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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 Young Lee's group at MIT might have found it.

Lee was aware of Wen's work and decided to look for such materials. Trawling through geology journals, his team spotted a candidate - a dark green crystal that geologists stumbled across in the mountains of Chile in 1972. "The geologists named it after a mineralogist they really admired, Herbert Smith, labelled it and put it to one side," says Lee. "They didn't realise the potential herbertsmithite would have for physicists years later."

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 his colleagues propose that such a system would be a string-net liquid.

Although herbertsmithite exists in nature, the mineral contains impurities that disrupt any string-net signatures, says Lee. So Lee's team made a pure sample in the lab. "It was painstaking," says Lee. "It took us a full year to prepare it and another year to analyse it."

The team measured the degree of magnetisation in the material, in response to an applied magnetic field. If herbertsmithite behaves like ordinary matter, they argue, then below about 26 \ufffdC the spins of its electrons should stop fluctuating - a condition called magnetic order. But the team found no such transition, even down to just a fraction above absolute zero.

They measured other properties, too, such as heat conduction. In conventional solids, the relationship between their temperature and their ability to conduct heat changes below a certain temperature, because the structure of the material changes. The team found no sign of such a transition in herbertsmithite, suggesting that, unlike other types of matter, its lowest energy state has no discernible order. "We could have created something in the lab that nobody has seen before," says Lee.

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