ABSTRACT
By using the microscopic description of a superfluid which was outlined in the preceding sections, one can construct the complete system of hydrodynamic equations valid in this case. The fundamental assumptions upon which our whole analysis will be based are the following: The ordered motion of the excitations carries along with it only part of the liquid, characterized by the “normal” density ρ n. The remaining part, the “superfluid,” is characterized by the density ρ S = ρ − ρ n, and performs an independent motion. This latter motion has the important property of being irrotational. Thus in a superfluid, there can exist two simultaneous but independent motions, one normal and the other superfluid, with velocities v n and v S, with () curl v s = 0 https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9780429502897/20d72e0b-9d39-4175-acc6-a943547edf8a/content/eq234.tif"/>
