The perfect liquid -- now even more perfect

January 17, 2012, Vienna University of Technology
The perfect liquid -- now even more perfect
Simulation of Quark-Gluon-Plasma (Copyright: CERN)

Ultra hot quark-gluon-plasma, generated by heavy-ion collisions in particle accelerators, is supposed to be the "most perfect fluid" in the world. Previous theories imposed a limit on how "liquid" fluids can be. Recent results at the Vienna University of Technology suggest that this limit can be broken -- making the world's "most perfect fluid" even more perfect.

How liquid can a fluid be? This is a question at the Vienna University of Technology have been working on. The "most perfect liquid" is nothing like water, but the extremely hot quark-gluon-plasma which is produced in heavy-ion collisions at the at CERN. New theoretical results at Vienna UT show that this quark-gluon plasma could be even less viscous than was deemed possible by previous theories. The results were published in and highlighted as an "editors' selection".

Highly viscous liquids (such as honey) are thick and have strong internal friction, quantum liquids, such as super fluid helium can exhibit extremely low viscosity. In 2004, theorists claimed that provided a lower bound for viscosity of fluids. Applying methods from , the lowest possible ratio of viscosity to the density was predicted to be ħ/4π (with the Planck-constant ħ). Even super fluid helium is far above this threshold. In 2005, measurements showed that quark-gluon-plasma exhibits a viscosity just barely above this limit. However, this record for low viscosity can still be broken, claims Dominik Steineder from the Institute for Theoretical Physics at Vienna UT. He obtained this remarkable result working as a PhD-student with Professor Anton Rebhan.

The viscosity of a quark-gluon plasma cannot be calculated directly. Its behavior is so complicated that very sophisticated methods have to be applied, says Anton Rebhan: "Using string theory, the quantum field theory of quark-gluon plasma can be related to the physics of black holes in higher dimensions. So we are solving equations from string theory and then transfer the results to the physics of the quark-gluon plasma." The previously established lower bound for viscosity was calculated in a very similar way. However, in these calculations the plasma was modeled to be symmetric and isotropic. "In fact, a plasma produced by a collision in a particle accelerator is not isotropic at the beginning", says Anton Rebhan. The particles are accelerated and collided along one specific direction – so the resulting plasma shows different properties, depending on the direction from which one looks at it.

The physicists at Vienna UT found a way to include this anisotropy in their equations – and surprisingly the limit for the viscosity can be broken in this new model. "The viscosity depends on several other physical parameters, but it can be lower than the number previously considered to be the absolute lower bound", Dominik Steineder explains. The on-going quark-gluon-experiments at will provide opportunities for testing the new theoretical predictions.

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2 / 5 (3) Jan 17, 2012
that article said absolutely nothing
2 / 5 (3) Jan 17, 2012
that article said absolutely nothing

3.3 / 5 (4) Jan 17, 2012
that article said absolutely nothing

about the most perfect liquid, beer.
1 / 5 (6) Jan 17, 2012
String theory remained unconfirmed during last forty years. Latest LHC results are disproving it instead.

To make it clear, I am quoting Daniel Friedan, a former string theorist at Rutgers from hep-th/0204131

"String theory has no credibility as a candidate theory of physics. Recognizing failure is a userful part of the scientific strategy. Only when failure is recognized can dead ends be abandoned and useable pieces of failed programs be recycled. Aside from possible utility, there is a responsibility to recognize failure. Recognizing failure
is an essential part of the scientific ethos. Complete scientific failure must be recognized eventually."

Apparently, the running train full of theorists cannot be stopped so easily - they need salaries, they must write articles..
1 / 5 (4) Jan 17, 2012
To sum up all the scientific mumbo jumbo in the article, they are saying "Scientists have found something more slippery than snot".

Simplified. Of course to produce this in any industrial quantities at this point is next to impossible.
2 / 5 (4) Jan 17, 2012
I am told String Theory is, so far, untestable. Plugging in equations, from an untestable theory, tells us what, exactly?
1 / 5 (2) Jan 17, 2012
The perfect liquid? It could be nothing else than the sauce that made that beof stew sublime.
4.2 / 5 (5) Jan 18, 2012
I am told String Theory is, so far, untestable. Plugging in equations, from an untestable theory, tells us what, exactly?

It tells us that we now have a test for string theory. If we can create a quark gluon plasma and the viscosity measured checks out with the predicted value then this is a pretty neat result.

The central part of the article is this:
New theoretical results at Vienna UT show that this quark-gluon plasma could be even less viscous than was deemed possible by previous theories.

Up to now string theory only got results that conformed to the standard model (or had so many free variables as to be tweakable until it conformed to those results). This seems to indicate that now there is an area where both theoretical frameworks make different predictions.

4.7 / 5 (3) Jan 18, 2012
The point of string theory research is to find testable predictions.

Even if string theory turns out to be incorrect, it's not like all these physicists have wasted their time.

Lots of very useful mathematical techniques and results have been developed along the way.

String theorist Edward Witten, for example, won the Fields Medal for his work on topology.

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