that describes the viscous interaction of a radiation-dominated, relativistic jet with its surrounding medium. In that analysis we treated the jet and its surroundings as two separate fluids, interacting with one another via small anisotropies in the comoving radiation field that are explicitly accounted for in the equations of radiation hydrodynamics in the viscous limit. This two-stream approximation, in agreement with the non-relativistic analysis of @ara92, demonstrates the manner in which the shear between the two fluids carves out a region of low density material within the boundary layer between them. We also deduced the dependence of the boundary layer thickness on the asymptotic properties of the jet and the ambient medium.

These models also show, however, that the contact discontinuity separating the jet and its surroundings, necessary for maintaining their respective identities, results in the likely unphysical vanishing of the density of scatterers along that surface of separation. The contact discontinuity also prevents the jet from entraining ambient material; since the jet in the two-stream model we considered had an infinite amount of momentum, any entrainment or lack thereof is formally inconsequential to the evolution of the system. However, realistic jets – those with finite extent – will almost 