Physical constant
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 contrasted with 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_{0}, 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 finestructure 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.^{[1]} Increasingly, however, physicists reserve the use of the term fundamental physical constant for dimensionless physical constants, such as the finestructure constant α.
Physical constant in the sense under discussion in this article should not be confused with other quantities called "constants" that are assumed to be constant in a given context without the implication that they are fundamental, such as the "time constant" characteristic of a given system, or material constants, such as the Madelung constant, electrical resistivity, heat capacity.
On November 16, 2018, the International Bureau of Weights and Measures voted to redefine several base units in the International System of Units (SI) by fixing the SI value of several physical constants, including the Planck constant, h, elementary charge, e, Boltzmann constant, k, Avogadro constant, N_{A}, and the speed of light, c. The new fixed values are based on the best measurements of the constants based on the earlier definitions, including the kilogram, to ensure minimal impact.
Contents
Choice of unitsEdit
Whereas the physical quantity indicated by a physical constant does not depend on the unit system used to express the quantity, the numerical values of dimensional physical constants do depend on choice of unit system. The term "physical constant" refers to the physical quantity, and not to the numerical value within any given system of units. For example, the speed of light is defined as having the numerical value of 299,792,458 in SI units, and as having the numerical value of 1 in natural units. While its numerical value can be defined at will by the choice of units, the speed of light itself is a single physical constant.
Any ratio between physical constants of the same dimensions results in a dimensionless physical constant, for example, the protontoelectron mass ratio. Any relation between physical quantities can be expressed as a relation between dimensionless ratios via a process known as nondimensionalisation.
The term of "fundamental physical constant" is reserved for physical quantities which, according to the current state of knowledge, are regarded as immutable and as nonderivable from more fundamental principles. Notable examples are the speed of light c, and the gravitational constant G.^{[2]}
The finestructure constant α is the best known dimensionless fundamental physical constant. It is the value of the elementary charge squared expressed in Planck units. This value has become a standard example when discussing the derivability or nonderivability of physical constants. Introduced by Arnold Sommerfeld, its value as determined at the time was consistent with 1/137. This motivated Arthur Eddington (1929) to construct an argument why its value might be 1/137 precisely, which related to the Eddington number, his estimate of the number of protons in the Universe.^{[3]} By the 1940s, it became clear that the value of the finestructure constant deviates significantly from the precise value of 1/137, refuting Eddington's argument.^{[4]}
With the development of quantum chemistry in the 20th century, however, a vast number of previously inexplicable dimensionless physical constants were successfully computed from theory. In light of that, some theoretical physicists still hope for continued progress in explaining the values of other dimensionless physical constants.
It is known that the Universe would be very different if these constants took values significantly different from those we observe. For example, a few percent change in the value of the fine structure constant would be enough to eliminate stars like our Sun. This has prompted attempts at anthropic explanations of the values of some of the dimensionless fundamental physical constants.
Natural unitsEdit
Using dimensional analysis, it is possible to combine dimensional universal physical constants to define a system of units of measurement that has no reference to any human construct. Depending on the choice and arrangement of constants used, the resulting natural units may have useful physical meaning. For example, Planck units, shown in the table below, use c, G, ħ, ε_{0} and k_{B} in such a manner to derive units relevant to unified theories such as quantum gravity.
Name Quantity Expression Value (SI units) Planck length Length (L) 229(38)×10^{−35} m^{[5]} 1.616 Planck mass Mass (M) 470(51)×10^{−8} kg^{[6]} 2.176 Planck time Time (T) 16(13)×10^{−44} s^{[7]} 5.391 Planck charge Electric charge (Q) 1.875 545 956(41) × 10^{−18} C^{[8]}^{[9]}^{[10]} Planck temperature Temperature (Θ) 808(33)×10^{32} K^{[11]} 1.416
Number of fundamental constantsEdit
The number of fundamental physical constants depends on the physical theory accepted as "fundamental". Currently, 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 (2011) lists 22 "unknown constants" in the fundamental theories, which give rise to 19 "unknown dimensionless parameters", as follows:
 the gravitational constant G,
 the speed of light c,
 the Planck constant h,
 the 9 Yukawa couplings for the quarks and leptons (equivalent to specifying the rest mass of these elementary particles),
 2 parameters of the Higgs field potential,
 4 parameters for the quark mixing matrix,
 3 coupling constants for the gauge groups SU(3) × SU(2) × U(1) (or equivalently, two coupling constants and the Weinberg angle),
 a phase for the QCD vacuum.
The number of 19 independent fundamental physical constants is subject to change under possible extensions of the Standard Model, notably by the introduction of neutrino mass (equivalent to seven additional constants, i.e. 3 Yukawa couplings and 4 lepton mixing parameters).^{[12]}
The discovery of variability in any of these constants would be equivalent to the discovery of "new physics".^{[13]}
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 LévyLeblond 1979, not all physical constants are of the same importance, with some having a deeper role than others. LévyLeblond 1979 proposed a classification schemes of three types of fundamental constant:
 A: characteristic of a particular system
 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 electromagnetic phenomena) with the development of classical electromagnetism, and finally a class C constant with the discovery of special relativity.^{[14]}
Tests on timeindependenceEdit
By definition, fundamental physical constants are subject to measurement, so that their being constant (independent on both the time and position of the performance of the measurement) is necessarily an experimental result and subject to verification.
Paul Dirac in 1937 speculated that physical constants such as the gravitational constant or the finestructure constant might be subject to change over time in proportion of the age of the universe. Experiments can in principle only put an upper bound on the relative change per year. For the finestructure constant, this upper bound is comparatively low, at roughly 10^{−17} per year (as of 2008).^{[15]}
The gravitational constant is much more difficult to measure with precision, and conflicting measurements in the 2000s have inspired the controversial suggestions of a periodic variation of its value in a 2015 paper.^{[16]} However, while its value is not known to great precision, the possibility of observing type Ia supernovae which happened in the universe's remote past, paired with the assumption that the physics involved in these events is universal, allows for an upper bound of less than 10^{−10} per year for the gravitational constant over the last nine billion years.^{[17]}
Similarly, an upper bound of the change in the protontoelectron mass ratio has been placed at 10^{−7} over a period of 7 billion years (or 10^{−16} per year) in a 2012 study based on the observation of methanol in a distant galaxy.^{[18]}^{[19]}
It is problematic to discuss the proposed rate of change (or lack thereof) of a single dimensional physical constant in isolation. The reason for this is that the choice of a system of units may arbitrarily select as its basis, making the question of which constant is undergoing change an artefact of the choice of units.^{[20]}^{[21]}
For example, in SI units, the speed of light has been given a defined value in 1983. Thus, it was meaningful to experimentally measure the speed of light in SI units prior to 1983, but it is not so now. Similar, since 2018, the Planck constant has been given a defined value, such that all SI base units are now expressed in terms of fundamental physical constants (the kilogram was the last physical object used to define a base unit until November 2018).
Tests on the immutability of physical constants look at dimensionless quantities, i.e. ratios between quantities of like dimensions, in order to escape this problem. Changes in physical constants are not meaningful if they result in an observationally indistinguishable universe. For example, a "change" in the speed of light c would be meaningless if accompanied by a corresponding change in the elementary charge e so that the ratio e^{2}/(4πε_{0}ħc) (the finestructure constant) remained unchanged.^{[22]}
Finetuned UniverseEdit
Some physicists have explored the notion that if the dimensionless physical constants had sufficiently different values, our Universe would be so radically different that intelligent life would probably not have emerged, and that our Universe therefore seems to be finetuned for intelligent life. The anthropic principle states a logical truism: the fact of our existence as intelligent beings who can measure physical constants requires those constants to be such that beings like us can exist. There are a variety of interpretations of the constants' values, including that of a divine creator (the apparent finetuning is actual and intentional), or that ours is one universe of many in a multiverse (e.g. the Manyworlds interpretation of quantum mechanics), or even that, if information is an innate property of the universe and logically inseparable from consciousness, a universe without the capacity for conscious beings cannot exist.
Table of physical constantsEdit
This section's factual accuracy may be compromised due to outofdate information. In particular: This section has not been updated to correspond with the 2019 redefinition of SI base units. (November 2018) 
Universal constantsEdit
Quantity

Symbol  Value^{[23]}^{[24]}  Relative standard uncertainty 

speed of light in vacuum  299 792 458 m⋅s^{−1}  defined  
Newtonian constant of gravitation  08(31)×10^{−11} m^{3}⋅kg^{−1}⋅s^{−2} 6.674  4.7 × 10^{−5}  
Planck constant  6.626 070 040(81) × 10^{−34} J⋅s  1.2 × 10^{−8}  
reduced Planck constant  1.054 571 800(13) × 10^{−34} J⋅s  1.2 × 10^{−8} 
Electromagnetic constantsEdit
Quantity

Symbol  Value^{[23]}^{[24]} (SI units)  Relative standard uncertainty 

magnetic constant (vacuum permeability)  4π × 10^{−7} N⋅A^{−2} = 1.256 637 061... × 10^{−6} N⋅A^{−2}  defined  
electric constant (vacuum permittivity)  8.854 187 817... × 10^{−12} F⋅m^{−1}  defined  
characteristic impedance of vacuum  376.730 313 461... Ω  defined  
Coulomb's constant  8.987 551 787 368 176 4 × 10^{9} kg⋅m^{3}⋅s^{−4}⋅A^{−2}  defined  
elementary charge  1.602 176 6208(98) × 10^{−19} C  6.1 × 10^{−9}  
Bohr magneton  9.274 009 994(57) × 10^{−24} J⋅T^{−1}  6.2 × 10^{−9}  
conductance quantum  7.748 091 7310(18) × 10^{−5} S  2.3 × 10^{−10}  
inverse conductance quantum  12 906.403 7278(29) Ω  2.3 × 10^{−10}  
Josephson constant  4.835 978 525(30) × 10^{14} Hz⋅V^{−1}  6.1 × 10^{−9}  
magnetic flux quantum  2.067 833 831(13) × 10^{−15} Wb  6.1 × 10^{−9}  
nuclear magneton  5.050 783 699(31) × 10^{−27} J⋅T^{−1}  6.2 × 10^{−9}  
von Klitzing constant  25 812.807 4555(59) Ω  2.3 × 10^{−10} 
Atomic and nuclear constantsEdit
Quantity

Symbol  Value^{[23]}^{[24]} (SI units)  Relative standard uncertainty  

Bohr radius  5.291 772 1067(12) × 10^{−11} m  2.3 × 10^{−9}  
classical electron radius  2.817 940 3227(19) × 10^{−15} m  6.8 × 10^{−10}  
electron mass  9.109 383 56(11) × 10^{−31} kg  1.2 × 10^{−8}  
Fermi coupling constant  1.166 3787(6) × 10^{−5} GeV^{−2}  5.1 × 10^{−7}  
finestructure constant  7.297 352 5664(17) × 10^{−3} = 1 / 137.035 999 139(31)  2.3 × 10^{−10}  
Hartree energy  4.359 744 650(54) × 10^{−18} J  1.2 × 10^{−8}  
proton mass  1.672 621 898(21) × 10^{−27} kg  1.2 × 10^{−8}  
quantum of circulation  3.636 947 5486(17) × 10^{−4} m^{2} s^{−1}  4.5 × 10^{−10}  
Rydberg constant  10 973 731.568 508(65) m^{−1}  5.9 × 10^{−12}  
Thomson cross section  6.652 458 7158(91) × 10^{−29} m^{2}  1.4 × 10^{−9}  
weak mixing angle  0.2223(21)  9.5 × 10^{−3}  
Efimov factor  22.7 
Physicochemical constantsEdit
Quantity

Symbol  Value^{[23]}^{[24]} (SI units)  Relative standard uncertainty  

Atomic mass constant  539040(20)×10^{−27} kg 1.660  ×10^{−8} 1.2  
Avogadro constant  140857(74)×10^{23} mol^{−1} 6.022  ×10^{−8} 1.2  
Boltzmann constant  64852(79)×10^{−23} J⋅K^{−1} 1.380  ×10^{−7} 5.7  
Faraday constant  485.33289(59) C⋅mol^{−1} 96  ×10^{−9} 6.2  
first radiation constant  3.741 771 790(46) × 10^{−16} W⋅m^{2}  1.2 × 10^{−8}  
for spectral radiance  1.191 042 953(15) × 10^{−16} W⋅m^{2}⋅sr^{−1}  1.2 × 10^{−8}  
Loschmidt constant  at = 273.15 K and = 101.325 kPa  2.686 7811(15) × 10^{25} m^{−3}  5.7 × 10^{−7}  
gas constant  4598(48) J⋅mol^{−1}⋅K^{−1} 8.314  ×10^{−7} 5.7  
molar Planck constant  3.990 312 7110(18) × 10^{−10} J⋅s⋅mol^{−1}  4.5 × 10^{−10}  
molar volume of an ideal gas  at = 273.15 K and = 100 kPa  2.271 0947(13) × 10^{−2} m^{3}⋅mol^{−1}  5.7 × 10^{−7}  
at = 273.15 K and = 101.325 kPa  2.241 3962(13) × 10^{−2} m^{3}⋅mol^{−1}  5.7 × 10^{−7}  
Sackur–Tetrode constant  at = 1 K and = 100 kPa  
−1.151 7084(14)  1.2 × 10^{−6} 
at = 1 K and = 101.325 kPa  −1.164 8714(14)  1.2 × 10^{−6}  
second radiation constant  1.438 777 36(83) × 10^{−2} m⋅K  5.7 × 10^{−7}  
Stefan–Boltzmann constant  367(13)×10^{−8} W⋅m^{−2}⋅K^{−4} 5.670  ×10^{−6} 2.3  
Wien displacement law constant  4.965 114 231...  7729(17)×10^{−3} m⋅K 2.897  ×10^{−7} 5.7  
WienBonal entropy displacement law constant^{[25]}  4.791 267 357...  3.002 9152(05) × 10^{−3} m⋅K  5.7 × 10^{−7} 
Adopted valuesEdit
Quantity  Symbol  Value (SI units)  Relative standard uncertainty  

conventional value of Josephson constant^{[26]}  4.835 979 × 10^{14} Hz⋅V^{−1}  defined  
conventional value of von Klitzing constant^{[27]}  25 812.807 Ω  defined  
molar mass  constant  1 × 10^{−3} kg⋅mol^{−1}  defined  
of carbon12  1.2 × 10^{−2} kg⋅mol^{−1}  defined  
standard acceleration of gravity (gee, freefall on Earth)  9.806 65 m⋅s^{−2}  defined  
standard atmosphere  101 325 Pa  defined  
Caesium standard (defines the SI second)  Δν_{Cs}  9 192 631 770 Hz  defined 
See alsoEdit
ReferencesEdit
 ^ "Archived copy". Archived from the original on 20160113. Retrieved 20160114. NIST
 ^ 2010 Values of the Constants Archived 20160113 at the Wayback Machine.; NIST, 2011.
 ^ A.S Eddington (1956). "The Constants of Nature". In J.R. Newman. The World of Mathematics. 2. Simon & Schuster. pp. 1074–1093.
 ^ H. Kragh (2003). "Magic Number: A Partial History of the FineStructure Constant". Archive for History of Exact Sciences. 57 (5): 395. doi:10.1007/s0040700200657.
 ^ "CODATA Value: Planck length". The NIST Reference on Constants, Units, and Uncertainty. US National Institute of Standards and Technology. June 2015. Retrieved 20170622.
2014 CODATA recommended values
 ^ "CODATA Value: Planck mass". The NIST Reference on Constants, Units, and Uncertainty. US National Institute of Standards and Technology. June 2015. Retrieved 20170622.
2014 CODATA recommended values
 ^ "CODATA Value: Planck time". The NIST Reference on Constants, Units, and Uncertainty. US National Institute of Standards and Technology. June 2015. Retrieved 20170622.
2014 CODATA recommended values
 ^ "CODATA Value: electric constant". The NIST Reference on Constants, Units, and Uncertainty. US National Institute of Standards and Technology. June 2015. Retrieved 20150925.
2014 CODATA recommended values
 ^ "CODATA Value: Planck constant over 2 pi". The NIST Reference on Constants, Units, and Uncertainty. US National Institute of Standards and Technology. June 2015. Retrieved 20150925.
2014 CODATA recommended values
 ^ "CODATA Value: speed of light in vacuum". The NIST Reference on Constants, Units, and Uncertainty. US National Institute of Standards and Technology. June 2015. Retrieved 20150925.
2014 CODATA recommended values
 ^ "CODATA Value: Planck temperature". The NIST Reference on Constants, Units, and Uncertainty. US National Institute of Standards and Technology. June 2015. Retrieved 20170622.
2014 CODATA recommended values
 ^ JeanPhilippe Uzan, "Varying Constants, Gravitation and Cosmology", Living Rev. Relativ., 14.2 (2011), p. 10f.
 ^ "Any constant varying in space and/or time would reflect the existence of an almost massless field that couples to matter. This will induce a violation of the universality of free fall. Thus, it is of utmost importance for our understanding of gravity and of the domain of validity of general relativity to test for their constancy." JeanPhilippe Uzan, "Varying Constants, Gravitation and Cosmology", Living Rev. Relativ., 14.2 (2011), 10f.
 ^ LévyLeblond, J.M. (1979). "The importance of being (a) Constant". In Toraldo di Francia, G. Problems in the Foundations of Physics, Proceedings of the International School of Physics 'Enrico Fermi' Course LXXII, Varenna, Italy, July 25 – August 6, 1977. New York: NorthHolland. pp. 237–263.
 ^ T. Rosenband; et al. (2008). "Frequency Ratio of Al^{+} and Hg^{+} SingleIon Optical Clocks; Metrology at the 17th Decimal Place". Science. 319 (5871): 1808–12. Bibcode:2008Sci...319.1808R. doi:10.1126/science.1154622. PMID 18323415.
 ^ J.D. Anderson; G. Schubert; V. Trimble; M.R. Feldman (April 2015), "Measurements of Newton's gravitational constant and the length of day" (PDF), EPL, 110: 10002, arXiv:1504.06604, Bibcode:2015EL....11010002A, doi:10.1209/02955075/110/10002
 ^ J. Mould; S. A. Uddin (20140410), "Constraining a Possible Variation of G with Type Ia Supernovae", Publications of the Astronomical Society of Australia, 31: e015, arXiv:1402.1534, Bibcode:2014PASA...31...15M, doi:10.1017/pasa.2014.9, archived from the original on 20140408
 ^ Bagdonaite, Julija; Jansen, Paul; Henkel, Christian; Bethlem, Hendrick L.; Menten, Karl M.; Ubachs, Wim (December 13, 2012). "A Stringent Limit on a Drifting ProtontoElectron Mass Ratio from Alcohol in the Early Universe". Science. 339: 46–48. Bibcode:2013Sci...339...46B. doi:10.1126/science.1224898. PMID 23239626. Retrieved December 14, 2012.
 ^ Moskowitz, Clara (December 13, 2012). "Phew! Universe's Constant Has Stayed Constant". Space.com. Archived from the original on December 14, 2012. Retrieved December 14, 2012.
 ^ Duff, M. J. (13 August 2002). "Comment on timevariation of fundamental constants". arXiv:hepth/0208093.
 ^ Duff, M. J.; Okun, L. B.; Veneziano, G. (2002). "Trialogue on the number of fundamental constants". Journal of High Energy Physics. 3: 023–023. arXiv:physics/0110060. Bibcode:2002JHEP...03..023D. doi:10.1088/11266708/2002/03/023.
 ^ Barrow, John D. (2002), The Constants of Nature; From Alpha to Omega  The Numbers that Encode the Deepest Secrets of the Universe, Pantheon Books, ISBN 0375422218 "[An] important lesson we learn from the way that pure numbers like α define the World is what it really means for worlds to be different. The pure number we call the fine structure constant and denote by α is a combination of the electron charge, e, the speed of light, c, and Planck's constant, h. At first we might be tempted to think that a world in which the speed of light was slower would be a different world. But this would be a mistake. If c, h, and e were all changed so that the values they have in metric (or any other) units were different when we looked them up in our tables of physical constants, but the value of α remained the same, this new world would be observationally indistinguishable from our World. The only thing that counts in the definition of worlds are the values of the dimensionless constants of Nature. If all masses were doubled in value you cannot tell, because all the pure numbers defined by the ratios of any pair of masses are unchanged."
 ^ ^{a} ^{b} ^{c} ^{d} The values are given in the socalled concise form; the number in parentheses after the mantissa is the standard uncertainty, which is the value multiplied by the relative standard uncertainty, and indicates the amount by which the least significant digits of the value are uncertain. For example, 75 is the standard uncertainty in "8.314 4621(75)", and means that the value is between 8.314 4546 and 8.314 4696.
 ^ ^{a} ^{b} ^{c} ^{d} Mohr, Peter J.; Newell, David B.; Taylor, Barry N. "CODATA Recommended Values of the Fundamental Physical Constants: 2014". arXiv:1507.07956v1 [physics.atomph].
 ^ DelgadoBonal, Alfonso (10 May 2017). "Entropy of radiation: the unseen side of light". Scientific Reports. 7 (1642). Bibcode:2017NatSR...7.1642D. doi:10.1038/s41598017016226.
 ^ This is the value adopted internationally for realizing representations of the volt using the Josephson effect.
 ^ This is the value adopted internationally for realizing representations of the ohm using the quantum Hall effect.
 Mohr, Peter J.; Taylor, Barry N.; Newell, David B. (2008). "CODATA Recommended Values of the Fundamental Physical Constants: 2006". Reviews of Modern Physics. 80 (2): 633–730. arXiv:0801.0028. Bibcode:2008RvMP...80..633M. doi:10.1103/RevModPhys.80.633.
 Barrow, John D. (2002), The Constants of Nature; From Alpha to Omega  The Numbers that Encode the Deepest Secrets of the Universe, Pantheon Books, ISBN 0375422218.
External linksEdit
 Sixty Symbols, University of Nottingham
 IUPAC  Gold Book