Quantum width of spherically symmetric black hole horizons is defined by signal escape timing and calculated in perturbative quantum gravity to often greatly exceed the Planck length, scaling as sqrt(l_P r_s^2 / sigma_perp) for Schwarzschild patches.
Holography and hydrodynamics: diffusion on stretched horizons
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abstract
We show that long-time, long-distance fluctuations of plane-symmetric horizons exhibit universal hydrodynamic behavior. By considering classical fluctuations around black-brane backgrounds, we find both diffusive and shear modes. The diffusion constant and the shear viscosity are given by simple formulas, in terms of metric components. For a given metric, the answers can be interpreted as corresponding kinetic coefficients in the holographically dual theory. For the near-extremal Dp, M2 and M5 branes, the computed kinetic coefficients coincide with the results of independent AdS/CFT calculations. In all the examples, the ratio of shear viscosity to entropy density is equal to \hbar/(4\pi k_B), suggesting a special meaning of this value.
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A holographic QCD model with dilaton-dependent Gauss-Bonnet corrections matches lattice thermodynamics and yields non-monotonic η/s plus a critical endpoint.