Chaotic motion in <B> pp</B>-wave spacetimes

We investigate geodesics in non-homogeneous vacuum pp-wave solutions and demonstrate their chaotic behaviour by rigorous analytic and numerical methods. For the particular class of solutions considered, distinct `outcomes' (channels to infinity) are identified, and it is shown that the boundary between different outcomes has a fractal structure. This seems to be the first example of chaos in exact radiative spacetimes.