Some unforeseen aspects of electromagnetic and gravitational waves.

The foundations of the theory of gravitational waves in full non-linear general relativity were laid down over 55 years ago by Bondi, Sachs, Penrose, Newman and others. This well-established framework provides the anchor for numerical simulations of compact binary coalescences that are heavily used to characterize sources by the LIGO-VIRGO collaboration. Yet surprising aspects of the theory as well as limitations of its current use continue to emerge. I will illustrate some of the resulting conceptual tension with a few examples.