Boson Interferometry after SPS and before RHIC
  We re--examine the connection between interferometry and the Wigner representation for source freeze--out, and continuous emission. At the operator level, two equivalent representations of the two--particle spectrum are found, which contradict the standard expression of kinetic theory. The discrepancy is resolved using two toy models. Further, we revisit interferometry in scale--invariant one--dimensional hydrodynamics, and argue that recent experimental results are evidence for a short kaon emission time. Using two exactly calculable models of two-- and three--dimensional flow, it is shown that the saddle point approximation, which is reasonable for one--dimensional flow, is no longer adequate. In these models the scaling law is altered, and we argue that such qualitative trends, together with other observables, are vital if one is to draw conclusions about the unknown source parameters. 
A. Makhlin, E. Surdutovich, G. Welke