A phonon — a particle-like unit of vibration that can describe sound at very small scales — has a very slight negative mass, and that means sound waves travel upward ever so slightly, said Rafael Krichevsky, a graduate student in physics at Columbia University.

Why are phonons massless?

They are massless Goldstone bosons resulting from the violation of the continuous translational symmetry of free space, by the crystal lattice. Phonons must carry momentum because they interact with electrons and change the momentum of the latter.

Do photons have mass?

Light is composed of photons, so we could ask if the photon has mass. The answer is then definitely "no": the photon is a massless particle. According to theory it has energy and momentum but no mass, and this is confirmed by experiment to within strict limits.