From: Sam Wormley on
What black holes teach about strongly coupled particles

http://ptonline.aip.org/journals/doc/PHTOAD-ft/vol_63/iss_5/29_1.shtml?type=PTALERT&bypassSSO=1

Classical systems with strongly interacting particles can, in principle,
have zero viscosity. But string theory and experiment suggest that in
the quantum world, the viscosity can be only so low.