String theory has finally made a prediction that can be tested with experiments — but in a completely unexpected realm of physics.

The theory has long been touted as the best hope for a unified "theory of everything," bringing together the physics of the vanishingly small and the mindbendingly large. But it has also been criticized and even ridiculed for failing to make any predictions that could be checked experimentally. It's not just that we don't have big enough particle accelerators or powerful enough computers; string theory's most vocal critics charge that no experiment could even be imagined that would prove it right or wrong, making the whole theory effectively useless.

Now, physicists at Imperial College London and Stanford University have found a way to make string theory useful, not for a theory of everything, but for quantum entanglement.

"We can use string theory to solve problems in a different area of physics," said theoretical physicist Michael Duff of Imperial College London. "In that context it's actually useful: We can make statements which you could in principle check by experiment." Duff and his colleagues describe their findings in a paper in Physical Review Letters September 2.

String theory suggests that matter can be broken down beyond electrons and quarks into tiny loops of vibrating strings. Those strings move and vibrate at different frequencies, giving particles distinctive properties like mass and charge. This strange idea could unite all the fundamental forces, explain the origins of fundamental particles and connect Einstein's general relativity to quantum mechanics. But to do so, the theory requires six extra dimensions of space and time curled up inside the four that we're used to.


To understand how these extra dimensions could hide from view, imagine a tightrope walker on a wire between two high buildings. To the tightrope walker, the wire is a one-dimensional line. But to a colony of ants crawling around the wire, the rope has a second dimension: its thickness. In the same way that the tightrope walker sees one dimension where the ants see two, we could see just three dimensions of space while strings see nine or ten.

Unfortunately, there's no way to know if this picture is real. But although string theorists can't test the big idea, they can use this vision of the world to describe natural phenomena like black holes.

Four years ago, while listening to a talk at a conference in Tasmania, Duff realized the mathematical description string theorists use for black holes was identical to the mathematical description of certain quantum systems, called quantum bits or qubits.

Qubits form the backbone of quantum information theory, which could lead to things like ultrafast computers and absolutely secure communication. Two or more qubits can sometimes be intimately connected in a quantum state called entanglement. When two qubits are entangled, changing one's state influences the state of the other, even when they're physically far apart.

"As I listened to his talk, I realized the kind of math he was using to describe qubit entanglement was very similar to mathematics I had been using some years before to describe black holes in string theory," Duff said. When he looked into it, the mathematical formulation of three entangled qubits turned out to be exactly the same as the description of a certain class of black holes.

In the new study, Duff and his colleagues push the similarity one step further. They used the mathematics of stringy black holes to compute a new way to describe four entangled qubits, an open question in quantum information theory.

"We made statements that weren't previously known using string theory techniques," Duff said. "Whether the result is some fundamental principle or some quirk of mathematics, we don't know, but it is useful for making statements about quantum entanglement."

What's more, these statements are precise and experimentally provable, unlike previous suggestions for ways to test string theory, Duff says.

"So in a way, there's bad news and good news in our paper," he said. "The bad news is, we're not describing the theory of everything. The good news is, we're making a very exact statement which is either right or wrong. There's no in between."

Duff emphasized that this is only a test of string theory as it relates to quantum entanglement, not as a description of the fundamental physics of the universe. The battle over string theory as a theory of everything rages on.

"Already I can imagine enemies sharpening their knives," Duff said.

And they are. A chorus of supporters and critics, including Nobel laureate and string theory skeptic Sheldon Glashow and string theorists John Schwarz of Caltech, James Gates of the University of Maryland, and Juan Maldacena and Edward Witten of the Institute for Advanced Study in Princeton agree that Duff's argument is "not a way to test string theory" and has nothing to do with a theory of everything.

Mathematician Peter Woit of Columbia University, author of the blog Not Even Wrong, thinks even claiming that the new paper is a test of quantum entanglement is going too far.

"Honestly, I think this is completely outrageous," he said. Even if the math is the same, he says, testing the quantum entangled system would only tell you how well you understand the math.

"The fact that the same mathematical structure appears in a quantum mechanical problem and some model of black holes isn't even slightly surprising," he said. "It doesn't mean that one is a test of the other."

Witten takes a more optimistic view of the theory's chances, pointing out that the mathematics of string theory have turned out to be coincidentally useful in other areas of physics before.

"In general, this kind of work shows that string theory is useful, and in fact by now it has been useful in many different ways," Witten said in an email to Wired.com.

"One might surmise that a physics theory that has proved to be useful in so many different areas of physics and math is probably on the right track," he added. "But that is another question."




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