IT is generally assumed, for any material, that sound travels faster i
n the solid phase than in the liquid. This is true at least for waves
of macroscopic wavelength, the dynamics of which depend on the elastic
properties of the medium and, hence, on the presence or absence of lo
ng-range order. But at wavelengths approaching the typical interpartic
le distance, the dynamics should become insensitive to the long-range
organization of the medium. Here we report inelastic X-ray scattering
measurements in water and ice, which show that sound waves with wavele
ngths between 0.5 and 3 nm propagate at the same velocity in both phas
es. The observed sound speed, which is greater than twice the hydrodyn
amic speed of sound in water, but less than that in ice, agrees with v
alues obtained in previous measurements of 'fast sound' in liquid D2O
(ref. 1) and H2O (ref. 2). These results show that, despite the fundam
ental structural and dynamical differences between water and ice, the
dynamical response of the two phases is strikingly similar at very sho
rt wavelengths.