Editing Physical constant (section)

== Relationship to units ==
=== Numerical values ===
Whereas the [[physical quantity]] indicated by a physical constant does not depend on the unit system used to express the quantity, the numerical values of dimensional physical constants do depend on choice of unit system. The term "physical constant" refers to the physical quantity, and not to the numerical value within any given system of units. For example, the speed of light is defined as having the numerical value of {{val|299792458}} when expressed in the [[SI unit]] metres per second, and as having the numerical value of 1 when expressed in the [[natural units]] [[Planck units|Planck length]] per Planck time. While its numerical value can be defined at will by the choice of units, the speed of light itself is a single physical constant.
[[File:SI Illustration Base Units and Constants Colour Full.svg|thumb|Illustration of the SI system of units, with base units and defining constants used to define them: '''s''' – the [[Hyperfine structure#Use in defining the SI second and meter|hyperfine structure transition frequency]] in the caesium atom for the '''[[second]]''', '''kg''' – Planck's constant for the '''[[kilogram]]''', '''mol''' – the [[Avogadro constant]] for the '''[[Mole (unit)|mole]]''', '''cd''' – the [[luminous efficacy]] of monochromatic radiation of frequency {{val|540|u=THz}} for the '''[[candela]]''', '''K''' – the [[Boltzmann constant]] for the '''[[kelvin]]''', '''A''' – [[elementary charge]] for the '''[[ampere]]''', '''m''' – the [[speed of light]] for the '''[[metre]]'''.]]

=== International System of Units ===
{{Main | SI base unit}}

Since [[2019 revision of the SI|2019 revision]], all of the units in the [[International System of Units]] have been defined in terms of fixed natural phenomena, including three fundamental constants: the speed of light in vacuum, ''c''; the Planck constant, ''h''; and the [[elementary charge]], ''e''.<ref name="SI9th">{{SIbrochure9th}}.</ref>{{rp|128}}

As a result of the new definitions, an SI unit like the [[kilogram]] can be written in terms of fundamental constants and one experimentally measured constant, Δ''ν''<sub>Cs</sub>:<ref name=SI9th/>{{rp|131}}
: 1&nbsp;kg = {{math|{{sfrac|({{val|299792458}}){{sup|2}}|({{val|6.62607015|e=-34}})({{val|9192631770}})}}{{sfrac|{{gaps|''h''|Δ''ν''<sub>Cs</sub>}}|''c''{{sup|2}}}}}}.

=== Natural units ===
{{main article |Natural units}}
It is possible to combine dimensional universal physical constants to define fixed quantities of any desired dimension, and this property has been used to construct various systems of natural units of measurement. Depending on the choice and arrangement of constants used, the resulting natural units may be convenient to an area of study. For example, Planck units, constructed from [[speed of light|''c'']], [[gravitational constant|''G'']], [[reduced Planck constant|''ħ'']], and [[Boltzmann constant|''k''<sub>B</sub>]] give conveniently sized measurement units for use in studies of [[quantum gravity]], and [[atomic units]], constructed from [[reduced Planck constant|''ħ'']], [[electron rest mass|''m''<sub>e</sub>]], [[elementary charge|''e'']] and 4''π''[[vacuum permittivity|''ε''<sub>0</sub>]] give convenient units in [[atomic physics]]. The choice of constants used leads to widely varying quantities.