the QGP must be due to nonperturbative and hence strong coupling physics that persists to at least $3T_c$.

The Minimal Viscosity of the QGP
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One intriguing theoretical possibility being explored in the literature [@Policastro:2002tn] is that the shear viscosity, $\eta$, in the strongly coupled QGP may saturate at a universal super-string bound, $\eta/\sigma=1/4\pi$. This conjectured duality between string theory and QCD may help to explain also the $\sim 20\%$ deviation of $P_{QCD}(T)$ from the ideal Stefan Boltzmann limit. The discovery of nearly perfect fluid flow of [*long wavelength*]{} modes with $p_T<1$ GeV at RHIC is certain to fuel more interest in this direction.

I propose that a simpler physical explanation of the lower bound on viscosity follows from the uncertainty principle, as derived in Eq.(3.3) of Ref. [@Danielewicz:ww]. Standard kinetic theory derivation of shear viscosity leads to $\eta=(\rho\langle p\rangle \lambda)/3$ where $\rho$ is the proper density, $\langle p \rangle$ is the average total momentum, and $\lambda$ is the momentum degradation transport mean free path. The uncertainty principle implies that quanta with average momentum components $\langle p \rangle$ cannot be localized to better than $\Delta x\sim
1/\langle p \rangle$. Therefore the momentum degradation mean free path cannot be defined more 