Newtonian gravitation can be written as the theory of a scalar field, Φ, which is the gravitational potential in joules per kilogram of the gravitational field g = −∇Φ, see Gauss's law for gravity ∇ ∇ 2 Φ Φ ( x → → , t ) = 4 π π G ρ ρ ( x → → , t ) where ρ is the mass density. The orbit of a free-falling particle satisfies x → → ¨ ¨ ( t ) = g → → = − − ∇ ∇ Φ Φ ( x → → ( t ) , t ) .