For most fluids observable on Earth, traditional fluid mechanics based on Newtonian mechanics is sufficient. However, as the fluid velocity approaches the speed of light or moves through strong gravitational fields, or the pressure approaches the energy density ( P ∼ ∼ ρ ρ ), these equations are no longer valid. Such situations occur frequently in astrophysical applications. For example, gamma-ray bursts often feature speeds only 0.01 % % less than the speed of light, and neutron stars feature gravitational fields that are more than 10 11 times stronger than the Earth's. Under these extreme circumstances, only a relativistic treatment of fluids will suffice.