Matter becomes very simple and universal under extreme conditions of extremely high temperature and pressure

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Scientists at Queen Mary University of London have made two discoveries about the behaviour of “supercritical matter” – the point at which the difference between a liquid and a gas appears to disappear.

While the behavior of matter at fairly low temperatures and pressures is well understood, images of matter at high temperatures and pressures are blurred. Above the critical point, the difference between liquid and gas appears to disappear, and supercritical matter is thought to become hot, dense, and homogeneous.

The researchers believe that in the supercritical state, there is still new physics to be discovered about this problem.

By applying two parameters – the heat capacity and the length of the wave’s propagation in the system, they made two key findings. First, they found that there is a fixed reversal point in between, where matter changes its physical properties—from liquid to gas. They also found that this reversal point is very close in all the systems studied, which tells us that supercritical matter is very simple and easy to understand.

Beyond a basic understanding of states of matter and phase transition diagrams, understanding supercritical matter has many practical applications; hydrogen and helium are supercritical in gas giant planets such as Jupiter and Saturn, and thus control their physical properties. In green environment applications, supercritical fluids have also been shown to be very effective in destroying hazardous wastes, but engineers increasingly need theoretical guidance to improve the efficiency of supercritical processes.

Kostya Trachenko, Professor of Physics at Queen Mary University of London, said: “The ubiquity of supercritical matter opens the way for a new physically transparent map of matter under extreme conditions. This is an exciting prospect from a fundamental principles point of view. Physics science and understanding and predicting supercritical properties in green environmental applications, astronomy, and other fields.

“This journey is underway and the future is likely to see exciting developments. For example, does it raise the question of whether fixed inversion points are related to conventional higher-order phase transitions? Is it possible to use existing ideas to describe phase transition theory, Or need something new and completely different? When we push the boundaries of what we know, we can identify these exciting new questions and start looking for answers.


The main problem with understanding supercritical matter is that the theory of gases, liquids and solids does not apply. It is unclear which physical parameters will reveal the most striking properties of the supercritical state.

With an early understanding of liquids at lower temperatures and pressures, the researchers used two parameters to describe supercritical matter.

1. The first parameter is a commonly used property: this is the heat capacity that shows how efficiently the system absorbs heat and contains basic information about the system’s degrees of freedom.

2. The second parameter is less common: it is the length that the wave can travel in the system. This length controls the phase space available for phonons. When this length reaches its minimum possible value and equals the interatomic separation, something very interesting happens.

The scientists found that in terms of these two parameters, the material under extreme conditions of high pressure and high temperature became very common.

This universality is twofold. First, the plot of heat capacity versus wave propagation length has a striking fixed inversion point, which corresponds to the transition between two physically distinct supercritical states: liquid-like and gas-like states. In crossing this inversion point, the supercritical material changes its key physical properties. The reversal point is important as a clear way of distinguishing between the two states – something that has been on the minds of scientists for some time.

Second, the location of this reversal point is very close in all types of systems studied. This second generality is distinctly different from all other known transition points. For example, two of these transition points—the triple point where all three states of matter (liquid, gas, solid) coexist and the critical point where the gas-liquid boiling line ends—are different in different systems. On the other hand, the same inversion point in all systems at extreme supercritical conditions tells us that supercritical matter is very simple.

Revealing and demonstrating this simplicity is the main result of the paper “Double Universality of Transition to Supercritical States,” published in scientific progress.

Molecular-scale phase boundaries: “primitive” liquid-gas transitions

More information:
C. Cockrell et al., Double universality of supercritical state transitions, scientific progress (2022). DOI: 10.1126/sciadv.abq5183.

Courtesy of Queen Mary University of London

Citation: Matter under extreme conditions of extreme temperature and pressure becomes very simple and common (12 Aug 2022) Retrieved 12 Aug 2022 from extreme-conditions-high-temperature-pressure.html

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