Editing Dimensionless physical constant (section)

{{Short description|Physical constant with no units}}
{{Use dmy dates|date=October 2024}}
{{For|dimensionless quantity|dimensionless quantity}}

In physics, a '''dimensionless physical constant''' is a [[physical constant]] that is [[dimensionless quantity|dimensionless]], i.e. a pure number having no units attached and having a numerical value that is independent of whatever [[system of units]] may be used.<ref>Stroke, H. H., ed., ''The Physical Review: The First Hundred Years'' ([[Berlin]]/[[Heidelberg]]: [[Springer Science+Business Media|Springer]], 1995), [https://books.google.com/books?id=3U2HSMHsouMC&pg=PA525&redir_esc=y#v=onepage&q&f=false p. 525].</ref>

The concept should not be confused with ''[[dimensionless number]]s'', that are not universally constant, and remain constant only for a particular phenomenon. In [[aerodynamics]] for example, if one considers one particular [[airfoil]], the [[Reynolds number]] value of the [[laminar–turbulent transition]] is one relevant dimensionless number of the problem. However, it is strictly related to the particular problem: for example, it is related to the airfoil being considered and also to the type of fluid in which it moves.

The term '''fundamental physical constant''' is sometimes used to refer to some {{em|universal}} dimensionless constants. Perhaps the best-known example is the [[fine-structure constant]], ''α'', which has an approximate value of {{sfrac|1|{{physconst|alphainv|round=3|ref=no}}}}.<ref>Vértes, A., Nagy, S., Klencsár, Z., Lovas, R. G., & Rösch, F., eds., ''Handbook of Nuclear Chemistry'', (Berlin/Heidelberg: Springer, 2011), [https://books.google.com/books?id=NQyF6KaUScQC&pg=PA367&redir_esc=y#v=onepage&q&f=false p. 367].</ref>