This paper by G. Policastro, D.T. Son, and A.O. Starinets explores the shear viscosity of strongly coupled $\mathcal{N} = 4$ supersymmetric Yang-Mills (SYM) plasma at finite temperatures. The authors leverage the AdS/CFT correspondence, which relates the SYM theory to classical ten-dimensional gravity on black three-branes, to compute the shear viscosity $\eta$. They show that $\eta$ is proportional to the area of the horizon of the black brane, specifically given by $\eta = \frac{\pi}{2} N^2 T^3$, where $N$ is the number of colors and $T$ is the temperature. This result is derived by relating the shear viscosity to the absorption cross section of low-energy gravitons falling perpendicularly onto near-extremal black three-branes. The authors also discuss the implications of this result for the behavior of the shear viscosity in the strong coupling limit, noting that it approaches a constant value. The paper highlights the challenges in computing transport coefficients in strongly coupled systems and suggests that the hydrodynamic theory may not be applicable at timescales or spatial distances much smaller than the relaxation time $T^{-1}$.This paper by G. Policastro, D.T. Son, and A.O. Starinets explores the shear viscosity of strongly coupled $\mathcal{N} = 4$ supersymmetric Yang-Mills (SYM) plasma at finite temperatures. The authors leverage the AdS/CFT correspondence, which relates the SYM theory to classical ten-dimensional gravity on black three-branes, to compute the shear viscosity $\eta$. They show that $\eta$ is proportional to the area of the horizon of the black brane, specifically given by $\eta = \frac{\pi}{2} N^2 T^3$, where $N$ is the number of colors and $T$ is the temperature. This result is derived by relating the shear viscosity to the absorption cross section of low-energy gravitons falling perpendicularly onto near-extremal black three-branes. The authors also discuss the implications of this result for the behavior of the shear viscosity in the strong coupling limit, noting that it approaches a constant value. The paper highlights the challenges in computing transport coefficients in strongly coupled systems and suggests that the hydrodynamic theory may not be applicable at timescales or spatial distances much smaller than the relaxation time $T^{-1}$.