The universe is a string-net liquid
15 March 2007
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In 1998, just after he won a share of the Nobel prize for physics, Robert Laughlin of Stanford University in California was asked how his discovery of "particles" with fractional charge, now called quasi-particles, would affect the lives of ordinary people. "It probably won't," he said, "unless people are concerned about how the universe works."
Well, people were. Xiao-Gang Wen at the Massachusetts Institute of Technology and Michael Levin at Harvard University ran with Laughlin's ideas and have come up with a prediction for a new state of matter, and even a tantalising picture of the nature of space-time itself. Levin presented their work at the Topological Quantum Computing conference at the University of California, Los Angeles, early this month.
The first hint that a new type of matter may exist came in 1983. "Twenty five years ago we thought we understood everything about how matter changes phase," says Wen. "Then along came an experiment that opened up a whole new world."
In the experiment, electrons moving in the interface between two semiconductors behaved as though they were made up of particles with only a fraction of the electron's charge. This so-called fractional quantum hall effect (FQHE) suggested that electrons may not be elementary particles after all. However, it soon became clear that electrons under certain conditions can congregate in a way that gives them the illusion of having fractional charge - an explanation that earned Laughlin, Horst Störmer and Daniel Tsui the Nobel prize (New Scientist, 31 January 1998, p 36).
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.
Helton 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 team member Young 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 Levin's model shows 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 Helton'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 °C 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.
The team plans further tests to visualise the position of individual electrons, looking for long-range entanglement by firing neutrons at the crystal and observing how they scatter. "We want to see the dynamics of the spin," says Lee. "If we tweak one [electron], we can see how the others are affected."
This intrigues Paul Fendley, a quantum computing specialist at the University of Virginia, Charlottesville (see "Silicon for a quantum age"). "It's reasonable to hope that we are seeing something exotic here," he says. "People are getting very excited about this."
Even if herbertsmithite is not a new state of matter, we shouldn't be surprised if one is found soon, as many teams are hunting for them, says Freedman. He says people wrongly assume that particle accelerators are the only places where big discoveries about matter can be made. "Accelerators are just recreating conditions after the big bang and repeating experiments that are old hat for the universe," he says. "But in labs people are creating [conditions] that are colder than anywhere that has ever existed in the universe. We are bound to stumble on something the universe has never seen before."