### Constantes fundamentales: la última frontera para el Sistema Internacional de Unidades

#### Abstract

The phrase ``one measure for all men and all times'' summarizes the ideal that would lead to the development of the International System of Units, SI. The evolution of the definitions of the SI units of measurement may be somehow considered as a series of approximations to such an ideal in which the units of measurement are invariant in time and space, immutable, and susceptible to equivalent realizations, reproducible, and accessible to be useful in measuring processes of practical, industrial, technological and scientific purposes. In the current state of the science and technology the fundamental constants appear as the last frontier to define the SI units of measurement. The abandonment of artifacts to support the definitions of SI units began in 1960 when the wavelength of a krypton 86 radiation was used to redefine the unit of length, the meter. Moreover, the time unit, defined in 1967 in terms of the separation of the ground state hyperfine levels of the Cesium-133 atom, was another important step towards the incorporation of the fundamental constants in the SI units. In 1982, the unit of length was established in terms of the speed of light in vacuum and time measurements. This clearly illustrates how the SI evolves towards the incorporation of fundamental constants, or combinations of them, to support the units of measurement. In some way or another, all units of measurement have gone through a process of redefinition with the notable exception of the mass unit, the kilogram. Currently, among the SI seven base units, the kilogram is the only one based on the properties of a specific artifact, namely a cylinder of platinum. However, this may change in the near future by setting a value without uncertainty for Planck's constant and by defining the kilogram in terms of electromagnetic forces generated in a balance, usually called ``watt balance''. Thus, the definition of the kilogram would be framed in the context of the quantum mechanics, since it would rely experimentally on the time unit, and therefore on the hyperfine separation of the ground state of Cesium-133 atom, on the realizations of the volt in terms of the Josephson effect and on the ohm in terms of the quantum Hall effect. Definitions for the kelvin in terms of the Boltzmann constant, the mole in terms of Avogadro's number and the candela in terms of luminous efficacy of monochromatic radiation will allow the fundamental constants to be the support of the SI in order to provide measurement systems with a very high accuracy to pursuit the scientific and technological advances to come.

#### Keywords

Fundamental constants; International System of Units

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**Revista Mexicana de Física**

**ISSN: 2683-2224** (on line), **0035-001X** (print)

Bimonthly publication of Sociedad Mexicana de Física, A.C.

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