TFNR - Fundamental Constants of Nature

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The constants of Nature are invariant physical quantities, fundamental (cause-indipendence), universal in nature (space-indipendence) and constant in time (time-independence). They are used in the basic equations that describe the fundamental physical phenomena. They can be dimensionless, or have dimensions.

The constants that are considered fundamental are the following (the set of constants varies according to the theoretical reference framework considered):

• the gravitational constant G
• the velocity of light in vacuum or speed of light (c)

the Planck constant (h) or (h bar)

• the charge of the electron
• the absolute value of which is the fundamental unit of electric charge (e)
• the mass of the electron (me)
• the fine-structure constant, symbolized by the Greek letter alpha

• 9 Yukawa couplings for the quarks and leptons (equivalent to specifying the rest mass of these elementary particles),
• 2 parameters of the Higgs field potential,
• 4 parameters for the quark mixing matrix,
• 3 coupling constants for the gauge groups SU(3) × SU(2) × U(1) (or equivalently, two coupling constants and the Weinberg angle),
• a phase for the QCD vacuum.

There are many physical constants in science, some of the most widely recognized being the speed of light in vacuum c, the gravitational constant G, the Planck constant h, the electric constant ε0, and the elementary charge e. Physical constants can take many dimensional forms: the speed of light signifies a maximum speed for any object and its dimension is length divided by time; while the fine-structure constant α, which characterizes the strength of the electromagnetic interaction, is dimensionless.

The term fundamental physical constant is sometimes used to refer to universal-but-dimensioned physical constants such as those mentioned above.[1] Increasingly, however, physicists only use fundamental physical constant for dimensionless physical constants, such as the fine-structure constant α.

Physical constant, as discussed here, should not be confused with other quantities called "constants", which are assumed to be constant in a given context without being fundamental, such as the "time constant" characteristic of a given system, or material constants (e.g., Madelung constant, electrical resistivity, and heat capacity).

Since May 2019, all of the SI base units have been defined in terms of physical constants. As a result, five constants: the speed of light in vacuum, c; the Planck constant, h; the elementary charge, e; the Avogadro constant, NA; and the Boltzmann constant, kB, have known exact numerical values when expressed in SI units. The first three of these constants are fundamental constants, whereas NA and kB are of a technical nature only: they do not describe any property of the universe, but instead only give a proportionality factor for defining the units used with large numbers of atomic-scale entities.