Figure 30

Figure 30: “Small wave fronts” of the light cone in the spacetime (156 ) of a plane gravitational wave. The picture shows the intersection of the light cone of Figure 29 with the lightlike hyperplane $u = constant$ for three different values of the constant: (a) exactly at the caustic (parabola), (b) at a larger value of $u$ (hyperbolic paraboloid), and (c) at a smaller value of $u$ (elliptic paraboloid). In each case, the hyperplane $u = constant$ does not intersect the one generator $λ$ tangent to $∂v$ ; all other generators are intersected transversely and exactly once.