STRING theory: you love it or loathe it.



To some it represents our best hope for a route to a "theory of everything"; others portray it as anything from a mathematically obtuse minefield to a quasi-religion that has precious little to do with science.

There might be a middle way. String theory's mathematical tools were designed to unlock the most profound secrets of the cosmos, but they could have a far less esoteric purpose: to tease out the properties of some of the most complex yet useful types of material here on Earth.

Both string theorists and condensed matter physicists

Those studying the properties of complex matter phases such as solids and liquids - are enthused by the development. "I am flabbergasted," says Jan Zaanen, a condensed matter theorist from the University of Leiden in the Netherlands. " The theory is calculating precisely what we are seeing in experiments."



If solid science does turn out to be the salvation of string theory, it would be the latest twist in a tangled history. String theory was formulated in the late 1960s to explain certain features of the strong nuclear force, one of four fundamental forces of nature. It holds that electrons, quarks and the like are not point-like particles but minuscule, curled-up, vibrating strings. No sooner had this idea emerged, though, than it lost ground to particle physicists' "standard model", which proved capable of describing not just the strong force but also the weak and electromagnetic forces - and did so far more intuitively through the interactions of point-like quantum particles.

Then string theory staged a dramatic comeback.
Gravity had resisted incorporation into the standard model, still being described by Einstein's general theory of relativity, a resolutely non-quantum theory. In the 1980s, it became clear that certain features of strings correspond perfectly to properties predicted for the graviton, a hypothetical quantum particle that would transmit the gravitational force. Suddenly it looked as though string theory could unite all of nature's workings into one grand quantum-physical scheme.
Holographic worlds.



If that's true, progress has been abysmally slow. "The string theorists were saying, 'Give us two more weeks and we will have explained all the big puzzles in the universe'," Zaanen observes. "That was 20 years ago."

The critical voices have in the meantime been getting more strident. They complain about string theory's weird, unverifiable predictions - for instance, that space-time has any number of dimensions, usually 10, rather than the three of space and one of time we see. Folding 10 dimensions down to four can be done in a mind-boggling 10500 ways, with no way of saying which of them corresponds to how our universe does it. As if that weren't enough, the energies needed to create the tiny strings the theory is woven from make them impossible to detect. To its detractors, string theory is long on mathematical elegance, but woefully short on real-world relevance.

A string-theory curiosity with the forbidding moniker of the anti-de-Sitter/conformal field theory correspondence (AdS/CFT for short) is at first glance a classic of the genre. Dreamed up in 1997 by Juan Maldacena, a young Argentinian physicist then working at Harvard University, it is a special case of what is known as the "holographic principle", floated by physicist Gerard 't Hooft of Utrecht University in the Netherlands and developed by Leonard Susskind at Stanford University in California in the early 1990s.

Their basic premise was this
much as a hologram you might find on your credit card encodes all the information for a 3D image in just two dimensions, a quantum theory in a certain number of dimensions that includes gravity can be encoded as an entirely different theory without gravity in one dimension fewer. The three spatial dimensions of our universe - along with gravity and us too - might, for instance, all be a holographic image generated from the interactions of particles on the cosmos's 2D boundary.

Maldacena took that idea further. He was trying to do something that had consumed some of the best minds in cosmology for decades: to reconcile the behaviour of black holes, which are a core prediction of general relativity, with quantum theory. One way to model black hole behaviour was to turn to multidimensional membranes known as D-branes that pop up in string theory. Like black holes in our cosmos, these curiosities are extremely heavy and capable of curving higher-dimensional space around them.

Maldacena's insight was to see that the goings-on on a D-brane could be described in two entirely equivalent, holographically related ways. The first comes from string theory: it includes gravity, and involves 10 dimensions. Of these, five are rolled up tightly while the other five are configured as an "anti-de-Sitter space" - one that warps back on itself like a saddle, rather than being broadly flat as our cosmos is assumed to be.