Quark-gluon plasma is a state of dense matter with the quarks and gluons being its constituents. Soon after the Big Bang the matter was just in such a phase. When the Universe was expanding and cooling down the quark-gluon plasma turned into hadrons (neutrons and protons), which further formed the atomic nuclei. Whilst both the viscosity and density of quark-gluon plasma are about 16 orders of magnitude larger than in water, an international team of physicists found that the ratio between the viscosity and density of the two types of fluids are the same.
“This study provides a fairly rare and delightful example of where we can draw quantitative comparisons between hugely disparate systems,” said Professor Matteo Baggioli, a researcher at the Universidad Autónoma de Madrid.
“Liquids are described by hydrodynamics, which leaves us with many open problems that are currently at the forefront of physics research.”
“Our result shows the power of physics to translate general principles into specific predictions about complex properties such as liquid flow in exotic types of matter like quark-gluon plasma.”
Since the early 2000s, physicists have been able to recreate quark-gluon plasma experimentally using large particle colliders, which has provided new insights into this exotic state of matter.
The ordinary matter is thought to have very different properties to the quark-gluon plasma found in the early beginnings of the Universe. For example, fluids like water are governed by the behavior of atoms and molecules that are much larger than the particles found in quark-gluon plasma, and are held together by weaker forces.
However, Professor Baggioli and colleagues found that despite these differences the ratio of viscosity and density, known as the kinematic viscosity, is close in both quark-gluon plasma and ordinary liquids.
This ratio is important because the fluid flow does not depend on viscosity alone but is governed by the Navier-Stokes equation which contains density and viscosity.
Therefore, if this ratio is the same for two different fluids these two fluids will flow in the same way even if they have very different viscosities and densities.
Importantly, it’s not just any liquid viscosity that coincides with the viscosity of quark-gluon plasma.
Indeed, liquid viscosity can vary by many orders of magnitude depending on temperature.
However, there is one very particular point where liquid viscosity has a nearly-universal lower limit.
Previous research found that in that limit, fluid viscosity is governed by fundamental physical constants such as the Planck constant and the nucleon mass.
It is these constants of nature that ultimately decide whether a proton is a stable particle, and govern processes like nuclear synthesis in stars and the creation of essential biochemical elements needed for life.
The study found that it is this universal lower limit of viscosity of ordinary fluids like water which turns out to be close to the viscosity of quark-gluon plasma.
“We do not fully understand the origin of this striking similarity yet but we think it could be related to the fundamental physical constants which set both the universal lower limit of viscosity for both ordinary liquids and quark-gluon plasma,” said Professor Kostya Trachenko, a researcher at Queen Mary University of London.
“It is conceivable that the current result can provide us with a better understanding of the quark-gluon plasma,” said Professor Vadim Brazhkin, a researcher at the Universidad Autónoma de Madrid.
“The reason is that viscosity in liquids at their minimum corresponds to a very particular regime of liquid dynamics which we understood only recently.”
“The similarity with the quark-gluon plasma suggests that particles in this exotic system move in the same way as in tap water.”
The results were published today in the journal SciPost Physics.
Kostya Trachenko et al. 2021. Similarity between the kinematic viscosity of quark-gluon plasma and liquids at the viscosity minimum. SciPost Phys 10, 118; doi: 10.21468/SciPostPhys.10.5.118
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