  An important question of theoretical physics is whether sound is able to
propagate in vacuums at all and if this is the case, then it must lead to the
reinterpretation of one zero-restmass particle which corresponds to
vacuum-sound waves. Taking the electron-neutrino as the corresponding particle,
its observed non-vanishing rest-energy may only appear for neutrino-propagation
inside material media. The idea may also influence the physics of dense matter,
restricting the maximum speed of sound, both in vacuums and in matter to the
speed of light.
