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.

It is as if waves do not like to stay still; on the contrary, they really like to move from the moment they are created. However, there are some instances in which perturbations remain spatially localized, ignoring their propagating counterparts entirely, even when sharing the same space. These forever localized perturbations are called BICs (from bound states in the continuum). I discuss these in a new article published in Physica Scripta.