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Why String Theory Still Offers Hope We Can Unify Physics

Evidence that the universe is made of strings has been elusive for 30 years, but the theory’s mathematical insights continue to have an alluring pull

(illustration by John Hersey)
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The final results that we found successfully incorporated various established features of particle physics and so were worthy of attention (and, for me, a doctoral dissertation), but were far from providing evidence for string theory. Naturally, our group and many others turned back to the list of allowed shapes to consider other possibilities. But the list was no longer short. Over the months and years, researchers had discovered ever larger collections of shapes that passed mathematical muster, driving the number of candidates into the thousands, millions, billions and then, with insights spearheaded in the mid-1990s by Joe Polchinski, into numbers so large that they’ve never been named. 

Against this embarrassment of riches, string theory offered no directive regarding which shape to pick. And as each shape would affect string vibrations in different ways, each would yield different observable consequences. The dream of extracting unique predictions from string theory rapidly faded.

From a public relations standpoint, string theorists had not prepared for this development. Like the Olympic athlete who promises eight gold medals but wins “only” five, theorists had consistently set the bar as high as it could go. That string theory unites general relativity and quantum mechanics is a profound success. That it does so in a framework with the capacity to embrace the known particles and forces makes the success more than theoretically relevant. Seeking to go even further and uniquely explain the detailed properties of the particles and forces is surely a noble goal, but one that lies well beyond the line dividing success from failure.

Nevertheless, critics who had bristled at string theory’s meteoric rise to dominance used the opportunity to trumpet the theory’s demise, blurring researchers’ honest disappointment of not reaching hallowed ground with an unfounded assertion that the approach had crashed. The cacophony grew louder still with a controversial turn articulated most forcefully by one of the founding fathers of string theory, the Stanford University theoretical physicist Leonard Susskind.

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In August 2003, I was sitting with Susskind at a conference in Sigtuna, Sweden, discussing whether he really believed the new perspective he’d been expounding or was just trying to shake things up. “I do like to stir the pot,” he told me in hushed tones, feigning confidence, “but I do think this is what string theory’s been telling us.”

Susskind was arguing that if the mathematics does not identify one particular shape as the right one for the extra dimensions, perhaps there isn’t a single right shape. That is, maybe all of the shapes are right shapes in the sense that there are many universes, each with a different shape for the extra dimensions.

Our universe would then be just one of a vast collection, each with detailed features determined by the shape of their extra dimensions. Why, then, are we in this universe instead of any other? Because the shape of the hidden dimensions yields the spectrum of physical features that allow us to exist. In another universe, for example, the different shape might make the electron a little heavier or the nuclear force a little weaker, shifts that would cause the quantum processes that power stars, including our sun, to halt, interrupting the relentless march toward life on Earth.

Radical though this proposal may be, it was supported by parallel developments in cosmological thinking that suggested that the Big Bang may not have been a unique event, but was instead one of innumerable bangs spawning innumerable expanding universes, called the multiverse. Susskind was suggesting that string theory augments this grand cosmological unfolding by adorning each of the universes in the multiverse with a different shape for the extra dimensions.

With or without string theory, the multiverse is a highly controversial schema, and deservedly so. It not only recasts the landscape of reality, but shifts the scientific goal posts. Questions once deemed profoundly puzzling—why do nature’s numbers, from particle masses to force strengths to the energy suffusing space, have the particular values they do?—would be answered with a shrug. The detailed features we observe would no longer be universal truths; instead, they’d be local bylaws dictated by the particular shape of the extra dimensions in our corner of the multiverse.

Most physicists, string theorists among them, agree that the multiverse is an option of last resort. Yet, the history of science has also convinced us to not dismiss ideas merely because they run counter to expectation. If we had, our most successful theory, quantum mechanics, which describes a reality governed by wholly peculiar waves of probability, would be buried in the trash bin of physics. As Nobel laureate Steven Weinberg has said, the universe doesn’t care about what makes theoretical physicists happy.    

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This spring, after nearly two years of upgrades, the Large Hadron Collider will crackle back to life, smashing protons together with almost twice the energy achieved in its previous runs. Sifting through the debris with the most complex detectors ever built, researchers will be looking for evidence of anything that doesn’t fit within the battle-tested “Standard Model of particle physics,” whose final prediction, the Higgs boson, was confirmed just before the machine went on hiatus. While it is likely that the revamped machine is still far too weak to see strings themselves, it could provide clues pointing in the direction of string theory. 

Many researchers have pinned their hopes on finding a new class of so-called “supersymmetric” particles that emerge from string theory’s highly ordered mathematical equations. Other collider signals could show hints of extra-spatial dimensions, or even evidence of microscopic black holes, a possibility that arises from string theory’s exotic treatment of gravity on tiny distance scales.

While none of these predictions can properly be called a smoking gun—various non-stringy theories have incorporated them too—a positive identification would be on par with the discovery of the Higgs particle, and would, to put it mildly, set the world of physics on fire. The scales would tilt toward string theory.

But what happens in the event—likely, according to some—that the collider yields no remotely stringy signatures?

Experimental evidence is the final arbiter of right and wrong, but a theory’s value is also assessed by the depth of influence it has on allied fields. By this measure, string theory is off the charts. Decades of analysis filling thousands of articles have had a dramatic impact on a broad swath of research cutting across physics and mathematics. Take black holes, for example. String theory has resolved a vexing puzzle by identifying the microscopic carriers of their internal disorder, a feature discovered in the 1970s by Stephen Hawking. 

Looking back, I’m gratified at how far we’ve come but disappointed that a connection to experiment continues to elude us. While my own research has migrated from highly mathematical forays into extra-dimensional arcana to more applied studies of string theory’s cosmological insights, I now hold only modest hope that the theory will confront data during my lifetime. 

Even so, string theory’s pull remains strong. Its ability to seamlessly meld general relativity and quantum mechanics remains a primary achievement, but the allure goes deeper still. Within its majestic mathematical structure, a diligent researcher would find all of the best ideas physicists have carefully developed over the past few hundred years. It’s hard to believe such depth of insight is accidental.

I like to think that Einstein would look at string theory’s journey and smile, enjoying the theory’s remarkable geometrical features while feeling kinship with fellow travelers on the long and winding road toward unification. All the same, science is powerfully self-correcting. Should decades drift by without experimental support, I imagine that string theory will be absorbed by other areas of science and mathematics, and slowly shed a unique identity. In the interim, vigorous research and a large dose of patience are surely warranted. If experimental confirmation of string theory is in the offing, future generations will look back on our era as transformative, a time when science had the fortitude to nurture a remarkable and challenging theory, resulting in one of the most profound steps toward understanding reality.

Editor's Note: The web headline has been changed to better reflect the article's content.

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