Published online 26 March 2008 | Nature 452, 392-393 (2008) | doi:10.1038/452392b

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Dimensions of space-time used to order potential universes.

A cosmos folded up (simulation — not actual size). A. J. HANSON, INDIANA UNIV.

Physicists' search for a theory of everything is entering territory more familiar to biologists: taxonomy. A small team of theorists is meeting in Tucson, Arizona, in April to discuss how to classify the billions upon billions of different possible universes created by string theory, which describes fundamental particles and forces as vibrating strings.

“String theory is notorious for having a lot of solutions,” says Keith Dienes, a physicist at the University of Arizona in Tucson. “We are having this big kick-off meeting to try and organize ourselves,” he says. The String Vacuum Project aims to begin placing the various solutions into broad categories. If it works, then theorists may finally begin honing down the version of the theory that best fits with the cosmos.

But that's a pretty big if. There's no guarantee that the different possibilities will be easy to classify, or that the solution matching our Universe will be easy to find, according to Nathan Seiberg, a string theorist at the Institute for Advanced Study in Princeton, New Jersey. “I wouldn't hold my breath,” he says, “but it is possible.”

String theory is beloved by theorists because it is one of the few paradigms that promises to merge quantum mechanics, which explains the behaviour of particles on a subatomic level, with general relativity, which describes how gravity shapes large entities such as galaxies. To do so, the theory uses 10 or 11 dimensions of space-time. To make the theory resemble the physical world, theorists get rid of the extra dimensions by folding them up.

Like a sheet of paper, there are a nearly infinite number of ways to fold the extra dimensions, and each leads to a different 'vacuum' or fundamental state of the Universe. Some vacua have four forces, like ours, whereas others might have six or eight or ten. The strengths of those forces, the masses and number of fundamental particles, and even the number of dimension in a given cosmos, can all vary according to how the extra dimensions are folded. The number of possibilities has been steadily rising in recent years and current sits at nearly 10500, although there could be still more.

In the 1980s and 1990s, theorists had hoped that there might be a vacuum 'selection process' — a natural extension of string theory that would select a single vacuum resembling our Universe. But more recently, some theorists have begun to believe that there indeed may be as many as 10500 universes out there. They argue that we just happen to live in our cosmos because it can support life as we know it (see Nature 439, 10–12 ; 2006).

To date, nobody's bothered to look at how many different kinds of vacua there may be, or whether they fit into any sort of systematic categories, according to Dienes. That will be the goal of the new String Vacuum Project. Like biologists, physicists on the project will try to classify the 10500 solutions into different taxonomic ranks, such as kingdoms and phyla.

Among the project's main goals is to make string theory more predictive, says Gordon Kane of the University of Michigan in Ann Arbor. A major criticism of the theory is that it fails to provide experimental tests that would support or falsify it. If the vacuum project succeeds, then physicists would be able to find the class of vacua that fit with our own Universe. They can use those solutions to make predictions about what might show up in experiments such as the Large Hadron Collider — a massive proton-smasher being built at CERN, the European particle-physics lab outside Geneva in Switzerland. Kane says that, if the project works, he believes that evidence supporting string theory could emerge “within a few weeks” of the accelerator's start-up.

But others are less optimistic about the vacuum project's chances of success. There's no guarantee that the 10500 possibilities predicted by the theory will be easily categorized, and it's likely they won't be, says Seiberg. “It's a very complicated system,” he says. Searching for the few solutions that match our own Universe will be worse than looking for a needle in a haystack, he adds. Still, he believes the project may yield interesting findings.

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Others are more sceptical. Sifting through a near-infinite number of vacua provides no deeper understanding of the theory, according to mathematician Peter Woit of Columbia University in New York City, who is frequently critical of string theory. The fact that physicists are interested in a statistical approach is a sign of desperation, he argues. “It would be better to say, 'okay, [string theory] doesn't work.'"

Dienes accepts these criticisms, and concedes that the plan is “absolutely controversial”. Nevertheless, he believes it is the best way forward. “The plethora of vacua is an urgent issue,” he says. “I want to understand our space of possibilities.”