where ξ = x 2 − x 1 is the separation vector between two geodesics, ⁠ D / ds ⁠ (not just ⁠ d / ds ⁠) is the covariant derivative, and R βγδ is the Riemann curvature tensor, containing the Christoffel symbols. In other words, the geodesic deviation equation is the equation of motion for masses in curved spacetime, analogous to the Lorentz force equation for charges in an electromagnetic field.