This review highlights some of the lessons that the holographic gauge/gravity duality has taught us regarding the behavior of the shear viscosity to entropy density in strongly coupled field theories. The viscosity to entropy ratio has been shown to take on a very simple universal value in all gauge theories with an Einstein gravity dual. Here we describe the origin of this universal ratio, and focus on how it is modified by generic higher derivative corrections corresponding to curvature corrections on the gravity side of the duality. In particular, certain curvature corrections are known to push the viscosity to entropy ratio below its universal value. This disproves a longstanding conjecture that such a universal value represents a strict lower bound for any fluid in nature. We discuss the main developments that have led to insight into the violation of this bound, and consider whether the consistency of the theory is responsible for setting a fundamental lower bound on the viscosity to entropy ratio.

SERA CREMONINI

Modern Physics Letters B

MPLB 为凝聚态物理、统计物理以及原子、分子和光学物理中重要和有用的研究成果的快速传播开辟了渠道。重点关注当前感兴趣的主题，例如冷原子和分子、新拓扑材料和相，以及新型低维材料。该杂志还包含一个简要评论部分，目的是发表关于最新实验发现和紧急新理论发展的简短报告。

Modern Physics Letters B provides a rapid dissemination channel for significant and useful research findings in condensed matter physics, statistical physics, and atomic, molecular, and optical physics. The journal focuses on current topics of interest, such as cold atoms and molecules, new topological materials and phases, and novel low-dimensional materials. It also includes a brief commentary section aimed at publishing short reports on recent experimental discoveries and urgent new theoretical developments.

《Modern Physics Letters B》为凝聚态物理、统计物理以及原子、分子和光学物理领域的重要研究成果提供了快速传播的渠道。该期刊重点关注当前的热门主题，如冷原子与分子、新型拓扑材料及其相态，以及新型低维材料。此外，期刊还设有简要评论部分，旨在发表关于最新实验发现和紧急新理论发展的简短报告。

Category	Quartile
物理与天体物理	4
物理与天体物理, 物理应用	4
物理与天体物理, 物理凝聚态物理	4
物理与天体物理, 物理数学物理	4

THE SHEAR VISCOSITY TO ENTROPY RATIO: A STATUS REPORT

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Journal Info

Category	Quartile
PHYSICS, MATHEMATICAL	2