Figure 8

Figure 8: The retarded, simultaneous, and advanced points on a world line $γ$ . The retarded point $′ x ≡ z(u)$ is linked to $x$ by a future-directed null geodesic. The simultaneous point $¯x ≡ z(t)$ is linked to $x$ by a spacelike geodesic that intersects $γ$ orthogonally. The advanced point $x′′ ≡ z(v)$ is linked to $x$ by a past-directed null geodesic.