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Units of measurement based on universal physical constants
In physics, natural units are physical units of measurement based only on universal physical constants. For example, the elementary chargee is a natural unit of electric charge, and the speed of lightc 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 1. 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 e and c, 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 e, c, etc., can be uniquely determined.
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: c and G 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 G, h, c, and kB 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, h.
The Hartree atomic unit system uses the following constants to have numeric value 1 in terms of the resulting units:
e, me, ?, ke.
Coulomb's constant, ke, is generally expressed as 1/4??0 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) a0 = 1 lA, orbital velocity = 1 lA?tA-1, angular momentum = 1 mA?lA?tA-1, ionization energy = 1/2mA?lA2?tA-2, 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 (c = 1/?lA?tA-1 ? 137 lA?tA-1) since an electron in hydrogen tends to move much slower than the speed of light. The gravitational constant is extremely small in atomic units (G ? 10-45mA-1?lA3?tA-2), 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 e2/2, 2me, ?, ke are used as the normalized constants, with resulting units
lR = a0 = (4??0)?2/mee2,
tR = 2(4??0)2?3/mee4,
mR = 2me,
qR = e⁄.
The vacuum permittivity ?0 is implicitly normalized, as is evident from the physicists' expression for the fine-structure constant, written ? = e2/(4?), which may be compared to the same expression in SI: ? = e2/(40?c).
c = mp = ? = 1; if rationalized, then is 1, if not, is 1 (in the original QCD units, e 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".
c = G = 1
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.
^However, if it is assumed that at the time the Gaussian definition of electric charge was used and hence not regarded as an independent quantity, the Coulomb constantke = 1/4??0 would be implicitly added to the list of defining constants, this would yield a charge unit .