Energy-momentum density and pressure relations for a relativistic ideal gas with a bulk motion

Abstract

We derive here, from first principles, the energy-momentum densities of a perfect fluid, in the form of an ideal molecular gas, in an inertial frame where the fluid possesses a bulk motion. We begin from the simple expressions for the energy density and pressure of a perfect fluid in the rest frame of the fluid, where the fluid constituents (gas molecules) may possess a random motion, but no bulk motion. From a Lorentz transformation of the velocity vectors of molecules, moving along different directions in the rest frame of the fluid, we compute their energy-momentum vectors and number densities in an inertial frame moving with respect to the rest frame of the liquid. From that we arrive at the energy-momentum density of the fluid in a frame where it has a bulk motion. This way we explicitly demonstrate how a couple of curious pressure-dependent terms make appearance in the energy-momentum density of a perfect fluid having a bulk motion. In addition to an ideal molecular gas, we compute the energy-momentum density for a photon gas also, which of course matches with the energy-momentum density expression obtained for a molecular gas having ultra-relativistic random motion.