Editing Dimensionless quantity (section)

== Dimensionless physical constants ==
{{main|Dimensionless physical constant}}

Certain universal dimensioned physical constants, such as the [[speed of light]] in vacuum, the [[universal gravitational constant]], the [[Planck constant]], the [[Coulomb constant]], and the [[Boltzmann constant]] can be normalized to 1 if appropriate units for [[time]], [[length]], [[mass]], [[electric charge|charge]], and [[temperature]] are chosen. The resulting [[system of units]] is known as the [[natural units]], specifically regarding these five constants, [[Planck units]]. However, not all [[physical constant]]s can be normalized in this fashion. For example, the values of the following constants are independent of the system of units, cannot be defined, and can only be determined experimentally:<ref>{{cite web |url=http://math.ucr.edu/home/baez/constants.html |title=How Many Fundamental Constants Are There? |author-last=Baez |author-first=John Carlos |author-link=John Carlos Baez |date=2011-04-22 |access-date=2015-10-07}}</ref>

* [[engineering strain]], a measure of physical deformation defined as a change in length divided by the initial length.

* [[fine-structure constant]], ''α'' ≈ 1/137 which characterizes the magnitude of the [[electromagnetic interaction]] between electrons.<ref name=":0">{{Cite journal |last=Navas |first=S. |last2=Amsler |first2=C. |last3=Gutsche |first3=T. |last4=Hanhart |first4=C. |last5=Hernández-Rey |first5=J. J. |last6=Lourenço |first6=C. |last7=Masoni |first7=A. |last8=Mikhasenko |first8=M. |last9=Mitchell |first9=R. E. |last10=Patrignani |first10=C. |last11=Schwanda |first11=C. |last12=Spanier |first12=S. |last13=Venanzoni |first13=G. |last14=Yuan |first14=C. Z. |last15=Agashe |first15=K. |date=2024-08-01 |title=Review of Particle Physics |url=https://link.aps.org/doi/10.1103/PhysRevD.110.030001 |journal=Physical Review D |language=en |volume=110 |issue=3 |doi=10.1103/PhysRevD.110.030001 |issn=2470-0010|hdl=20.500.11850/695340 |hdl-access=free }}</ref>
* ''β'' (or ''μ'') ≈ 1836, the [[proton-to-electron mass ratio]]. This ratio is the [[rest mass]] of the [[proton]] divided by that of the [[electron]]. An analogous ratio can be defined for any [[elementary particle]].
* [[Strong force]] coupling strength ''α''<sub>s</sub> ≈ 1.
* The tensor-to-scalar ratio <math>r</math>, a ratio between the contributions of tensor and scalar modes to the primordial power spectrum observed in the [[Cosmic microwave background|CMB]].<ref name=":0" />
* The [[Immirzi parameter|Immirzi-Barbero]] parameter <math>\gamma</math>, which characterizes the area gap in [[loop quantum gravity]].<ref>{{Cite book |last=Rovelli |first=Carlo |url=https://www.cambridge.org/core/books/quantum-gravity/9EEB701AAB938F06DCF151AACE1A445D |title=Quantum Gravity |date=2004 |publisher=Cambridge University Press |isbn=978-0-521-71596-6 |series=Cambridge Monographs on Mathematical Physics |location=Cambridge |doi=10.1017/cbo9780511755804}}</ref>
* [[emissivity]], which is the ratio of actual emitted radiation from a surface to that of an [[Black body|idealized surface]] at the same [[temperature]]