On the completeness of gravitational pp-wave spacetimes: A counterexample to the {Ehlers--Kundt} conjecture

This paper concerns the geometry of gravitational pp-wave spacetimes. We give a counterexample to the Ehlers–Kundt conjecture. In fact, we show that geodesic completeness is generic, in the sense of Baire category, among gravitational pp-waves. We also define a concept of ``universal pp-wave’', i.e. a pp-wave whose metric approximates that of every other gravitational pp-wave arbitrarily closely on arbitrarily large compact sets, and we show that there are geodesically complete universal pp-waves. Our machinery also suffices to give a new proof of the polynomial case of the Ehlers–Kundt conjecture (previously proved by Flores and Sánchez).