· The conundrum in the title is whether to treat radiation in the lab frame or the comoving frame in a radiation-hydrodynamic problem
· Several of the difficulties are associated with combining a somewhat relativistic treatment of radiation with a non-relativistic treatment of hydrodynamics
· The principal problem is a tradeoff between easily obtaining the correct diffusion limit and describing free-streaming radiation with the correct wave speed
· The computational problems of the comoving-frame formulation in more than one dimension, and the difficulty of obtaining both exact conservation and full u/c accuracy argue against this method
· As the interest in multi-D increases, as well as the power of computers, the lab-frame method is becoming more attractive
· The Monte Carlo method combines the advantages of both lab-frame and comoving-frame approaches, its only disadvantage being cost
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