The non-linear dynamics of gravitational wave propagation in spacetime can contain drastic new phenomenology that is absent from the linearised theory. In this talk, I will probe the non-linear radiative regime of Horndeski gravity by making use of disformal field redefinition. I will discuss how disformal transformations alter the properties of congruences of geodesics and in particular how they can generate disformal gravitational waves at the fully non-linear level. I will illustrate this effect by presenting a new exact radiative solution in Horndeski gravity describing a scalar pulse. Analysing the non-linear dynamics of this new radiative solution will show that it contains tensorial gravitational waves generated by a purely time-dependent scalar monopole. This intriguing result is made possible by the higher-order nature of Horndeski gravity.
