# Gravitational wave defocussing in quadratic gravity

## TL;DR

This paper shows that in quadratic gravity models with conformal symmetry, gravitational waves experience defocussing, leading to signal blurring over finite timescales, which could be used to constrain new physics energy scales.

## Contribution

It reveals the failure of Huygens' principle in quadratic gravity and connects gravitational wave signal blurring to high-energy physics scales.

## Key findings

- Huygens' principle fails in quadratic gravity models.
- Gravitational wave signals blur over finite timescales.
- Green's function reduces to 2D wave equation at small scales.

## Abstract

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.