abstract index notation planetary orbits Newtonian constant of gravitation matter theory energy momentum energy–momentum tensor speed of light light weak gravity gravitation
On the right-hand side, 



κ


{\displaystyle \kappa }

 is a constant and 




T

μ
ν




{\displaystyle T_{\mu \nu }}

 is the energy–momentum tensor. All tensors are written in abstract index notation. Matching the theory's prediction to observational results for planetary orbits or, equivalently, assuring that the weak-gravity, low-speed limit is Newtonian mechanics, the proportionality constant 



κ


{\displaystyle \kappa }

 is found to be 



κ
=



8
π
G


c

4






{\textstyle \kappa ={\frac {8\pi G}{c^{4}}}}

, where 



G


{\displaystyle G}

 is the Newtonian constant of gravitation and 



c


{\displaystyle c}

 the speed of light in vacuum. When there is no matter present, so that the energy–momentum tensor vanishes, the results are the vacuum Einstein equations,
