# 2019 redefinition of the SI base units

**2019 Redefinition of the SI Base Units**

The **2019 redefinition of the SI base units** represents a significant and historic revision of the International System of Units (SI), which had been the cornerstone of international agreement on units of measurement since its inception. This redefinition was adopted by the 26th General Conference on Weights and Measures (CGPM) on May 20, 2019, and it marked a move towards a system entirely based on physical constants rather than physical artifacts. This change aimed to improve the SI in terms of its universality, stability, and accessibility.

## Background[edit | edit source]

The SI system is built on seven base units: the meter (m) for length, the kilogram (kg) for mass, the second (s) for time, the ampere (A) for electric current, the kelvin (K) for temperature, the mole (mol) for the amount of substance, and the candela (cd) for luminous intensity. Prior to 2019, the definitions of these units were based on a mix of constants and physical artifacts, such as the International Prototype of the Kilogram for mass.

## Rationale for Redefinition[edit | edit source]

The rationale behind the redefinition was twofold: to eliminate the reliance on physical artifacts, which can deteriorate or change over time, and to base the SI on the unchanging properties of nature. This ensures that the units of measurement are the same everywhere in the universe and remain stable over time.

## Changes in Definitions[edit | edit source]

The 2019 redefinition involved changes to the definitions of the kilogram, ampere, kelvin, and mole, with the definitions of the meter, second, and candela having already been based on physical constants. The new definitions use the Planck constant (h) for the kilogram, the elementary charge (e) for the ampere, the Boltzmann constant (k) for the kelvin, and the Avogadro constant (N_A) for the mole.

### Kilogram[edit | edit source]

The kilogram is now defined by fixing the numerical value of the Planck constant to exactly 6.62607015×10^−34 kg⋅m^2⋅s^−1. This definition replaces the International Prototype of the Kilogram, a platinum-iridium cylinder stored in France.

### Ampere[edit | edit source]

The ampere is defined by fixing the numerical value of the elementary charge to exactly 1.602176634×10^−19 C (coulombs), which directly ties the unit of electric current to a fundamental property of nature.

### Kelvin[edit | edit source]

The kelvin is defined by fixing the numerical value of the Boltzmann constant to exactly 1.380649×10^−23 J⋅K^−1 (joules per kelvin), linking temperature to energy.

### Mole[edit | edit source]

The mole is defined by fixing the numerical value of the Avogadro constant to exactly 6.02214076×10^23 mol^−1, defining the amount of substance directly in terms of the number of particles.

## Impact[edit | edit source]

The 2019 redefinition of the SI base units has had a profound impact on science, technology, and industry. It has provided a more stable and universally accessible foundation for precise measurements. This redefinition ensures that the SI system will continue to serve humanity by facilitating technological innovation, economic development, and the advancement of science.

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