Assuming that sound waves are the most important excitations in helium-4 at low temperatures, he showed that helium-4 flowing past a wall would not spontaneously create excitations if the flow velocity was less than the sound velocity. Source: Internet
Because the group velocity for such a field is superluminal, naively it appears that its excitations propagate faster than light. Source: Internet
By our current understanding, these elementary particles are excitations of the quantum fields that also govern their interactions. Source: Internet
From the momentum and flow velocity of the excitations he could then define a "normal fluid" density, which is zero at zero temperature and increases with temperature. Source: Internet
From the point of view of quantum field theory, particles are identical if and only if they are excitations of the same underlying quantum field. Source: Internet
Inelastic scattering is useful for probing such excitations of matter, but not in determining the distribution of scatterers within the matter, which is the goal of X-ray crystallography. Source: Internet