Editing Physical constant (section)

== Number of fundamental constants ==
The number of fundamental physical constants depends on the [[physical theory]] accepted as "fundamental". Currently, <!--"As of the later 20th to 21st century"--> this is the theory of [[general relativity]] for gravitation and the [[Standard Model]] for electromagnetic, weak and strong nuclear interactions and the matter fields. Between them, these theories account for a total of 19 independent fundamental constants. There is, however, no single "correct" way of enumerating them, as it is a matter of arbitrary choice which quantities are considered "fundamental" and which as "derived". Uzan<ref name=UzanVaryingConstants/> lists 22 "fundamental constants of our standard model" as follows: 
* the [[gravitational constant]] ''G'', 
* the [[speed of light]] ''c'',
* the [[Planck constant]] ''h'',
* the 9 [[Yukawa couplings]] for the [[quark]]s and [[lepton]]s (equivalent to specifying the [[rest mass]] of these [[elementary particles]]),
* 2 parameters of the [[Higgs field]] potential,
* 4 parameters for the [[Cabibbo–Kobayashi–Maskawa matrix|quark mixing matrix]], 
* 3 coupling constants for the [[gauge group]]s [[Standard Model (mathematical formulation)|SU(3) × SU(2) × U(1)]] (or equivalently, two coupling constants and the [[Weinberg angle]]),
* a phase for the [[QCD vacuum|quantum chromodynamics vacuum]].
The number of 19 independent fundamental physical constants is subject to change under possible [[Physics beyond the Standard Model|extensions of the Standard Model]], notably by the introduction of [[neutrino mass]] (equivalent to seven additional constants, i.e. 3 Yukawa couplings and 4 [[Pontecorvo–Maki–Nakagawa–Sakata matrix|lepton mixing]] parameters).<ref name=UzanVaryingConstants>{{cite journal | url= | doi=10.12942/lrr-2011-2| title=Varying Constants, Gravitation and Cosmology| journal=Living Reviews in Relativity| volume=14| year=2011| last1=Uzan| first1=Jean-Philippe| issue=1| pages=2| doi-access=free| pmid=28179829| pmc=5256069| arxiv=1009.5514| bibcode=2011LRR....14....2U}}</ref>

The discovery of variability in any of these constants would be equivalent to the discovery of "[[new physics]]".<ref name=UzanVaryingConstants/>

The question as to which constants are "fundamental" is neither straightforward nor meaningless, but a question of interpretation of the physical theory regarded as fundamental; as pointed out by {{harvnb|Lévy-Leblond|1977}}, not all physical constants are of the same importance, with some having a deeper role than others. {{harvnb|Lévy-Leblond|1977}} proposed a classification schemes of three types of constants: 
* A: physical properties of particular objects
* B: characteristic of a class of physical phenomena
* C: universal constants
The same physical constant may move from one category to another as the understanding of its role deepens; this has notably happened to the speed of light, which was a class A constant (characteristic of [[light]]) when it was first measured, but became a class B constant (characteristic of [[electromagnetism|electromagnetic phenomena]]) with the development of [[classical electromagnetism]], and finally a class C constant with the discovery of [[special relativity]].<ref>{{cite journal |last1=Lévy-Leblond |first1=J. |title=On the conceptual nature of the physical constants |journal=La Rivista del Nuovo Cimento |series=Series 2|date=1977 |volume=7 |issue=2 |pages=187–214|doi=10.1007/bf02748049|bibcode=1977NCimR...7..187L |s2cid=121022139 }}{{cite book|last=Lévy-Leblond |first=J.-M. |chapter=The importance of being (a) Constant |editor1-last=Toraldo di Francia |editor1-first=G. |title=Problems in the Foundations of Physics, Proceedings of the International School of Physics 'Enrico Fermi' Course LXXII, Varenna, Italy, July 25 – August 6, 1977 |pages=237–263 |publisher=NorthHolland |location=New York |date=1979}}</ref>