When one mentions “waves,” we immediately think of a perturbation that propagates. This applies to waves on the shore, sound waves in the air or electromagnetic waves that we use to transmit information via optical fibers. The same idea of propagating perturbations applies to gravitational waves (GWs), which entered the mainstream media a decade ago after their first direct detection. These are perturbations of the elastic fabric that we are all embedded in, called spacetime.