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 test 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 10500 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.
The Search for Supersymmetry
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.
Measuring Extra Dimensions
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.
The Holographic Principle and Superconductors
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.
Editor's picks for further reading
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.
Can science fiction influence the course of real science?
By “science fiction,” I don’t mean fantasy—vampires, werewolves, elf princesses, that kind of thing. Science fiction may seem fantastical, but even its most fantastic elements are driven by real science.
The obvious predictions of science fiction are all around us, from iPads to cell phones and various other electronic wonders that we treat as disposable. My 2-year-old son entertains himself with toys that are more technologically sophisticated than the first computer I ever owned. The next phase in casually transforming us all into cyborgs may be fully-immersive augmented reality, at least if Google has anything to say about it.
Science fiction isn’t just a sneak preview of future gadgets, though. For scientists, it is an inspiration machine. The theoretical physicist and TV personality Michio Kaku recalls watching "Flash Gordon" in his youth and realizing that the real hero of the series wasn’t the handsome, athletic Flash: it was the brilliant scientist Dr. Zarkov. As Kaku recounts in his book "Physics of the Future," “[Dr. Zarkov] invented the rocket ship, the invisibility shield, the power source for the city in the sky, etc. Without the scientist, there is no future.”
Dr. Wernher von Braun (center), then Chief of the Guided Missile Development Division at Redstone Arsenal, Alabama, discusses a "bottle suit" model with Dr. Heinz Haber (left), an expert on aviation medicine, and Willy Ley, a science writer on rocketry and space exploration. Source: NASA, via the Wikimedia Commons
To Stephen Hawking
, science fiction offers a kind of exercise for the imagination. As he wrote in the forward to Lawrence Krauss
’ 1995 classic "The Physics of Star Trek":
“Science fiction [...] is not only good fun but it also serves a serious purpose, that of expanding the human imagination. We may not yet be able to boldly go where no man (or woman) has gone before, but at least we can do it in the mind.[…] There is a two-way trade between science fiction and science. Science fiction suggests ideas that scientists incorporate into their theories, but sometimes science turns up notions that are stranger than any science fiction.”
Yet even some of those strange notions, like Einstein’s theory of general relativity, were anticipated by science fiction. As Krauss pointed out in "Hiding in the Mirror," the very first page of H.G. Wells’ "The Time Machine," published in 1895, included an explanation from the unnamed time traveler about how objects require existence in time as well as space. To modern ears, his description sounds a lot like Einstein’s vision of space and time.
Yet at the same time that Wells was presaging Einstein, some physicists believed that science was turning the final pages in the book of nature. In 1900, the scientist Lord Kelvin famously declared that physics was nearly complete—that we only needed to solve two minor lingering problems to know all there was to know about the universe. As it turned out, resolving those two problems did not usher in the end of physics—it led directly to the theory of relativity and quantum theory, as well as all of the scientific discoveries and technology that’s come about from them: television, nuclear energy, computers, transistors, cell phones...you get the idea. So while physicists thought that we were nearing the end of a journey, science fiction writers, with their fantastical stories of time travel and robots, showed that we were just at the beginning. And the science fiction writers were right.
In the aftermath of this quantum revolution, science fiction doubled down. This was the era of pulp adventures like the "Flash Gordon" serials that inspired Michio Kaku. Science fiction authors like Isaac Asimov, Robert Heinlein, and Arthur Clarke were the vanguard of a generation of science fiction authors who also had strong scientific backgrounds.
And it wasn’t just science fiction authors writing about the future. Theoretical physicist S. James Gates, Jr. recounts how his father brought home four non-fiction books in the late 1950s, all written by the science writer Willy Ley. With titles like "Space Pilots," "Man-Made Satellites," "Space Stations," and "Space Travel," these books brought scientific credibility to dreams of mankind’s star-faring future and inspired Gates to pursue the sciences. (Lest we think the only benefits of science fiction are intellectual, in a recent interview for the radio program "On Being", Gates also relates how Isaac Asimov’s "Lucky Starr" books helped him cope with his mother’s death.) Today Gates serves as director for the Center for String and Particle Theory at University of Maryland.
This isn’t to say that science fiction gets everything right, of course. For one thing, the golden age of sci-fi was full of laser pistols and flying cars that never quite made it into the mainstream. (At least, not yet.) In "The Amazing Story of Quantum Physics," physicist James Kakalios explains that this pulp-era futurism went awry by over-estimating the amount of energy we’d have access to. Turns out it takes a lot of energy to build a laser pistol or a flying car!
But the deepest error in science fiction—and the one that most rankles physicists like Gates—is how easy it often makes scientific accomplishment look. “I know from a life in science that nothing could be farther from the truth. The effort to advance science is one of the most monumental struggles I have witnessed in my life. Progress is usually painfully slow.”
David Brin, a physicist who now has a successful career as a science fiction author, agrees:
“The most annoying thing is when sci-fi or fantasy stories get the process of science all wrong. When, for plot reasons and just to get the heroes in jeopardy, they show science and scientists behaving in ways that are paranoid, incurious, conniving, unscrupulous, and addicted to secrecy....But science is about doing things in the open. And that's when horrible mistakes get pointed out, in advance.”
Science fiction has its fair share of mad scientists slinking about in gloomy dystopias. But more often, it is an optimistic genre. When science fiction author Robert J. Sawyer was recently asked about his top five science fiction predictions, for instance, his top pick was that there was a future. During the Cold War decades, it was science fiction that offered a hopeful vision of the future. Gates actually attributes much of the success of science fiction to the zeitgeist of this post-World War II era. “The challenge of Sputnik only turbo-charged these conditions and the kind of science fiction produced in this climate was almost guaranteed to have caught my attention.”
The finest science fiction is inspired by the same thing that has inspired the greatest science discoveries throughout the ages: optimism for the future. As I read today’s science fiction, I worry that many modern authors do not seek to inspire the way they once did. Brin points out that “images of a can-do, problem-solving humanity seem to be offered less and less,” despite his own best efforts to buck this trend.
Like Gates, I was strongly influenced by Isaac Asimov, who first inspired my interest in science, fiction, and the future. Which authors have inspired you with their hopeful visions of the future?
Editor's picks for further reading
Bulletin of the Atomic Scientists: The Science Fiction Effect
In this essay, Laura Kahn explores the connection between science fiction and science fact.
Inside NOVA: Cinema Science: Time Travel
In this blog post, explore the real science behind time travel as seen in science fiction films.
Smithsonian’s Surprising Science: NASA Picks Best and Worst Sci-Fi Movies
Find out which films get the science right—and which ones get it very, very wrong.
Technology Review: The Best Hard Science Fiction Books of All Time