The constants listed here are known values of physical constants expressed in SI units; that is, physical quantities that are generally believed to be universal in nature and thus are independent of the unit system in which they are measured. Many of these are redundant, in the sense that they obey a known relationship with other physical constants and can be determined from them.
Symbol | Quantity | Value [lower-alpha 1] [lower-alpha 2] | Relative standard uncertainty | Ref [1] |
---|---|---|---|---|
speed of light in vacuum | 299792458 m⋅s−1 | 0 | [2] | |
Planck constant | 6.62607015×10−34 J⋅Hz−1 | 0 | [3] | |
reduced Planck constant | 1.054571817...×10−34 J⋅s | 0 | [4] | |
vacuum magnetic permeability | 1.25663706212(19)×10−6 N⋅A−2 | 1.5×10−10 | [5] | |
characteristic impedance of vacuum | 376.730313668(57) Ω | 1.5×10−10 | [6] | |
vacuum electric permittivity | 8.8541878128(13)×10−12 F⋅m−1 | 1.5×10−10 | [7] | |
Boltzmann constant | 1.380649×10−23 J⋅K−1 | 0 | [8] | |
Newtonian constant of gravitation | 6.67430(15)×10−11 m3⋅kg−1⋅s−2 | 2.2×10−5 | [9] | |
Coulomb constant | 8.9875517923(14)×109 N⋅m2⋅C−2 | 1.5×10−10 | [10] | |
cosmological constant | 1.089(29)×10−52 m−2 | 0.027 | [11] | |
Stefan–Boltzmann constant | 5.670374419...×10−8 W⋅m−2⋅K−4 | 0 | [12] | |
first radiation constant | 3.741771852...×10−16 W⋅m2 | 0 | [13] | |
first radiation constant for spectral radiance | 1.191042972...×10−16 W⋅m2⋅sr−1 | 0 | [14] | |
second radiation constant | 1.438776877...×10−2 m⋅K | 0 | [15] | |
[lower-alpha 3] | Wien wavelength displacement law constant | 2.897771955...×10−3 m⋅K | 0 | [16] |
[lower-alpha 4] | Wien frequency displacement law constant | 5.878925757...×1010 Hz⋅K−1 | 0 | [17] |
Wien entropy displacement law constant | 3.002916077...×10−3 m⋅K | 0 | [18] | |
elementary charge | 1.602176634×10−19 C | 0 | [19] | |
conductance quantum | 7.748091729...×10−5 S | 0 | [20] | |
inverse conductance quantum | 12906.40372... Ω | 0 | [21] | |
von Klitzing constant | 25812.80745... Ω | 0 | [22] | |
Josephson constant | 483597.8484...×109 Hz⋅V−1 | 0 | [23] | |
magnetic flux quantum | 2.067833848...×10−15 Wb | 0 | [24] | |
fine-structure constant | 7.2973525693(11)×10−3 | 1.5×10−10 | [25] | |
inverse fine-structure constant | 137.035999084(21) | 1.5×10−10 | [26] | |
electron mass | 9.1093837015(28)×10−31 kg | 3.0×10−10 | [27] | |
muon mass | 1.883531627(42)×10−28 kg | 2.2×10−8 | [28] | |
tau mass | 3.16754(21)×10−27 kg | 6.8×10−5 | [29] | |
proton mass | 1.67262192369(51)×10−27 kg | 3.1×10−10 | [30] | |
neutron mass | 1.67492749804(95)×10−27 kg | 5.7×10−10 | [31] | |
top quark mass | 3.0784(53)×10−25 kg | 1.7×10−3 | [32] | |
proton-to-electron mass ratio | 1836.15267343(11) | 6.0×10−11 | [33] | |
W-to-Z mass ratio | 0.88153(17) | 1.9×10−4 | [34] | |
weak mixing angle | 0.22290(30) | 1.3×10−3 | [35] | |
electron g-factor | −2.00231930436256(35) | 1.7×10−13 | [36] | |
muon g-factor | −2.0023318418(13) | 6.3×10−10 | [37] | |
proton g-factor | 5.5856946893(16) | 2.9×10−10 | [38] | |
quantum of circulation | 3.6369475516(11)×10−4 m2⋅s−1 | 3.0×10−10 | [39] | |
Bohr magneton | 9.2740100783(28)×10−24 J⋅T−1 | 3.0×10−10 | [40] | |
nuclear magneton | 5.0507837461(15)×10−27 J⋅T−1 | 3.1×10−10 | [41] | |
classical electron radius | 2.8179403262(13)×10−15 m | 4.5×10−10 | [42] | |
Thomson cross section | 6.6524587321(60)×10−29 m2 | 9.1×10−10 | [43] | |
Bohr radius | 5.29177210903(80)×10−11 m | 1.5×10−10 | [44] | |
Hartree energy | 4.3597447222071(85)×10−18 J | 1.9×10−12 | [45] | |
Rydberg unit of energy | 2.1798723611035(42)×10−18 J | 1.9×10−12 | [46] | |
Rydberg constant | 10973731.568160(21) m−1 | 1.9×10−12 | [47] | |
Fermi coupling constant | 1.1663787(6)×10−5 GeV−2 | 5.1×10−7 | [48] | |
Avogadro constant | 6.02214076×1023 mol−1 | 0 | [49] | |
molar gas constant | 8.31446261815324 J⋅mol−1⋅K−1 | 0 | [50] | |
Faraday constant | 96485.3321233100184 C⋅mol−1 | 0 | [51] | |
molar Planck constant | 3.9903127128934314×10−10 J⋅s⋅mol−1 | 0 | [52] | |
atomic mass of carbon-12 | 1.99264687992(60)×10−26 kg | 3.0×10−10 | [53] | |
molar mass of carbon-12 | 11.9999999958(36)×10−3 kg⋅mol−1 | 3.0×10−10 | [54] | |
atomic mass constant | 1.66053906660(50)×10−27 kg | 3.0×10−10 | [53] | |
molar mass constant | 0.99999999965(30)×10−3 kg⋅mol−1 | 3.0×10−10 | [55] | |
molar volume of silicon | 1.205883199(60)×10−5 m3⋅mol−1 | 4.9×10−8 | [56] | |
hyperfine transition frequency of 133Cs | 9192631770 Hz | 0 | [57] | |
While the values of the physical constants are independent of the system of units in use, each uncertainty as stated reflects our lack of knowledge of the corresponding value as expressed in SI units, and is strongly dependent on how those units are defined. For example, the atomic mass constant is exactly known when expressed using the dalton (its value is exactly 1 Da), but the kilogram is not exactly known when using these units, the opposite of when expressing the same quantities using the kilogram.
Some of these constants are of a technical nature and do not give any true physical property, but they are included for convenience. Such a constant gives the correspondence ratio of a technical dimension with its corresponding underlying physical dimension. These include the Boltzmann constant , which gives the correspondence of the dimension temperature to the dimension of energy per degree of freedom, and the Avogadro constant , which gives the correspondence of the dimension of amount of substance with the dimension of count of entities (the latter formally regarded in the SI as being dimensionless). By implication, any product of powers of such constants is also such a constant, such as the molar gas constant .
In physics, an electronvolt is the measure of an amount of kinetic energy gained by a single electron accelerating from rest through an electric potential difference of one volt in vacuum. When used as a unit of energy, the numerical value of 1 eV in joules is equivalent to the numerical value of the charge of an electron in coulombs. Under the 2019 redefinition of the SI base units, this sets 1 eV equal to the exact value 1.602176634×10−19 J.
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.
The dalton or unified atomic mass unit is a non-SI unit of mass defined as 1/12 of the mass of an unbound neutral atom of carbon-12 in its nuclear and electronic ground state and at rest. The atomic mass constant, denoted mu, is defined identically, giving mu = 1/12m(12C) = 1 Da.
In physics, the fine-structure constant, also known as the Sommerfeld constant, commonly denoted by α, is a fundamental physical constant which quantifies the strength of the electromagnetic interaction between elementary charged particles.
The Boltzmann constant is the proportionality factor that relates the average relative thermal energy of particles in a gas with the thermodynamic temperature of the gas. It occurs in the definitions of the kelvin and the gas constant, and in Planck's law of black-body radiation and Boltzmann's entropy formula, and is used in calculating thermal noise in resistors. The Boltzmann constant has dimensions of energy divided by temperature, the same as entropy. It is named after the Austrian scientist Ludwig Boltzmann.
The Bohr radius is a physical constant, approximately equal to the most probable distance between the nucleus and the electron in a hydrogen atom in its ground state. It is named after Niels Bohr, due to its role in the Bohr model of an atom. Its value is 5.29177210903(80)×10−11 m.
The hartree, also known as the Hartree energy, is the unit of energy in the atomic units system, named after the British physicist Douglas Hartree. Its CODATA recommended value is Eh = 4.3597447222071(85)×10−18 J = 27.211386245988(53) eV.
The atomic units are a system of natural units of measurement that is especially convenient for calculations in atomic physics and related scientific fields, such as computational chemistry and atomic spectroscopy. They were originally suggested and named by the physicist Douglas Hartree. Atomic units are often abbreviated "a.u." or "au", not to be confused with similar abbreviations used for astronomical units, arbitrary units, and absorbance units in other contexts.
The elementary charge, usually denoted by e, is a fundamental physical constant, defined as the electric charge carried by a single proton or, equivalently, the magnitude of the negative electric charge carried by a single electron, which has charge −1 e.
In atomic physics, the Bohr magneton is a physical constant and the natural unit for expressing the magnetic moment of an electron caused by its orbital or spin angular momentum. In SI units, the Bohr magneton is defined as
In spectroscopy, the Rydberg constant, symbol for heavy atoms or for hydrogen, named after the Swedish physicist Johannes Rydberg, is a physical constant relating to the electromagnetic spectra of an atom. The constant first arose as an empirical fitting parameter in the Rydberg formula for the hydrogen spectral series, but Niels Bohr later showed that its value could be calculated from more fundamental constants according to his model of the atom.
The magnetic flux, represented by the symbol Φ, threading some contour or loop is defined as the magnetic field B multiplied by the loop area S, i.e. Φ = B ⋅ S. Both B and S can be arbitrary, meaning that the flux Φ can be as well but increments of flux can be quantized. The wave function can be multivalued as it happens in the Aharanov-Bohm effect or quantized as in superconductors. The unit of quantization is therefore called magnetic flux quantum.
The Sackur–Tetrode equation is an expression for the entropy of a monatomic ideal gas.
The conductance quantum, denoted by the symbol G0, is the quantized unit of electrical conductance. It is defined by the elementary charge e and Planck constant h as:
The molar mass constant, usually denoted by Mu, is a physical constant defined as one twelfth of the molar mass of carbon-12: Mu = M(12C)/12. The molar mass of any element or compound is its relative atomic mass multiplied by the molar mass constant.
The Planck constant, or Planck's constant, denoted by , is a fundamental physical constant of foundational importance in quantum mechanics: a photon's energy is equal to its frequency multiplied by the Planck constant, and the wavelength of a matter wave equals the Planck constant divided by the associated particle momentum.
In particle physics, the electron mass is the mass of a stationary electron, also known as the invariant mass of the electron. It is one of the fundamental constants of physics. It has a value of about 9.109×10−31 kilograms or about 5.486×10−4 daltons, which has an energy-equivalent of about 8.187×10−14 joules or about 0.511 MeV.
In 2019, four of the seven SI base units specified in the International System of Quantities were redefined in terms of natural physical constants, rather than human artifacts such as the standard kilogram. Effective 20 May 2019, the 144th anniversary of the Metre Convention, the kilogram, ampere, kelvin, and mole are now defined by setting exact numerical values, when expressed in SI units, for the Planck constant, the elementary electric charge, the Boltzmann constant, and the Avogadro constant, respectively. The second, metre, and candela had previously been redefined using physical constants. The four new definitions aimed to improve the SI without changing the value of any units, ensuring continuity with existing measurements. In November 2018, the 26th General Conference on Weights and Measures (CGPM) unanimously approved these changes, which the International Committee for Weights and Measures (CIPM) had proposed earlier that year after determining that previously agreed conditions for the change had been met. These conditions were satisfied by a series of experiments that measured the constants to high accuracy relative to the old SI definitions, and were the culmination of decades of research.
In particle physics and physical cosmology, Planck units are a system of units of measurement defined exclusively in terms of four universal physical constants: c, G, ħ, and kB. Expressing one of these physical constants in terms of Planck units yields a numerical value of 1. They are a system of natural units, defined using fundamental properties of nature rather than properties of a chosen prototype object. Originally proposed in 1899 by German physicist Max Planck, they are relevant in research on unified theories such as quantum gravity.
In physics, natural unit systems are measurement systems for which certain physical constants have been set to 1 through nondimensionalization of physical units. For example, the speed of light c may be set to 1, and it may then be omitted, equating mass and energy directly E=m rather than using c as a conversion factor in the typical mass–energy equivalence equation E=mc2. A purely natural system of units has all of its dimensions collapsed, such that the physical constants completely define the system of units and the relevant physical laws contain no conversion constants.
t-quark mass (direct measurements): 172.69(30) GeV/c2