Figure 8

Figure 8: The retarded, simultaneous, and advanced points on a world line g. 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 g orthogonally. The advanced point x'' =_ z(v) is linked to x by a past-directed null geodesic.