Figure 29: Past light cone of an event in the spacetime (156) of a plane gravitational wave. The picture was produced with profile functions and . Then there is focusing in the -direction and defocusing in the -direction. In the (2 + 1)-dimensional picture, with the -coordinate not shown, the past light cone is completely refocused into a single point , with the exception of one generator . It depends on the profile functions whether there is a second, third, and so on, caustic. In any case, the generators leave the boundary of the chronological past when they pass through the first caustic. Taking the -coordinate into account, the first caustic is not a point but a parabola (“astigmatic focusing”) (see Figure 30). An electromagnetic plane wave (vanishing Weyl tensor rather than vanishing Ricci tensor) can refocus a light cone, with the exception of one generator, even into a point in 3 + 1 dimensions (“anastigmatic focusing”) (cf. Penrose [259] where a hand-drawing similar to the picture above can be found).