How does the liquid flow

17.01.2012 09:46

The most fluid liquid in the universe

Dr. Florian Aigner Public Relations Office
technical University of Vienna

The quark-gluon plasma, which was theoretically investigated at the Technical University of Vienna, could be more fluid than previous theories allow. The lower limit for the viscosity of liquids previously considered valid can be broken.

How fluid is the perfect liquid? Particle physicists at the Vienna University of Technology are concerned with this question. The “most liquid of all liquids” is not water, but the extremely hot quark-gluon plasma, which is produced during high-energy particle collisions in the Large Hadron Collider at CERN. New calculation results at the Vienna University of Technology now show: This quark-gluon plasma can be significantly thinner than was previously thought possible. This result was published on January 12th in the journal "Physical Review Letters" and was awarded an "Editors' Selection" as a particularly noteworthy publication

As fluid as physics allows

How thick or thin a substance flows is indicated by the viscosity: Viscous liquids (such as honey) are thick and have strong internal friction forces, thin liquids have a low viscosity, and quantum liquids such as superfluid helium can reach extremely low viscosities. In 2004, a theoretical result after the quantum theory was supposed to require an absolute lower limit for viscosity caused a sensation. Using string theory methods, the ratio of viscosity to entropy density (a measure of the “disorder” in a liquid) was? / 4? (with the Planck constant?) calculated as the lowest possible bound. While superfluid helium remains well above this limit, a value just above this limit was measured on the quark-gluon plasma in 2005. In certain cases, however, this record for viscosity can still be beaten by a quark-gluon plasma, as Dominik Steineder from the Institute for Theoretical Physics at the Vienna University of Technology discovered in the course of his Dotor work together with Professor Anton Rebhan

Black holes and particle collisions

The viscosity of a quark-gluon plasma cannot be calculated directly. Its behavior is so complicated that you have to resort to very special tricks, as Anton Rebhan explains: “With the help of string theory, the quantum field theory of quark-gluon plasmas can be related to the physics of black holes in higher dimensions. So we solve equations from string theory and then transfer the results to the quark-gluon plasma. ”The lower limit for the viscosity that was previously considered valid was calculated in a very similar way. However, in previous calculations it was assumed that the plasma is symmetrical and looks the same from all sides - that is, it is “isotropic”, as they say in physics. “A plasma that is created in a collision in a particle accelerator is not isotropic at the very beginning,” emphasizes Anton Rebhan. These particles are finally accelerated along a certain direction and brought to collision - the resulting quark-gluon plasma therefore shows different properties, depending on the direction from which it is viewed.

Lower limit broken

The TU physicists now found a way to incorporate this directional dependency into the formulas - and, completely surprisingly, it turned out that the viscosity is no longer restricted by this. "The viscosity also depends on a few other physical parameters - but it can be lower than the value that was previously thought to be the absolute lower limit," explains Dominik Steineder. The quark-gluon-plasma experiments now started at CERN will allow these theoretical predictions to be tested.

In any case, word of the remarkable research result quickly got around in the physics community even before the publication was officially published: Dominik Steineder, currently still a PhD student at TU Wien, received two job offers from research institutes abroad just before the publication of the paper and will now Year at the CEA Saclay near Paris, where quark-gluon plasma is also theoretically researched. Steineder assures us that he will continue to work with the Vienna University of Technology from there in the future.

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Inquiry note:
Prof. Anton Rebhan
Institute for Theoretical Physics
technical University of Vienna
Wiedner Hauptstrasse 8-10, 1040 Vienna
[email protected]

Features of this press release:
Physics / astronomy
research results