With our growing understanding of fundamental constants, the system of defining physical measures commonly used for international trade, business, and science is set to be revamped.

The metric system was developed during the French Revolution and is the beginning of the current International System of Units (SI, Système International d'Unités in French). The current SI was officially created in 1960.

The new system of weights and measures followed a suggestion made a century ago by John Wilkins, using the meter as the basic unit of measure to determine length, volume, and mass.

The meter is inspired by a natural constant thought to exist: one ten-millionth of the meridian that runs through Paris from the North Pole to the Equator.

After defining the units of volume and mass, the kilogram was defined as the mass of one liter of distilled water at 4 °C. The two platinum work standards for length and mass based on these definitions were later deposited in the Paris Archives de la République in 1799. In the words of the Marquis de Condorcet, a brand new system of measurement was created “for all times and for all peoples”.

Three international organizations, the General Conference on Weights and Measures (CGPM), the International Committee on Weights and Measures (CIPM), and the International Bureau of Weights and Measures were established 76 years after the signing of the Meter Convention (BIPM). They are technically tasked with protecting the SI and still continue to do so.

The International System of Units (SI) is a vibrant, dynamic system that adapts to new information and measurement needs, but is at times slow compared to the tremendous pace of scientific progress. For example, in the 18th and 19th centuries, natural philosophers and scientists discovered that they needed new units of measurement when they tried to use the system of length, mass, and time (time defined by astronomical observations) to measure newly discovered phenomena such as magnetism and electricity, and the concept of energy.

Pioneers of new science, such as Carl Friedrich Gauss, Wilhelm Weber, James Clerk Maxwell, and Lord Kelvin, contributed to the expansion of the system and helped establish the theoretical basis for a coherent system with basic mechanical units from which derived units could be derived if needed.

The coherent derived units were products of the base units' powers of 1 with a pre-factor, and the system provided detailed explanations of how to instantiate the base units through measurement.

The chronology in Figure 1 shows that, despite many revisions, the SI still includes this core set of units, including 7 base units (and associated definitions for their implementation) plus 22 derived units with unique names and symbols. However, there is growing international consensus to develop the SI once again to reflect the modern understanding of the physical world. Rather than define seven base units and consistently derived units, the proposed framework for future SI will assume exact values for the seven fundamental constants of nature against which all SI units will be realized. The basic units and their meanings are no longer used.

## Creating a System of Units

All physical quantities and the equations connecting these quantities, i.e. accepted principles of physics, must be taken into account when developing a unit system to express all physical measurements.

This is a simple example of a relationship; where F force, m mass, acceleration a, velocity v, length x and time t are all quantities and the relationships are Newton's second law of motion and fundamental dynamics.

By carefully choosing a subset of the independent fundamental quantities, accepted principles of physics can be used to infer other values as functions of the selected subset.

While the essential quantities can be chosen in a variety of ways, they should all be comprehensive and not superfluous. For example, if all we know about the physical world were equation 1 (six quantities, three constraints), choosing force or mass and any two of the other five quantities would result in a separate set of three fundamental quantities.

But we're not done yet. To fully describe the system of units, we need to give each base quantity a unique reference quantity.

The basic amount of mass in the current SI – the international prototype of the kilogram – is an example of such unique artifacts that can serve as a reference quantity (IPK). alternatively, in energy equivalence relations

Planck's constant h, speed of light c, elementary particle charge e, and Boltzmann's constant k can also be used as references, as they are invariants with known values.

Time, length, mass, electric current, thermodynamic temperature, amount of matter, and light intensity are the seven fundamental quantities that make up the current SI. These definitions serve as special reference quantities. In other words, the definitions of base units – seconds, meters, kilograms, ampere, kelvin, mole and candela – are reference quantities in the current SI.

Source: physicstoday – **David Newwell**

David Newell is chair of the CODATA Fundamental Constants Task Group and a physicist at the National Institute of Standards and Technology in Gaithersburg, Maryland.

Günceleme: 27/10/2022 18:44

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