Your desk is made up of individual, distinct atoms, but from far away its surface appears smooth. This is the basis of every model of the physical universe. It’s possible to describe the overall state of things without becoming too involved in all the intricate interactions among every electron and atom.
Many physicists were skeptical when they discovered that a new theory state of matter existed, whose microscopical features persist across scales.
“When I first heard about fractons, I said there’s no way this could be true, because it completely defies my prejudice of how systems behave,” said Nathan Seiberg, a theoretical physicist at the Institute for Advanced Study in Princeton, New Jersey. But I was mistaken. I realized I had been living in denial.”
The theoretical possibility of fractons surprised physicists in 2011. These strange state of matter are leading to new theoretical frameworks for fundamental physics that may help them solve some of their most difficult problems.
Fractons are quasiparticles–particle-like entities that emerge out of complicated interactions between many elementary particles inside a material. But fractons are bizarre even compared to other exotic quasiparticles, because they are totally immobile or able to move only in a limited way. They can’t stop moving because there is nothing they can do in the environment. This means that fractons’ microstructure influences how they behave over large distances.
It’s shocking. For me it is the weirdest phase of matter,” said Xie Chen, a condensed-matter theorist at the California Institute of Technology.
Jeongwan Haah was a Caltech graduate student in 2011. He sought out unusual forms of matter so that they could be used for quantum memories. He discovered a new phase of matter that he could use to secure quantum memory. Because of its strangely inmovable quasiparticles, the phase was quickly noticed by other physicists.
These particles seemed to be only fractions of the total number of particles that could move together. In 2015, Haah, along with Sagar Vijay, Liang Fu, discovered more phases with similar properties. They were then able to move in combination. (An earlier, overlooked paper by Claudio Chamon is now credited with the original discovery of fracton behavior.)
You can see the extraordinary nature of fracton phases by looking at a simpler particle like an electron moving through a substance. One way that physicists have come to understand electron movement is by assuming space is full of electron-positron pairs, which momentarily pop into or out of existence. The positron, the electron’s antiparticle of opposite charge, appears on top of one such pair and it annihilates. The pair’s electron is now displaced from its original electron. We can’t distinguish between them, so all that we see is one electron moving.
Instead, imagine two pairs of antiparticles and one of their antiparticles not being able to arise from the vacuum. Only squares are possible. A square could be formed if one antiparticle is placed on top of the first particle. This would annihilate that corner. The vacuum then creates a second square, where one side of it annihilates the other. The second side of the square, which is also composed of an antiparticle and a particle, then disappears. The resultant movement is that of a particle-antiparticle pair moving sideways in a straight line. This world is an example of a “fracton” phase. A single particle can only move in a restricted way, while a pair may easily move.
Haah code is a more extreme example of this phenomenon: particles can only move when they are summoned by new particles in endless repeating patterns known as fractals. Imagine four particles in a square. But if you zoom in on each corner, you will find another four-particle square that is close to the corners. You can zoom in again on any corner to find another square. To make such a structure in vacuum, it takes so much energy that you can’t move this kind of fracton. As the environment cannot disrupt their delicate states, this allows for very stable qubits (the bits of quantum computing) to be kept in the system.
It is difficult to explain fractons as a continuous smooth stream from faraway because of their immovability. Particles can move easily, so if they are left alone, eventually, they will find equilibrium. This is defined as their bulk properties, such as temperature and pressure. Particles lose their initial location. However, fractons cannot move along specific lines and planes or at certain points. This motion must be described by keeping track of the locations of the fractons. The phases can’t shake their microscopical nature or follow the standard continuum descriptions.
Vijay at UC Santa Barbara, a theoreticalist and theorist on fractons said that their resolute microscopic behaviour makes it difficult to “imagine examples of fractons” and think about possibilities. How can we describe these states of matter without a consistent description?
Chen stated that “we’re missing an enormous chunk of things.” Chen said, “We don’t know how to explain them or what they are.”
Although fractons are not yet made in the laboratory, that could change. Certain crystals with immovable defects have been shown to be mathematically similar to fractons. The theoretical landscape of fractons has expanded beyond anyone’s expectations, with new models appearing every month.
“Probably in the near future someone will take one of these proposals and say, ‘OK, let’s do some heroic experiment with cold atoms and exactly realize one of these fracton models,'” said Brian Skinner, a condensed-matter physicist at Ohio State University who has devised fracton models.
Even though they were not experimentally realized, Seiberg was alarmed by the possibility that fractons could exist. He is a world-renowned expert on quantum field theory and the theoretical framework within which nearly all physical phenomena can be described.
Quantum field theory depicts discrete particles as excitations in continuous fields that stretch across space and time. It’s the most successful physical theory ever discovered, and it encompasses the standard model of particle physics–the impressively accurate equation governing all known elementary particles.
“Fractons don’t fit within this framework. Seiberg stated that the framework was not complete.
Other reasons exist to believe that quantum field theory may not be complete. For one, it does not account for gravity. Seiberg, along with other scientists, see new possibilities for a quantum gravity theory if they are able to explain fractons within the framework of quantum field theory.
Seiberg stated that “Fractons can be dangerous because of their discreteness, which could ruin all the structure we have.” It’s up to you whether you think it’s a problem or you see it as an opportunity.
He and his colleagues are developing novel quantum field theories that try to encompass the weirdness of fractons by allowing some discrete behavior on top of a bedrock of continuous spacetime.
He said that quantum field theory was a delicate structure and therefore he would prefer to keep the rules in place. We are on thin ice and hoping to reach the other side.
Reprinted original story with permission of Quanta Magazine. This independent, editorially-controlled publication is part of the Simons Foundation and aims to increase public knowledge of science. It covers research trends and developments in math and other life and physical sciences.
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Publited at Sun, 01 August 2021 12:06.32 +0000