The gravitational field is conservative and can therefore be expressed as the negative gradient of the gravitational potential: g = − − ∇ ∇ V , ∇ ∇ ⋅ ⋅ g = ∇ ∇ ⋅ ⋅ ( − − ∇ ∇ V ) = − − ∇ ∇ 2 V , ⟹ ⟹ ∇ ∇ 2 V = − − ∇ ∇ ⋅ ⋅ g .