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.