# Gravitational coupling constant

In physics, a gravitational coupling constant is a constant characterizing the gravitational attraction between a given pair of elementary particles. The electron mass is typically used, and the associated constant typically denoted αG. It is a dimensionless quantity, with the result that its numerical value does not vary with the choice of units of measurement, only with the choice of particle.

## Definition

αG is typically defined[citation needed] in terms of the gravitational attraction between two electrons. More precisely,

$\alpha _{\mathrm {G} }={\frac {Gm_{\mathrm {e} }^{2}}{\hbar c}}=\left({\frac {m_{\mathrm {e} }}{m_{\mathrm {P} }}}\right)^{2}\approx 1.7518\times 10^{-45}$ where:

In Planck units, where G = c = ħ = 1, the expression becomes the square of the electron mass

$\alpha _{\mathrm {G} }=m_{\mathrm {e} }^{2}\ .$ This shows that the gravitational coupling constant can be thought of as the analogue of the fine-structure constant (also expressed in Planck units, extended to include the normalization 4πε0 = 1):

$\alpha =e^{2}\approx 7.29735\times 10^{-3}\ .$ While the fine-structure constant measures the electrostatic repulsion between two particles with equal charge, the magnitude of which is equal to the square of the elementary charge, this gravitational coupling constant measures the gravitational attraction between two electrons. This is one manner of expressing that "Gravity is a far weaker force than the electromagnetic interaction" since αG is 42 orders of magnitude smaller than α.[clarification needed]

## Measurement and uncertainty

There is no known way of measuring αG directly, and CODATA does not report an estimate of its value. The above estimate is calculated from the CODATA values of me and mP.

While me is known to one part in 3000000000 and ħ has an exact value by definition, mP is only known to one part in 100000 (mainly because G is known to only one part in 50000). Hence αG is known to only four significant digits. By contrast, the fine structure constant α can be measured via the anomalous magnetic dipole moment of electron with a precision of a few parts per 1010. Also, the meter and second are now defined in a way such that c has an exact value by definition. Hence the precision of αG depends only on that of G, and me.

## Related definitions

Let μ = mp/me = 1836.15267247(80) be the dimensionless proton-to-electron mass ratio, the ratio of the rest mass of the proton to that of the electron. Other definitions of αG that have been proposed in the literature differ from the one above merely by a factor of μ or its square;

• If αG is defined using the mass of one electron, me, and one proton (mp = μme), then αG = 1.752×10−45μ = 3.217×10−42, and α/αG ≈ 1039. α/αG defined in this manner is C in Eddington (1935: 232), with Planck's constant replacing the "reduced" Planck constant;
• (4.5) in Barrow and Tipler (1986) tacitly defines α/αG as e2/Gmpme ≈ 1039. Even though they do not name the α/αG defined in this manner, it nevertheless plays a role in their broad-ranging discussion of astrophysics, cosmology, quantum physics, and the anthropic principle;
• N in Rees (2000) is α/αG = α/1.752×10−45μ2 = α/5.906×10−39 ≈ 1036, where the denominator is defined using a pair of protons.

## Discussion

There is an arbitrariness in the choice of which particle's mass to use (whereas α is a function of the elementary charge, αG is normally a function of the electron rest mass). In this article αG is defined in terms of a pair of electrons unless stated otherwise. And while the relationship between αG and gravitation is somewhat analogous to that of the fine-structure constant and electromagnetism, the important difference is that the standard definition of αG describes a ratio in terms of electron mass alone, whereas the fine-structure constant relates to the elementary charge, which is a quantum that is independent of the choice of particle.

The electron is a stable particle possessing one elementary charge and one electron mass. Hence the ratio α/αG measures the relative strengths of the electrostatic and gravitational forces between two electrons. Expressed in natural units (so that G = c = ħ = ε0 = 1), the constants become α = e2/ and αG = me2/, resulting in a meaningful ratio α/αG = (e/me)2
. Thus the ratio of the electron charge to the electron mass (in natural units) determines the relative strengths of electromagnetic and gravitational interaction between two electrons.

α is 43 orders of magnitude greater than αG calculated for two electrons (or 37 orders, for two protons). The electrostatic force between two charged elementary particles is vastly greater than the corresponding gravitational force between them. The gravitational attraction among elementary particles, charged or not, can hence be ignored. Gravitation dominates for macroscopic objects because they are electrostatically neutral to a very high degree.

αG has a simple physical interpretation: it is the square of the electron mass, measured in units of Planck mass. By virtue of this, αG is connected to the Higgs mechanism, which determines the rest masses of the elementary particles. αG can only be measured with relatively low precision, and is seldom mentioned in the physics literature.

Because

$\alpha _{\text{G}}={\frac {Gm_{\text{e}}^{2}}{\hbar c}}=\left(t_{\text{P}}\omega _{\text{C}}\right)^{2}$ where tP is the Planck time, αG is related to ωC, the Compton angular frequency of the electron.