certainly engulf more material as they expand into their surroundings; because the total amount of momentum in the system, which is realistically finite, must be conserved, that entrainment will then result in an overall deceleration of the outflow that cannot be captured in the two-stream model.

In view of these unphysical properties of the two-stream treatment – the vanishing of the mass density of scatterers along the contact discontinuity and the lack of entrainment – we present here an alternative boundary layer scenario to describe the interaction of a relativistic, radiation-dominated jet with its surroundings. This “free-streaming jet” model, which has a well-known counterpart in the non-relativistic, incompressible limit [(see Chapter 10, Section 12 of]{} @kun08[)]{}, assumes that the jet is injected through a narrow opening into a static, homogenous medium, and that far enough from that opening the entire system can be modeled as a single fluid that is independent of the details at the injection point. By considering the jet and the ambient medium as one fluid we obviate the need for a contact discontinuity, which we demonstrate allows the density of scatterers to remain finite throughout the flow and for the jet to entrain material.

In section 