A recent article published in Physical Review B by Nicolis and Penco arrives at an unconventional physical assertion: that non-relativistic sound waves carry gravitational mass.
This claim should sound odd to anyone who has taken an undergraduate mechanics course, as one of the first physical assertions about waves is that they transport energy through matter (which possesses gravitational mass), but not having any mass of it's own. The original paper focuses on the focused effect of such behavior on condensed matter solids, though they authors assert the result is more general.
If we assume their result to be true, then I find the following train of thought interesting. Given a sound wave propagating through a solid that itself possesses gravitational mass, to what extent do the sound wave and the medium interact gravitationally? Are there any circumstances (very "loose" media, such as IGM) under which that interaction would play an important role in the dynamics of the resulting wave?
Though the above questions are tempting - I'll wait until Nicolis and Penco either experimentally observe their proposed effect, or can present a more general version of their initial results. Either way - interesting!