Fig. 13: Dispersion relation in solid (open circles) and liquid (filled circles) water as obtained from the fits of the inelastic X-ray scattering data. The red line indicates the best linear fit from Q = 4 to 10 nm-1and the slope corresponds to the speed of sound of 3200 ± 100 m/s. The blue line has a slope corresponding to 4000 m/s, i.e. the low frequency velocity of sound in ice Ih. The orange line has a slope corresponding to 1500 m/s, i.e. the hydrodynamic sound velocity in liquid water.