Israeli Physicists Create Thought-provoking Model for Material That Never Melts

The model describing a material where order doesn’t disappear as heat is applied also has implications for our understanding of the early universe

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Smelting iron
Smelting iron. 'The article demonstrates that there are models of matter that break symmetry even at high temperatures.'
Gid'on Lev
Gid'on Lev

The accepted science is that the more (almost) any given matter is heated, the more disrupted its internal order becomes. It melts, or evaporates. Now a model developed by researchers from the Hebrew University in Jerusalem and the University of Kentucky contradicts that notion, and may have implications for the development of superconductors that will help to create green energy

Take an iceberg. Anywhere in the world, if the temperature rises beyond zero (Celsius), it will melt, no matter how big it is. Melting is not limited to ice. If it’s hot enough, the crystalline order of the material’s atoms is disrupted and the molecules start to move randomly, which means: it’s melting or evaporating. But this may not be universal. Possibly, not all substances melt in the heat.

For almost 50 years scientists have been trying to develop theoretical models describing substances that can be heated without changing the internal order of the atoms comprising them. So far the equations all led to the conclusion that every matter will ulitimately melt or evaporate. But researchers at the Hebrew University of Jerusalem and the University of Kentucky have created just such a model, which was published last week in the journal Physical Review Letters.

“The article contains an impressive and thought-provoking achievement, because it demonstrates that there are models of matter that break symmetry even at high temperatures,” said Prof. Amos Yarom of the physics department of the Technion – Israel Institute of Technology, who was not involved in the study.

The assumption is that the more matter is heated, the more its internal order disappears. But order and disorder are expressions that are difficult to quantify universally. For that reason, the researchers focused on a specific symmetry that is easier to quantify.

As iron melts in mounting heat, its internal order lessens.

Symmetry is defined according to the number of points of view from which the system looks the same – in other words, from all of them, its physical features are identical. The more such points there are, the more symmetrical the system.

Dr. Michael Smolkin of Hebrew University’s Racah Institute of Physics developed the model with doctoral candidate Noam Chai and Prof. Anatoly Dymarsky of the University of Kentucky.

“If you look at any crystal, on the microscopic level it has an organized structure. If we draw the structure as a two-dimensional network, like graph paper, symmetry tells us which activities can be carried out on the grid without it being possible to realize that something was done,” explains Smolkin. “In a crystalline structure, symmetrical activities are very limited. Graph paper can be moved like that only in a very specific way, for example it can be turned at a 90-degree angle. But if you take water rather than a crystalline substance, at any angle that we turn a bucket of water we see no change. So if we heat ice, we obtain more freedom to do things to the matter without creating a change, and the symmetry increases.”

In other words, according to the accepted thinking, the more a system is heated, the more its order declines and its symmetry increases. This claim applies to all the known physical systems, but the researchers wanted to examine whether there could be a system in which this doesn’t happen. For this they tapped the theory of quantum fields, which combines quantum theory with the theory of special relativity. Physicists use it in order to create models of substances, which means, to describe their characteristics, behavior and interactions.

“Creating a model of matter means writing the substance in mathematical language – writing the fundamental laws dictating the behavior of the particles composing the substance and how they create interactions with one another,” explains Chai. “In effect, finding a model is the greatest challenge in physics.”

Anywhere in the world, if the temperature rises beyond zero (Celsius), it will melt, no matter how big it is

An iceberg melting.

Over the years physicists have developed several dozen models. Many of them describe familiar substances, but there are also models that are purely hypothetical.

“Physics is based on laboratory experiments from which the laws of nature are derived,” says Chai. “But in theoretical physics the experiments are only in the mind – we play mathematically with examples that can’t necessarily be measured in the laboratory, in order to discover the limits of the possible in the context of the laws of nature.” By means of such experiments researchers can estimate the possibility of the existence of unfamiliar substances, and later try to develop them in the laboratory.

The enigma of Rochelle salt

In the present study the researchers asked whether it is possible – in the context of the known laws of physics – that a substance won’t melt. In other words, if it’s possible that the crystalline order won’t disappear, even at extremely high temperatures. There are countless examples demonstrating that order declines with a rise in temperature, and therefore that seems to be a law of nature.

But Russian Jewish physicist Lev Landau, a Nobel Prize laureate, found an opposite example already over 50 years ago: The chemical Rochelle salt (potassium sodium tartrate) is a crystal whose structure changes when heated: the order increases and the symmetry declines.

Aside from the consensus regarding the connection between symmetry and temperature, the new study is likely to undermine another basic idea, regarding the existence of a unifying force in nature

In the case of Rochelle salt this is a temporary process, which takes place only within a limited temperature range, beyond which the crystal melts. In the present study the researchers demonstrated that theoretically there is a possibility of the existence of a material in which heating does not lead to an increase in its symmetry, at any temperature range.

In terms of physics, matter is particles with specific characteristics. In the model they created, the researchers examined which characteristics and interactions, which can be introduced into equations of the known laws of nature, would lead to a result indicating a substance that doesn’t melt.

“We ask ourselves what we want to find and then we try to find the way in equations,” is how Chai describes the work method of theoretical physicists. “Usually we look for more than one way in order to ascertain that the calculation that was done is correct. It’s like doing two independent experiments and getting the same result.”

He said that in spite of the image of theoretical physicists as scientists who work alone all day long, the process is quite interactive: “We meet once every few days and discuss the results, look for inaccuracies and failures and raise questions. Usually these discussions lead to ideas that in most cases would not have come up had we worked individually. The idea for the present study also began like that.”

Chai noted that in the present study all the equations were developed “with pen and paper,” although they also made some use of sophisticated computerized tools that helped to solve complex differential and integral equations. Using this method the researchers were able to develop equations that reflected non-melting matter.

'Creating a model of matter means writing the substance in mathematical. In effect, finding a model is the greatest challenge in physics'

“We found a very concrete example in which that happens,” says Smolkin. “It’s not clear whether it can be implemented in the laboratory, but it’s not very far from the laboratory, because in order to build it we started with an existing and known system of a substance in a certain state, to which we added a new structure. The phenomenon was obtained based on the equations.”

Chai says that the matter they received in the equations is quite similar to substances that are familiar to science from the family of super materials. Examples of such materials are super liquids, liquids that flow without friction, and superconductors: materials in which the electrons move around without any resistance. At present we know of several super-liquids and superconductors, which exist only at very low temperatures.

Research groups the world over are trying to increase the critical temperature of these materials so that they will work at room temperature. Such a development would enable tremendous savings in the global energy economy, due to the possibility of delivering electric current without losing energy along the way.

Chai stresses that the purpose of the new study was not to promote a solution for global energy problems, but to reach a more profound understanding of the laws of nature. “By means of our model we have broken a consensus that was accepted for years in the scientific community,” he says. In addition, he noted that the new study also gives hope for finding materials whose order is maintained at high temperatures. These materials are likely to be superconductors, which would preserve the characteristics of superconductors under any conditions. “That could be a green solution for the energy crisis, because we’ll be able to create less electricity and to burn far less fuel along the way,” says Chai.

Smolkin adds: “If in the end it’s possible to create such a material, that would be a big revolution. But at the moment it’s only a dream. At this stage we’ve discovered an interesting phenomenon: that the laws [of nature] don’t forbid the existence of such a material. The next question is how far it is from reality.”

However, Yarom says: “Along with the achievement in the article, there may be a problem with the model that the researchers are proposing. The model is based on quantum mechanics, which is a theory that doesn’t provide precise forecasts but only probabilities for existence in various states. A physical theory is expected to be unitary, in other words, that the sum of probabilities of being in all the possible states will be one. If Smolkin and his partners prove that their theory is unitary, that would strengthen the model they’ve built.” The researchers noted that they are currently working on such proof.

Aside from the consensus regarding the connection between symmetry and temperature, the new study is likely to undermine another basic idea, regarding the existence of a unifying force in nature. Existing physical theory describes four fundamental forces in nature: the strong force (which is responsible for binding subatomic quarks together in clusters to make more familiar subatomic particles, such as protons and neutrons), the weak force (which is responsible for radioactive decay), the electromagnetic force, and the gravitational force.

Each force is of different intensity and each has its own method of operation. The strong force operates with greater intensity the farther it is from the source that activates it (a bit like rubber, which the more it is stretched, the more force it activates), and the other forces lose their power the farther they are from the source activating them.

In the 20th century, physicists Steven Weinberg, Sheldon Glashow and Abdus Salam – who were awarded the Nobel Prize in Physics in 1979 for their work – demonstrated that beginning with sufficiently high energy, the electromagnetic force and the weak force behave identically and in effect become a single force: the electroweak force. In that state the symmetry of all the natural forces is greater, because there are more identical points of view of the system in which the forces operate.

The unification identified theoretically by Weinberg and his partners was later confirmed empirically, and theoretical unifications between the four forces were added, which are predicted by mathematical models under certain conditions. These conditions could be the ones that prevailed at the time the universe was created.

For example, the usual assumption among physicists is that in the early universe, which was extremely hot, all the forces of nature behaved identically and symmetrically. In other words, the young universe operated based on a single force. Understanding this force is likely to be the key to a unified theory of everything, the holy grail for physicists.

To date no additional unifications have been confirmed by experimentation, with the exception of the unification predicted by Weinberg and his partners, because that requires huge particle accelerators that could imitate the conditions of the early universe, which was very hot. But physicists continue to seek ways to confirm them. Because the new study suggests that the laws of quantum mechanics and special relativity do not require nature to increase symmetry with an increase in temperature, even if it is extreme temperature of the kind that existed in the early universe, a unification of forces is not essential.

In other words, in addition to shattering the consensus that heat reduces order, the new study undermines the perception that a single force operated in the early universe.

“The unification of forces means that there’s more symmetry,” says Smolkin, “and therefore if nature has chosen not to prevent the possibility of breaking the symmetry even when energy increases, the dream of the unification of forces may be incorrect.” However, Smolkin notes that all the existing observations are described well by the standard model, which is the accepted system of laws for describing the behavior of basic particles. According to this model, symmetry increases as energy is increased. That’s why many scientists believe that at a high temperature the universe is more symmetrical. But the standard model doesn’t tell the whole story,” he adds. “For example, it lacks an explanation for dark matter and dark energy. That’s why there is a chance that if we discover the model beyond the standard model, maybe we’ll find something surprising about the behavior of symmetry at a very high temperature. But it’s a mystery.”

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