Gravitational wave defocussing in quadratic gravity

We demonstrate that Huygens' principle for gravitational waves fails in quadratic gravity models that exhibit conformal symmetry at high energies. This results in the blurring of gravitational wave signals over finite timescales related to the energy scale of new physics $M_{\star}$. Furthermore, on very small scales the gravitational wave Green's function reduces to that of the wave equation in two dimensions. In principle, $M_{\star}$ could be constrained directly from gravitational wave observations.