The notion of a perfect fluid arises in many fields of physics. The term can be applied to any system that is in local equilibrium and has negligible shear viscosity η. In everyday life, viscosity is a familiar property associated with the tendency of a substance to resist flow. From a microscopic perspective, it is a diagnostic of the strength of the interactions between a fluid’s constituents. The shear viscosity measures how disturbances in the system are transmitted to the rest of the system through interactions. If those interactions are strong, neighboring parts of the fluid more readily transmit the disturbances through the system (see figure 1). Thus low shear viscosities indicate significant interaction strength. The ideal gas represents the opposite extreme—it is a system with no interactions and infinite shear viscosity.
Perfect fluids are easy to describe, but few substances on Earth actually behave like them. Although often...
