68 Chapter 2: Motivation ratio, η/s . By comparing the correlation function for the low energy scatter- ing cross section used in the motivation for the correspondence to the Kubo formula for the viscosity of a strongly-coupled SYM plasma [333] one finds that where [334] found that the cross section for extended p -branes has a universal low energy limit of the area of the horizon. Using this with Eq. (2.67), one quite trivially arrives at the famous strong coupling limit of viscosity to entropy The result led to another conjecture [335], that this is in fact a universal lower bound. As noted in Section 2.1.1, qualitative estimates of the viscosity at Rhic [121] appear to require just such values of η/s ∼ . 1 ∼ 1 / 4 π . This gives great impetus to look for additional observables calculable in AdS/CFT. In fact we can learn a great deal by extending the AdS/CFT conjecture further by breaking some of the field theory supersymmetry encoded in the S 5 part of the geometry. Specifically the introduction of M ≪ N D 7 branes that fill space from the asymptotic 3 + 1 D Minkowski boundary to some finite distance above r = 0 and wrapping an equatorial S 3 ⊂ S 5 corresponds to introducing M fundamental flavors in the gauge theory [336]. The M ≪ N requirement allows the D 3 branes to backreact and warp spacetime in the large N limit while the D 7 branes act only as probes. An open string ending η s = 1 4 π . (2.69) η = σ$_{abs}$ (0) 16 πG$_{N}$ = A 16 πG$_{N}$ , (2.68)