For decades, physicists have been trying to combine quantum physics and general relativity into a single, unified theory. One of the leading contenders is string theory, an elegant vision in which matter and the very forces of nature are vibrating and interacting filaments of energy. It sounds great, but there’s a problem: No one can really figure out a way to
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test
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string theory.

Now, we may finally be on the verge of experimentally confirming—or refuting—some key facets of string theory.

String theory describes nature on extremely small size scales and high energies that are all but inaccessible to modern physics. The ideal experiment would provide direct evidence of these strings behaving in ways uniquely predicted by the theory, but that’s not as easy as it sounds, for two reasons. First, Heisenberg’s uncertainty principle puts some fundamental limits on how precise measurements can be. On the tiny scale of string theory, these limits may make it impossible to point at data and declare, “Right there, that’s a string!” as we can (or can approach) with the Higgs boson .

Complicating matters further, string theory has so many variants that there are very few unique predictions from the theory, so scientists don’t even know what to look for. Because of the flexibility physicists have in defining the exact parameters of string theory in the high energy realm, some have predicted that there might be as many as 10
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different variants of the theory, far too many to explore one by one.

Still, there are three fundamental pieces of the theory that could be put to the test in the near future. These results would not “prove” string theory, but could certainly be claimed as successes by many string theorists—and would help define some of those parameters.

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The Search for Supersymmetry
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Supersymmetry is one of the central concepts of string theory. Without supersymmetry, string theory is unable to describe the full range of particles observed in our universe. It can deal with photons, but not electrons. Supersymmetry bridges this divide. It predicts that all of the known particles possess supersymmetric partner particles , or superpartners. These superpartners are unstable and mostly vanished when the universe cooled down from the dense soup of the early universe, but as we crash particles together at ever-higher energies the Large Hadron Collider, we should eventually stumble upon them.

Actually, we may already have our first evidence that can lead us toward confirming supersymmetry, with the potential discovery of the Higgs boson. Supersymmetry predicts not just a single Higgs, but an entire family of Higgs-like particles. In her slim volume “ Higgs Discovery: The Power of Empty Space ,” Harvard theoretical physicist Lisa Randall describes a variant in which “some superpartners have big masses, whereas others do not.” As Randall explains it, under supersymmetry, “… if the Higgs boson exists, it is most likely part of a larger sector of new particles.” So if scientists are successful at discovering multiple Higgs-like particles, it’s very possible that we’ll end up with direct experimental evidence to support supersymmetry.

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Measuring Extra Dimensions
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String theory also claims that the universe contains extra dimensions, curled up on the same very tiny distances at which the strings exist—and subject to those pesky uncertainty principle limitations.

Or at least that’s the traditional stance. In 1998, a group of string theorists put forth the bold idea that these extra dimensions may not be so miniscule after all. They suggested at the time that they could potentially be as large as a millimeter! At this size scale, the LHC might have had a chance of exposing them despite the uncertainty principle.

Unfortunately, December 2010 results from the Large Hadron Collider have placed serious constraints on this intriguing model. If extra dimensions do exist, they must be smaller than a millimeter—but perhaps could still be large enough to be detected at the LHC. If discovered, the properties of these extra dimensions could help narrow in on the correct version of string theory.

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The Holographic Principle and Superconductors
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An important idea at work in string theory is the holographic principle , especially a version called the Maldacena duality, named for the theorist Juan Maldacena ,who first proposed it in 1997.

The Maldacena duality is is a specific way of relating two theories that, at first glance, seem quite different mathematically. You can sort of picture this by imagining a box that contains an entire three-dimensional universe. (For the purposes of this analogy, I’m just going to ignore the dimension of time.) Now imagine that the box’s two-dimensional surface contains information about what’s going on inside the box. The holographic principle basically tells us that the description on the two-dimensional surface can contain all of the same information as in the whole three-dimensional universe itself. There is a perfect correspondence between these two models.

Maldacena’s duality proved that if you had a quantum theory without gravity on a surface, it corresponded to a full string theory and gravity on the space contained within the boundary: a huge boon to theorists, but not something that anyone—including Maldacena himself—would have thought had real practical applications.

And that just goes to show how little “anyone” knows when predicting the future course of science. In 2009, physicists showed that Maldacena’s duality could describe behaviors in high-temperature superconductors. While physicists understand low-temperature superconductors, they still couldn’t explain how materials become superconductive at warmer temperatures. They knew it was linked to electrons entering a quantum critical state, which is the quantum phase change that turns the material into a superconductor, but couldn’t fully understand or model this. As condensed matter theorist Jan Zaanen described the situation , “It has always been assumed that once you understand this quantum critical state, you can also understand high temperature superconductivity. But, although the experiments produced a lot of information, we hadn’t the faintest idea of how to describe this phenomenon.”

Then Zaanen’s team tried to explain quantum critical states with string theory . They created a string theory model, then applied the Maldacena duality to get a related version of the model—one which matched the experimental results surprisingly well. Maldacena has called this the most impressive and surprising outcome of his conjecture.

But for Zaanen, it is just the beginning. It “should be … viewed as the starting point of a novel line of enquiry for [the Maldacena duality] in general,” says Zaanen. Ideally, this approach will eventually result in testable predictions that could become the focus of experiments.

Even if, ultimately, the results of these experiments do not support string theory, they will have proven something important: That the pursuit of an interesting idea—even a wrong idea—can yield amazing insight into how the universe works.

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Go Deeper
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Editor’s picks for further reading
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FQXi:
Tying Down the Multiverse with String

Physicists Andrei Linde and Renata Kallosh are working at the intersection of string theory and cosmology.

NOVA:
The Elegant Universe

Author-physicist Brian Greene presents the nuts, bolts, and sometimes outright nuttiness of string theory in this four-part NOVA special.

Not Even Wrong:
Is String Theory Testable?

On his blog “Not Even Wrong,” mathematician and physicist Peter Woit takes a critical eye to string theory.