Universal and unchanging physical quantity

Physical constant

Physical constant

A physical constant, sometimes fundamental physical constant or universal constant, is a <a href="/en/Physical_quantity" title="Physical quantity" class="wl">physical quantity</a> that is generally believed to be both universal in nature and have <a href="/en/Constant_(mathematics)" title="Constant (mathematics)" class="wl">constant</a> value in time. It is distinct from a <a href="/en/Mathematical_constant" title="Mathematical constant" class="wl">mathematical constant</a>, which has a fixed numerical value, but does not directly involve any physical measurement.

<div class="shortdescription nomobile noexcerpt noprint searchaux" style="display:none" about="#mwt1">Universal and unchanging physical quantity</div>
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There are many physical constants in science, some of the most widely recognized being the <a href="/en/Speed_of_light" title="Speed of light" class="wl">speed of light</a> in vacuum c, the <a href="/en/Gravitational_constant" title="Gravitational constant" class="wl">gravitational constant</a> G, the <a href="/en/Planck_constant" title="Planck constant" class="wl">Planck constant</a> h, the <a href="/en/Electric_constant" title="Electric constant" class="mw-redirect wl">electric constant</a> ε0, and the <a href="/en/Elementary_charge" title="Elementary charge" class="wl">elementary charge</a> e. Physical constants can take many <a href="/en/Dimensional_analysis" title="Dimensional analysis" class="wl">dimensional</a> forms: the speed of light signifies a maximum <a href="/en/Speed" title="Speed" class="wl">speed</a> for any object and its <a href="/en/Dimensional_analysis" title="Dimensional analysis" class="wl">dimension</a> is <a href="/en/Length" title="Length" class="wl">length</a> divided by <a href="/en/Time" title="Time" class="wl">time</a>; while the <a href="/en/Fine-structure_constant" title="Fine-structure constant" class="wl">fine-structure constant</a> α, which characterizes the strength of the <a href="/en/Electromagnetic_interaction" title="Electromagnetic interaction" class="mw-redirect wl">electromagnetic interaction</a>, is <a href="/en/Dimensionless" title="Dimensionless" class="mw-redirect wl">dimensionless</a>.

The term fundamental physical constant is sometimes used to refer to universal-but-dimensioned physical constants such as those mentioned above.<a href="#cite_note-1" style="counter-reset: mw-Ref 1;">[1]</a> Increasingly, however, physicists only use fundamental physical constant for <a href="/en/Dimensionless_physical_constant" title="Dimensionless physical constant" class="wl">dimensionless physical constants</a>, 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 "<a href="/en/Time_constant" title="Time constant" class="wl">time constant</a>" characteristic of a given system, or <a href="/en/List_of_materials_properties" title="List of materials properties" class="wl">material constants</a> (e.g., <a href="/en/Madelung_constant" title="Madelung constant" class="wl">Madelung constant</a>, <a href="/en/Electrical_resistivity_and_conductivity" title="Electrical resistivity and conductivity" class="wl">electrical resistivity</a>, and <a href="/en/Heat_capacity" title="Heat capacity" class="wl">heat capacity</a>).

Since May 2019, all of the <a href="/en/SI_base_unit" title="SI base unit" class="wl">SI base units</a> have been defined in terms of physical constants. As a result, five constants: <a href="/en/The_speed_of_light" title="The speed of light" class="mw-redirect wl">the speed of light</a> in vacuum, c; the <a href="/en/Planck_constant" title="Planck constant" class="wl">Planck constant</a>, h; the <a href="/en/Elementary_charge" title="Elementary charge" class="wl">elementary charge</a>, e; the <a href="/en/Avogadro_constant" title="Avogadro constant" class="wl">Avogadro constant</a>, NA; and the <a href="/en/Boltzmann_constant" title="Boltzmann constant" class="wl">Boltzmann constant</a>, 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.

A physical constant, sometimes fundamental physical constant or universal constant, is a physical quantity that is generally believed to be both universal in nature and have constant value in time. It is distinct from a mathematical constant, which has a fixed numerical value, but does not directly involve any physical measurement. 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. Increasingly, however, physicists only use fundamental physical constant for dimensionless physical constants, such as the fine-structure constant α. 