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In physics, natural units are physical units of measurement based only on universal physical constants. For example, the elementary charge is a natural unit of electric charge, and the speed of light is a natural unit of speed. A purely natural system of units has all of its units usually defined such that the numerical values of the selected physical constants in terms of these units are exactly . These constants may then be omitted from mathematical expressions of physical laws, and while this has the apparent advantage of simplicity, it may entail a loss of clarity due to the loss of information for dimensional analysis. It precludes the interpretation of an expression in terms of fundamental physical constants, such as and , unless it is ''known'' which units (in dimensionful units) the expression is supposed to have. In this case, the reinsertion of the correct powers of , , etc., can be uniquely determined.

Systems of natural units

Planck units

The Planck unit system uses the following constants to have numeric value 1 in terms of the resulting units: : where is the speed of light, is the reduced Planck constant, is the gravitational constant, and is the Boltzmann constant. Planck units are a system of natural units that is not defined in terms of properties of any prototype, physical object, or even elementary particle. They only refer to the basic structure of the laws of physics: and are part of the structure of spacetime in general relativity, and captures the relationship between energy and frequency which is at the foundation of quantum mechanics. This makes Planck units particularly useful and common in theories of quantum gravity, including string theory. Planck units may be considered "more natural" even than other natural unit systems discussed below, as Planck units are not based on any arbitrarily chosen prototype object or particle. For example, some other systems use the mass of an electron as a parameter to be normalized. But the electron is just one of 16 known massive elementary particles, all with different masses, and there is no compelling reason, within fundamental physics, to emphasize the electron mass over some other elementary particle's mass. Planck considered only the units based on the universal constants , , , and B to arrive at natural units for length, time, mass, and temperature, but no electromagnetic units. The Planck system of units is now understood to use the reduced Planck constant, , in place of the Planck constant, .Tomilin, K. A., 1999,
Natural Systems of Units: To the Centenary Anniversary of the Planck System
, 287–296.

Stoney units

The Stoney unit system uses the following constants to have numeric value 1 in terms of the resulting units: :, where is the speed of light, is the gravitational constant, is the Coulomb constant, and is the elementary charge. George Johnstone Stoney's unit system preceded that of Planck. He presented the idea in a lecture entitled "On the Physical Units of Nature" delivered to the British Association in 1874. Stoney units did not consider the Planck constant, which was discovered only after Stoney's proposal. Stoney units are rarely used in modern physics for calculations, but they are of historical interest.

Atomic units

The Hartree atomic unit system uses the following constants to have numeric value 1 in terms of the resulting units: : Coulomb's constant, , is generally expressed as when working with this system. These units are designed to simplify atomic and molecular physics and chemistry, especially the hydrogen atom, and are widely used in these fields. The Hartree units were first proposed by Douglas Hartree. The units are designed especially to characterize the behavior of an electron in the ground state of a hydrogen atom. For example, in Hartree atomic units, in the Bohr model of the hydrogen atom an electron in the ground state has orbital radius (the Bohr radius) 0 = 1 , orbital velocity = 1 ⋅, angular momentum = 1 ⋅⋅, ionization energy =  ⋅⋅, etc. The unit of energy is called the Hartree energy in the Hartree system. The speed of light is relatively large in Hartree atomic units ( =  ⋅ ≈ 137 ⋅) since an electron in hydrogen tends to move much slower than the speed of light. The gravitational constant is extremely small in atomic units ( ≈ 10−45 ⋅⋅), which is due to the gravitational force between two electrons being far weaker than the Coulomb force between them. A less commonly used closely related system is the system of Rydberg atomic units, in which are used as the normalized constants, with resulting units = = , = , = 2, = .

Natural units (particle and atomic physics)

The natural unit system, used only in the fields of particle and atomic physics, uses the following constants to have numeric value 1 in terms of the resulting units: :, where is the speed of light, e is the electron mass, is the reduced Planck constant, and 0 is the vacuum permittivity. The vacuum permittivity 0 is implicitly normalized, as is evident from the physicists' expression for the fine-structure constant, written , which may be compared to the same expression in SI: .

Quantum chromodynamics units

:; if rationalized, then $\epsilon_0$ is 1, if not, $4 \pi \epsilon_0$ is 1 (in the original QCD units, is 1 instead.) The electron rest mass is replaced with that of the proton. ''Strong units'', also called quantum chromodynamics (QCD) units, are "convenient for work in QCD and nuclear physics, where quantum mechanics and relativity are omnipresent and the proton is an object of central interest".

Geometrized units

: The geometrized unit system, used in general relativity, is an incompletely defined system. In this system, the base physical units are chosen so that the speed of light and the gravitational constant are set equal to unity. Other units may be treated however desired. Planck units and Stoney units are examples of geometrized unit systems.

Summary table

where: * is the fine-structure constant, ≈ 0.007297, *A dash (–) indicates where the system is not sufficient to express the quantity.

* Anthropic units * Dimensional analysis * Dimensionless physical constant * SI units * N-body units * Physical constant * Astronomical system of units * Planck units * Units of measurement

Notes and references