Published online 19 July 2009 | Nature | doi:10.1038/news.2009.699


String theory hints at explanation for superconductivity

Baroque field gets fresh lease of life in condensed-matter physics.

stringsString theory - more than just a 'theory of everything'?Alamy

Until recently, string theory — long heralded as a 'theory of everything' — hadn't been particularly good at explaining anything.

But at a workshop this month at the Kavli Institute for Theoretical Physics in Santa Barbara, California, scientists have been using the theory to make progress in tackling one of the biggest puzzles in condensed-matter physics: the origin of high-temperature superconductivity.

String theory suggests that vibrating strings that exist in 10 dimensions underpin the observable Universe. Although that basic premise is still very much in doubt — and as yet impossible to test experimentally — some of the mathematical tools used in string theory have in the past few years been applied to describe the behaviour of hot particle plasmas and supercooled lattices of atoms.

The latest claim for string theory is that it is a key tool in explaining the normal behaviour of materials that conduct electricity without any resistance at relatively high temperatures. The theory that explains conventional superconductivity at temperatures close to absolute zero is well-developed — but the theory that explains the behaviour of a second class of materials, which can superconduct at temperatures as high as 70 K, remains something of a mystery. By explaining the normal behaviour of these materials just above their superconducting temperature, string theorists hope to get a better handle on high-temperature superconductivity itself.

"It suggests that we are on the verge of understanding a new state of matter using a string-theory description," says Subir Sachdev, a condensed-matter theorist at Harvard University in Cambridge, Massachusetts, who co-organized the workshop. At the workshop, Sachdev and his collaborators circulated a paper, as yet not even a preprint, in which they stake their claim to a string-theory model for high-temperature superconductors.

Finding new applications for the mathematics of string theory is reinvigorating the field, says Harvard University postdoctoral researcher Sean Hartnoll, another workshop co-organizer. "It now has the feeling of being a melting pot of ideas."

Baroque complexity

String theory started out in the late 1960s as a tool to explain the strong forces between nuclear atomic particles, but was replaced in the 1970s by the more successful quantum chromodynamics (QCD) theory. String theory went off in its own direction, acquiring ever more baroque layers of mathematical complexity. Some physicists found it anathema that the only way the resulting theories could be tested required energies far higher than those achievable in particle accelerators.

But in 2005, string theory did find its way, albeit indirectly, into one accelerator: the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory in New York. Scientists discovered that string theory could be as useful as QCD in explaining the strong nuclear forces involved in a quark–gluon plasma. This new state of matter, comprising the basic constituents of protons and neutrons, was created in the hot mash-ups of gold ions generated at the RHIC. The key to this discovery was a mathematical technique in string theory that embodies the principles of holography, in which information contained in a higher dimension can be embedded in one fewer dimensions — just as a three-dimensional image can be stored within a flat, two-dimensional hologram.

Since then, researchers such as Sachdev and Hartnoll have extended the holographic techniques to the cooler regimes of condensed matter. The same string-theory tools have helped to explain the behaviour of quantum critical points — the changes in matter cooled close to absolute zero when quantum mechanical effects start to dominate its behaviour.

That in turn has allowed physicists to describe the quantum behaviour of a variety of systems, including laser-induced lattices of supercooled atoms, and now high-temperature superconductivity.

Noted string theory critic Peter Woit, a mathematician at Columbia University in New York City, says that using string theory as a tool in these ways could be useful, but they are not tests of string theory itself. "Just because a model works in one context, doesn't mean you can unify physics and get a fundamental theory of reality," he says.

Joseph Polchinski, a string theorist at the Kavli Institute and the third conference organizer, argues that if the same string-theory tools used to describe black holes can help explain the behaviour of electrons in a metal, the crossover will enable string theory applications in one area to benefit other fields

The excitement is catching on, he adds. The institute received 110 applications for just 30 spots at the workshop — the toughest workshop to get into in his memory. Quite a feat given that when it was organized 18 months ago there were fewer than a dozen papers published on the topic. "It was clearly a good gamble," says Polchinski. "It's clear there's interesting new science here." 


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  • #60597

    Damn, when I read that article I realize how basic my knowledge of physics really is. I'd love to understand quantum mechanics JustinI

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