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
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

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

“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
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

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
and string theorists John Schwarz of Caltech, James Gates
of the University of Maryland, and Juan
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

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

“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.”

Via Universe

Image: Entangled string. Flickr/Whatknot

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