(note, however, that the overall sign is arbitrary, and that Newman & Penrose worked with a "timelike" metric signature of ( + , − − , − − , − − ) ). In empty space, the Einstein Field Equations reduce to R α α β β = 0 . From the definition of the Weyl tensor, we see that this means that it equals the Riemann tensor, C α α β β γ γ δ δ = R α α β β γ γ δ δ . We can make the standard choice for the tetrad at infinity: