In 1938 Petr Kapitza, investigating properties of the low temperature phase of liquid 4
He, discovered that viscosity of the liquid is more than 104
times smaller, than that of all known liquids. Kapitza concluded, that the liquid is in a new state of matter, a “superfluid”, which to some extend analogous to superconductors. Landau (1941) explained the phenomenon and predicted several its unusual properties. It occurs, that in a superfluid in any point of space at finite temperatures simultaneously exists two flow with different velocities. One is “normal” and has finite viscosity and the second one is “superfluid”, which viscosity is exactly zero. This results in presence of two type of sound, which were discovered experimentally in 1946.