Nearly perfect fluidity: from cold atomic gases to hot quark gluon plasmas

Abstract

Shear viscosity is a measure of the amount of dissipation in a simple fluid. In kinetic theory shear viscosity is related to the rate of momentum transport by quasiparticles, and the uncertainty relation suggests that the ratio of shear viscosity η to entropy density s in units ofh/k B is bounded by a constant. Here,h is Planck's constant and k B is Boltzmann's constant. A specific bound has been proposed on the basis of string theory where, for a large class of theories, one can show that η/s ≥h/(4πk B ). We will refer to a fluid that saturates the string theory bound as a perfect fluid. In this review we summarize theoretical and experimental information on the properties of the three main classes of quantum fluids that are known to have values of η/s that are smaller thanh/k B . These fluids are strongly coupled Bose fluids, in particular liquid helium, strongly correlated ultracold Fermi gases, and the quark gluon plasma. We discuss the main theoretical approaches to transport properties of these fluids: kinetic theory, numerical simulations based on linear response theory, and holographic dualities. We also summarize the experimental situation, in particular with regard to the observation of hydrodynamic behavior in ultracold Fermi gases and the quark gluon plasma.

Effective actions for anomalous hydrodynamics

“…The reasons for this are many-fold: theoretical results from the study of holographic fluids via the fluid/gravity correspondence [1], experimental results for strongly coupled systems in the hydrodynamic limit such as the quark-gluon plasma or cold atoms at unitarity, cf., [2] and developments in condensed matter theory have driven much of the recent progress. Spurred by these and other developments we have now come to appreciate that the microscopic details of the underlying quantum dynamics leave behind indelible signatures in hydrodynamic transport.…”

“…2 In addition accumulation of evidence from holography [14][15][16] and analysis of free field theories [17] lead to a near-complete picture of anomalous transport induced by flavour anomalies (see also [18,19] for related discussions and [20,21] for general symmetry based group-theoretic approach).…”

Thermal photons in QGP and non-ideal effects

“…Non zero value of bulk viscosity refers to Eqs. [18][19] and non zero shear viscosity is calculated from Eq. [20].…”

“…The ratio of the shear viscosity η to the entropy density s i.e. η/s is around 1/4π which is the smallest for any known liquid in the nature [19]. In fact the arguments based on AdS-CFT suggest that the values of η/s can not become * jeet@prl.res.in † hm@prl.res.in ‡ skv@prl.res.in lower than 1/4π.…”

Minimal submanifolds asymptotic to AdS 4 × S 2 in AdS 5 × S 5

“…The AdS/CF T correspondence [1][2][3][4] has provided the physics community with an unique perspective in the attempt to understand the various phases of strongly coupled quantum field theory, and together with the probe-brane extension thereof [5][6][7], it has been providing the theoretical playground that renders possible toy-model building for a wide domain of interests, ranging from the quark-gluon plasma [8,9], unitary Fermi gas [10,11], to high-T c superconductor [12,13].…”