In a recent study, scientists concluded that an exotic form of matter that existed in the very early universe probably flows just like water.
Throughout history, people in many places have thought of water as sacred or as having existed almost since the beginning of creation. The matter known as quark-gluon plasma is thought to be a hot soup of elementary particles that formed just a few microseconds after the Big Bang, and it is believed to have been everywhere in the universe at the beginning.
According to modern physics, the ratio between a fluid’s viscosity (that is, how runny it is) and its density decides how it flows.
As an exotic form of matter, quark-gluon plasma is unlike any form of matter found in our daily lives. However, the researchers found that the ratio between its viscosity and density is similar to that of tap water, as both of the measurements are about 16 orders of magnitude larger than those of water.
Professor Kostya Trachenko, professor of physics at Queen Mary University of London and author of the paper, said in a statement: “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.”
The matter we normally encounter is made of atoms. These tiny particles consist of a nucleus orbited by electrons. The nucleus is made of protons and neutrons, which consist of more fundamental particles called quarks and gluons.
In most cases, quarks and gluons stick together and form protons and neutrons. However, if the temperature is very high, about one million times hotter than the center of our sun, quarks and gluons can break free and form quark-gluon plasma.
According to the Big Bang theory, the very early universe was filled with incredibly hot quark-gluon plasma. In only a few microseconds, the plasma cooled, creating the building blocks of matter.
In the past 20 years or so, scientists have been able to recreate quark-gluon plasma in lab environments using large particle colliders, providing new insights into this type of matter.
The research team found that the ratio of viscosity and density, known as kinematic viscosity, is similar between quark-gluon plasma and ordinary liquids, despite huge differences in many of their physical properties.
According to fluid dynamics, fluid flow is governed by the Navier-Stokes equation, which is sensitive to this ratio. As long as this ratio is the same, the two fluids will flow in the same way even if they have very different viscosities and densities.
“This study provides a fairly rare and delightful example of where we can draw quantitative comparisons between hugely disparate systems,” Professor Matteo Baggioli from the Universidad Autónoma de Madrid said in the statement. “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.”
The statement pointed out that not just any liquid viscosity is the same as that of quark-gluon plasma, but there is a particular point where liquid viscosity has an almost universal lower limit. Some previous studies have suggested that at this limit, the viscosity is governed by fundamental physical constants such as the Planck constant.
These constants govern the properties of our universe, such as whether a proton is a stable particle and the creation of biochemical elements needed for life. The new study found that this universal lower limit of ordinary fluids like water is close to the viscosity of quark-gluon plasma.
“It is conceivable that the current result can provide us with a better understanding of the quark-gluon plasma,” Professor Vadim Brazhkin from the Russian Academy of Sciences said in the statement. “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 new research is described in a paper published in the journal SciPost Physics.