Editing Physical constant (section)

{{short description|Universal and unchanging physical quantity}}

A '''physical constant''', sometimes '''fundamental physical constant''' or '''universal constant''', is a [[physical quantity]] that cannot be explained by a theory and therefore must be measured experimentally. 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]] ''ε''<sub>0</sub>, and the [[elementary charge]] ''e''. Physical constants can take many [[dimensional analysis|dimensional]] forms: the speed of light signifies a maximum [[speed]] for any object and its [[Dimensional analysis|dimension]] is [[length]] divided by [[time]]; while the [[proton-to-electron mass ratio]] is [[dimensionless]].

The term "fundamental physical constant" is sometimes used to refer to universal-but-dimensioned physical constants such as those mentioned above.<ref>{{cite web |url=http://physics.nist.gov/cuu/Constants/ |title=Fundamental Physical Constants from NIST |access-date=2016-01-14 |url-status=live |archive-url=https://web.archive.org/web/20160113222630/http://physics.nist.gov/cuu/Constants/ |archive-date=2016-01-13 }} NIST</ref> Increasingly, however, physicists reserve the expression for the narrower case of [[dimensionless physical constant|dimensionless universal physical constant]]s, such as the [[fine-structure constant]] ''α'', which characterizes the strength of the [[electromagnetic interaction]].

Physical constants, as discussed here, should not be confused with [[empirical constant]]s, which are [[coefficient]]s or [[parameter]]s assumed to be constant in a given context without being fundamental.<ref name="ISO80000-1">{{cite web | website=iso.org |title=ISO 80000-1:2022 Quantities and units — Part 1: General | url=https://www.iso.org/obp/ui/#iso:std:iso:80000:-1:ed-2:v1:en | access-date=2023-08-31}}</ref> Examples include the [[characteristic time]], [[characteristic length]], or [[characteristic number (physics)|characteristic number]] (dimensionless) of a given system, or [[material constant]]s (e.g., [[Madelung constant]], [[Electrical resistivity and conductivity|electrical resistivity]], and [[heat capacity]]) of a particular material or substance.<!-- and because even if "in principle" they could be derived from the Standard Model, they cannot be in practice and still have to be measured-->