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.
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