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.
