Calculations of the shear viscosity of the quark-gluon plasma (QGP), observed in heavy ions collisions, are performed via the AdS/CFT correspondence [ref1]. One finds that the shear viscosity of the CFT, on the boundary of an AdS space time with a black hole in its interior, is equal to the shear viscosity of the fluid theory living on the black hole horizon surface. This quantity in turn can be calculated with considering the cross-section for graviton absorption [ref2] with the horizon in the limit that the frequency? (low-energy limit).
The end results suggest that the fluid on the horizon (and its dual fluid the QGP) is as close to being a "perfect" fluid as Nature allows. What is found is that the shear viscosity? Of the horizon is proportional to its surface entropy density s. It is conjectured that there is a lower bound:
\frac{\eta}{s} \ge \frac{1}{4\pi}
(In natural units) for the viscosity to entropy ratio for any hydro dynamical system)
These results would seem to imply that the evolution of the QGP cannot be given by a unitary theory. Possibly this is just a naive misinterpretation of the physics on my part. If unitarily in quantum gravity is presumed to be sacrosanct then how does one reconcile the dissipative behaviour of the field theory of a QGP and a black hole horizon with the demands of unitarily?