# nLab gravitational constant

Newtons gravitational constant

### Context

#### Gravity

gravity, supergravity

# Newton's gravitational constant

## Idea

Newton's gravitational constant provides a unit conversion between units of spacetime and units of mass/energy.

## Motivation

According to Einstein's theory of general relativity, the Einstein tensor $G$ of the pseudoRiemannian metric $g$ on spacetime is proportional to the stress-energy tensor $T$ of the matter and radiation in spacetime. (For simplicity, let any cosmological constant or other dark energy be included in $T$.) The simplest way to state this proportion is as an equality:

$G = T$

or (with tensor indices)

$G_{a,b} = T_{a,b} .$

However, conventional units of measurement don't allow this equality, and so we write

$G_{a,b} = k T_{a,b} ,$

where the constant $k$ is about $1.67732 \times 10^{-9} \m^3 \kg^{-1} \s^{-2}$ in SI unit?s. (Depending on how one handles the conversion between the space and time components of $G$ and $T$, there could be extra factors of the speed of light constant $2.99792458 \times 10^8 \m \s^{-1}$.)

This factor is essentially the gravitational constant; however, the gravitational constant is traditionally taken to be smaller by a factor of $8 \pi$:

$G = 6.67408(31) \times 10^{-11} \m^3 \kg^{-1} \s^{-2},$

where the standard uncertainty (about 46ppm) is in parentheses. Yes, the same letter is used for this constant as for the Einstein tensor! Although this yields

$G_{a,b} = 8 \pi G T_{a,b}$

in general relativity, it gives the simplest formula for the fictional force? of gravity in the nonrelativistic limit:

$F = G \frac{m_1 m_2}{r^2}$

for the force exerted by either mass $m_i$ on the other at a distance $r$. (It was in this context that Newton used the constant.)

## Measurement

There is good stuff to say here about how we only know this constant to about 6 significant digits.