On the right-hand side, κ κ is a constant and T μ μ ν ν 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 κ is found to be κ κ = 8 π π G c 4 , where G is the Newtonian constant of gravitation and 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,