String theory could be ruled out by experiments at the Large Hadron Collider (LHC), a particle accelerator scheduled to open by the end of 2007, a new study says. The finding offers a new approach for testing this potential "theory of everything", a goal that has so far proven elusive.

According to string theory, particles like electrons and photons are actually tiny, vibrating strings. The beauty of the theory is that it accounts for all of the known forces – including gravity, which the standard model of physics does not. But its critics have complained that there is essentially no way to test it.

Strong evidence for string theory could come from the observation of short-lived, mini black holes at the LHC (see Watching God play dice: The Large Hadron Collider). But the chance of their appearing is extremely small, so a failure to see them would not be a death blow for the theory.

In 2006, string theorist Allan Adams of MIT in Cambridge, US, and others offered a more promising check. They showed that some particle collisions could reveal whether certain fundamental assumptions underlying string theory are wrong.

Now, another team has shown that the energies needed to reveal such effects are achievable at the LHC, which is being built in Geneva, Switzerland. The team was led by Jacques Distler of the University of Texas in Austin, US.

High energies

One of string theory's assumptions comes from Einstein's theory of relativity – that the speed of light is the same for all observers, a principle called Lorentz invariance.

This principle – and three others underlying string theory – determine how strongly particles called W bosons, which transmit the weak nuclear force, interact.

If these interactions are below the strength calculated by Distler's team, it would signal that one of the assumptions built into string theory is incorrect and that therefore string theory itself is wrong, the researchers say.

"They did a very important thing," Adams told New Scientist.

Quantised space

If string theory does seem to be ruled out, physicists will have to find another theory of everything that can explain the LHC observations. "If we see these violations, people will start working very feverishly on some sort of alternative that will produce these violations," Distler told New Scientist.

That alternative may turn out to be a theory called loop quantum gravity, which posits that space itself is quantised into tiny chunks. Some physicists argue that loop quantum gravity does not satisfy Lorentz invariance. "So that's one possible direction people might go," Distler says.

Although the test could in principle rule out string theory if violations are found, both Distler and Adams suspect that the results will turn out to respect the four assumptions, leaving string theory as a viable candidate for the theory of everything.

Quantum World - Learn more about a weird world in our comprehensive special report.

Journal reference: Physical Review Letters (in press)

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