Figure 18: Instantaneous wave fronts in the spacetime of a non-transparent Barriola–Vilenkin monopole with . The picture shows in 3-dimensional space the intersections of the past light cone of some event with four hypersurfaces , at values . Only one half of each instantaneous wave front and of the monopole is shown. When the wave front passes the monopole, a hole is pierced into it, then a tangential caustic develops. The caustic of each instantaneous wave front is a point, the caustic of the entire light cone is a spacelike curve in spacetime which projects to part of the axis in 3-space.