will come into force predictably next May 20, the anniversary of the Metro Treaty of 1875.