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When a force is applied parallel to the top surface of a solid, it resists the applied force due to the internal frictional forces between the layers of the solid known as shearing resistance. However, when the force is removed, the shearing forces restore the original shape of the solid. Other deformation forces also cause temporary changes in shape if the forces are not beyond a threshold magnitude. Solids tend to retain their shape, making the study of their rest and motion easier. Beyond...

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An Analog Macroscopic Technique for Studying Molecular Hydrodynamic Processes in Dense Gases and Liquids

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Is there a "most perfect fluid" consistent with quantum field theory?

Thomas D Cohen¹

¹Department of Physics, University of Maryland, College Park, Maryland 20742, USA.

Physical Review Letters

|August 7, 2007

Summary

A conjecture proposed a universal lower bound for the ratio of shear viscosity to entropy density (eta/s) in fluids. This study demonstrates a counterexample in quantum chromodynamics (QCD), suggesting no such universal bound exists for metastable fluids.

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Area of Science:

High-energy physics
Fluid dynamics
Quantum field theory

Background:

A conjecture suggests a universal lower bound for the ratio of shear viscosity to entropy density (eta/s) for all fluids.
Theoretical counterexamples exist for nonrelativistic systems by increasing species number.
The study investigates if relativistic quantum field theory preserves this universal bound.

Purpose of the Study:

To determine if relativistic quantum field theory (QFT) imposes a universal lower bound on the shear viscosity to entropy density ratio (eta/s).
To explore the possibility of constructing a counterexample to the eta/s bound within a QFT framework.

Main Methods:

Analysis of a metastable gas of heavy mesons within a controlled regime of Quantum Chromodynamics (QCD).
Application of conservative assumptions to evaluate the system's properties within QFT.

Main Results:

A metastable gas of heavy mesons in QCD provides a realization of a theoretical counterexample to the eta/s bound.
This system is consistent with a well-defined underlying relativistic quantum field theory.

Conclusions:

Quantum field theory does not appear to impose a universal lower bound on the ratio of shear viscosity to entropy density (eta/s).
Metastable fluids, specifically heavy meson gases in QCD, can violate the conjectured eta/s bound.