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Wen suspected that the effect could be an example of a new type of matter. Different phases of matter are characterised by the way their atoms are organised. In a liquid, for instance, atoms are randomly distributed, whereas atoms in a solid are rigidly positioned in a lattice. FQHE systems are different. "If you take a snapshot of the position of electrons in an FQHE system they appear random and you think you have a liquid," says Wen. But step back, and you see that, unlike in a liquid, the electrons dance around each other in well-defined steps.
It is as if the electrons are entangled. Today, physicists use the term to describe a property in quantum mechanics in which particles can be linked despite being separated by great distances. Wen speculated that FQHE systems represented a state of matter in which entanglement was an intrinsic property, with particles tied to each other in a complicated manner across the entire material.
This led Wen and Levin to the idea that there may be a different way of thinking about matter. What if electrons were not really elementary, but were formed at the ends of long "strings" of other, fundamental particles? They formulated a model in which such strings are free to move "like noodles in a soup" and weave together into huge "string-nets".
Light and matter unified
The pair ran simulations to see if their string-nets could give rise to conventional particles and fractionally charged quasi-particles. They did. They also found something even more surprising. As the net of strings vibrated, it produced a wave that behaved according to a very familiar set of laws - Maxwell's equations, which describe the behaviour of light. "A hundred and fifty years after Maxwell wrote them down, here they emerged by accident," says Wen.
That wasn't all. They found that their model naturally gave rise to other elementary particles, such as quarks, which make up protons and neutrons, and the particles responsible for some of the fundamental forces, such as gluons and the W and Z bosons.
From this, the researchers made another leap. Could the entire universe be modelled in a similar way? "Suddenly we realised, maybe the vacuum of our whole universe is a string-net liquid," says Wen. "It would provide a unified explanation of how both light and matter arise." So in their theory elementary particles are not the fundamental building blocks of matter. Instead, they emerge from the deeper structure of the non-empty vacuum of space-time.
"Wen and Levin's theory is really beautiful stuff," says Michael Freedman, 1986 winner of the Fields medal, the highest prize in mathematics, and a quantum computing specialist at Microsoft Station Q at the University of California, Santa Barbara. "I admire their approach, which is to be suspicious of anything - electrons, photons, Maxwell's equations - that everyone else accepts as fundamental."
Other theories that try to explain the same phenomena abound, of course; Wen and Levin realise that the burden of proof is on them. It may not be far off. Their model predicts specific arrangements of atoms in the new state of matter, which they dub the "string-net liquid", and Joel Helton's group at MIT might have found it.
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Herbertsmithite (pictured) is unusual because its electrons are arranged in a triangular lattice. Normally, electrons prefer to line up so that their spins are in the opposite direction to that of their immediate neighbours, but in a triangle this is impossible - there will always be neighbouring electrons spinning in the same direction. Wen and Levin's model shows that such a system would be a string-net liquid.
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Wen suspected that the effect could be an example of a new type of matter. Different phases of matter are characterised by the way their atoms are organised. In a liquid, for instance, atoms are randomly distributed, whereas atoms in a solid are rigidly positioned in a lattice. FQHE systems are different. "If you take a snapshot of the position of electrons in an FQHE system they appear random and you think you have a liquid," says Wen. But step back, and you see that, unlike in a liquid, the electrons dance around each other in well-defined steps.
It is as if the electrons are entangled. Today, physicists use the term to describe a property in quantum mechanics in which particles can be linked despite being separated by great distances. Wen speculated that FQHE systems represented a state of matter in which entanglement was an intrinsic property, with particles tied to each other in a complicated manner across the entire material.
This led Wen and Levin to the idea that there may be a different way of thinking about matter. What if electrons were not really elementary, but were formed at the ends of long "strings" of other, fundamental particles? They formulated a model in which such strings are free to move "like noodles in a soup" and weave together into huge "string-nets".
Light and matter unified
The pair ran simulations to see if their string-nets could give rise to conventional particles and fractionally charged quasi-particles. They did. They also found something even more surprising. As the net of strings vibrated, it produced a wave that behaved according to a very familiar set of laws - Maxwell's equations, which describe the behaviour of light. "A hundred and fifty years after Maxwell wrote them down, here they emerged by accident," says Wen.
That wasn't all. They found that their model naturally gave rise to other elementary particles, such as quarks, which make up protons and neutrons, and the particles responsible for some of the fundamental forces, such as gluons and the W and Z bosons.
From this, the researchers made another leap. Could the entire universe be modelled in a similar way? "Suddenly we realised, maybe the vacuum of our whole universe is a string-net liquid," says Wen. "It would provide a unified explanation of how both light and matter arise." So in their theory elementary particles are not the fundamental building blocks of matter. Instead, they emerge from the deeper structure of the non-empty vacuum of space-time.
"Wen and Levin's theory is really beautiful stuff," says Michael Freedman, 1986 winner of the Fields medal, the highest prize in mathematics, and a quantum computing specialist at Microsoft Station Q at the University of California, Santa Barbara. "I admire their approach, which is to be suspicious of anything - electrons, photons, Maxwell's equations - that everyone else accepts as fundamental."
Other theories that try to explain the same phenomena abound, of course; Wen and Levin realise that the burden of proof is on them. It may not be far off. Their model predicts specific arrangements of atoms in the new state of matter, which they dub the "string-net liquid", and Joel Helton's group at MIT might have found it.
>
Herbertsmithite (pictured) is unusual because its electrons are arranged in a triangular lattice. Normally, electrons prefer to line up so that their spins are in the opposite direction to that of their immediate neighbours, but in a triangle this is impossible - there will always be neighbouring electrons spinning in the same direction. Wen and Levin's model shows that such a system would be a string-net liquid.
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