### The Elegant Universe

When I received my first hardbound copy of *The Elegant Universe*, fresh off the press, the Large Hadron Collider in Geneva, Switzerland, was still about a decade away from powering up. Back then, of course, no one had any inkling that a mere nine days after the collider finally turned on, in September of 2008, it would abruptly shut down. A tiny loose joint led to an electrical overload that cascaded into some of the superconducting magnets warming and a section of equipment buckling. The damage shut the machine down for about a year, but in late 2009 the Large Hadron Collider came roaring back, propelling protons at nearly light-speed in opposite directions around its 27-kilometer tunnel—among the emptiest and coldest stretches in the solar system—and guiding them into collisions at energies far beyond any previously achieved in the laboratory.

To coincide with the commissioning of the collider, I wrote a *New York Times* op-ed summarizing the major reasons for building it: find the Higgs particle, confirm supersymmetry, provide evidence for extra dimensions of space, encounter something completely unexpected. I was careful not to give equal likelihood to each of the goals that had coaxed the world to spend billions of dollars on the collider (they are listed above in descending order of anticipated likelihood), and I also included another potential outcome: that we find nothing at all. I noted that finding nothing would rank among the most radical outcomes, as it would require some long-cherished and deeply convincing ideas to be jettisoned. But it would also be the most challenging outcome to report: an exclamation point in the headline "Collider Finds Nothing!" would coax a hurried click from physicists but surely from no one else.

As the world knows, the collider was kind to physicists and journalists alike. On July 4, 2012, at a stirring press conference at the Geneva facility, spokespersons Fabiola Gianotti and Joe Incandela were surrounded by hundreds of researchers, including an eighty-three-year-old Peter Higgs, as they officially announced the discovery of the elusive particle bearing his name. The camera covering the event caught an emotional Higgs (the person), a tear welling in his eye. Higgs wasn't the only researcher responsible for the prediction of the Higgs particle—scientists who deserve credit include François Englert (who subsequently shared the Nobel Prize with Higgs), Robert Brout, Gerald Guralnik, Carl Hagen, Tom Kibble, and no doubt others too. But Higgs's journey represents the yellowest of brick roads that theorists dream of following.

In the 1960s, Higgs and others were struggling to understand how various particles that were central to the emerging standard model of particle physics would acquire mass. The conundrum was that symmetry principles underlying the theoretical equations seemed to require the particles to be massless, much like the photon, but the data showed clearly that the particles were massive. Faced with mounting tension between theory and experiment, Higgs found common ground: if space were filled with a field, much as a sauna is filled with steam, the equations could preserve their massless symmetry while particles would pick up mass from an environmental influence—a kind of exotic friction as the particles plowed through the Higgs mist.

Higgs realized that if his proposal were correct, this invisible space-filling field would have a visible signature: a tiny but dense "nugget" of the field would show up as a particle—the Higgs particle. And how to knock that nugget loose (more precisely, convert energy into a particle excitation of the Higgs field)? One way would be to slam protons together in energetic collisions that would thwack the Higgs field much as a belly flop thwacks water, creating a spray of particles that, according to the calculations, should contain the Higgs. Nearly a half century after Peter Higgs wrote his paper, this is indeed how the Large Hadron Collider succeeded in producing Higgs particles.

The odyssey from a puzzle pitting theory against experiment to a mathematical proposal for reconciliation and then, most importantly, to experimental data confirming the mathematics is the very odyssey theorists hold in the highest regard. However beautiful our equations, however seductive our ideas, they are relevant to describing reality only when experiment establishes that they are. The confirmation of the Higgs particle ranks among the most important modern examples of mathematics illuminating the road to reality, and it led many scientists to anticipate that the discovery of yet more particles predicted by mathematical studies, would not be far behind.

The "super" in superstring theory refers to a deep mathematical symmetry called supersymmetry. It is only the supersymmetric version of string theory that has a chance of describing our world. Supersymmetry requires a host of particles well beyond those that scientists have so far discovered. Indeed, for every particle so far revealed, supersymmetry requires a partner particle: for the electron, the predicted partner is the selectron, for the neutrino there should be the neutrino, for quarks there should be squarks. These "sparticles" are the "superpartners" of the known particles.

Although supersymmetry was developed in the framework of string theory, physicists subsequently developed a version that applies to quantum field theory, and more particularly to the phenomenally successful quantum field theory of the known particles, the standard model. The supersymmetric version of the standard model makes the same prediction of superpartner particles, which is why even non-string theorists ranked finding them as a major goal of the Large Hadron Collider. But, you might ask, without the motivation of string theory, why would physicists be at all interested in modifying the triumphant standard model to include supersymmetry?

One reason is that with supersymmetry the strengths of the electromagnetic, weak nuclear, and strong nuclear forces converge to very nearly the same value on short distance scales, an important clue toward unification. But a more basic motivation for supersymmetry is entwined with the Higgs saga. Long before the Higgs particle was detected experimentally, physicists completed calculations showing that the Higgs mass would be particularly sensitive to the quantum influences of the many other particles with which it interacted. Indeed, the quantum influences would drive the Higgs mass billions and billions of times larger than the value subsequently revealed by the Large Hadron Collider (the measured mass of the Higgs particle is about 125 times the mass of the proton). Supersymmetry offered a solution to the mismatch: the superpartner particles would influence the Higgs mass too, but according to the calculations, these new contributions would cancel those coming from ordinary particles, thwarting the push toward a far larger Higgs mass.

Over the years, many physicists deemed this role for supersymmetry deeply compelling. Many, too, were charmed by supersymmetry's mathematical structure, which naturally culminates a long history of incorporating symmetry principles into physics. I remember well the pervasive feeling back in the 1980s during my graduate school days that supersymmetry was among those ideas that were simply too beautiful to be wrong. And so it seemed just a matter of time before the superpartner particles would be produced, the problem with the Higgs mass resolved, and an intrinsic quality of string theory confirmed. Champagne bottles in physics departments the world over were poised to pop.

So, all these years later, where do we stand?

Well, more than a decade since the Large Hadron Collider successfully found the Higgs, the headline summarizing subsequent progress would indeed read "Collider Finds Nothing!" The hunt continues, as an exhaustive search for the superpartners is a long and arduous task. But what are the implications if the trend continues and the superpartner particles fail to turn up?

Most concretely, physicists will once again face the puzzle of the Higgs mass: without the cancelations provided by superpartner particles, some other mechanism must be identified to keep the Higgs mass from exploding. But of most concern to our focus here, what would the absence of superpartner particles mean for string theory?

Well, if we could conclude that string theory was ruled out, I'd be thrilled. Not thrilled that string theory was wrong, but thrilled to have definitively settled the question of string theory's relevance to describing reality. I stress this point because a scientist's close association with a theory can easily be misconstrued as a commitment to supporting the theory even in the face of damning evidence. Let me be explicit: My commitment is not to string theory, or to any other proposal for nature's deep workings. My commitment is to advancing science toward truth. If data, or in this case the absence of data, could rule out string theory, that would be a resolute truth-directed step, and that is a step I would enthusiastically take. Don't get me wrong, I would be disappointed that string theory's mathematics did not realize the theory's promise of providing a unified, predictive, and confirmed description of reality. But that sentiment would pale in comparison to the profound appreciation for learning now—not a year from now or ten years from now or twenty-five years from now—that the string theory community's time and energy needed to pivot.

Perhaps one day we will have such evidence ruling out string theory, but the failure to find supersymmetry will not be the source. The reason is one of scale. To probe smaller and smaller distances we need larger and larger energies. In string theory, the central scale is the Planck scale, which in terms of length is about a billionth of a billionth the size of an atomic nucleus, and in terms of energy is about a hundred billion billion times the mass of a proton. By contrast, at the Large Hadron Collider's highest setting, it can probe lengths only as small as a thousandth the size of an atomic nucleus, with an energy that's only as large as ten thousand times the mass of a proton. And so, to probe the Planck scale we'd need an accelerator about a million billion times more powerful than the Large Hadron Collider.

If we had such a Planckian-scale collider, and if string theory is a correct theory of nature, supersymmetry would manifest in the data. But with today's coarse technology (coarse compared to the Planck scale) the supersymmetric quality of string theory can easily be obscured, much as the quantum qualities of the atomic world are obscured on the comparatively coarse scales of everyday life. Concretely, it might be that the superpartner particles exist but their masses lie well beyond the collider's reach, explaining why we've not detected them.

This is not a new realization. Since the earliest research on supersymmetry, theorists recognized that the fundamental equations did not specify the mass of the superpartner particles, and so there was always a chance that the Large Hadron Collider would lack the power to produce them.

Although I and many colleagues considered that chance to be small, there were theorists at that time who emphasized that string theory seems to favor superheavy superpartners, far too massive for the Large Hadron Collider or even any remotely realistic next-generation colliders to produce. It is worth spending a moment to appreciate their reasoning, especially now that the data are tipping things in their favor.

All the shapes I discussed in *The Elegant Universe*—Calabi-Yau shapes—share the property that they keep focus on supersymmetry, preventing it from being obscured as we examine the theory's implications at energies well below the Planck scale. In fact, in the original research paper that introduced Calabi-Yau shapes into string theory, the authors required that the extra dimensions protect supersymmetry, ensuring that their tiny geometry thins the quantum fog that would otherwise shroud the superpartners, and with that stipulation the mathematics led them to Calabi-Yau shapes.

But there are a great many other possibilities beyond Calabi-Yau shapes that string theory can happily use (that is, shapes that solve the equations of string theory) and such shapes do not protect the supersymmetric quality from being obscured. For these choices of the extra dimensions, string theory does not predict that superpartner particles will be within reach of the Large Hadron Collider, or for that matter any collider we could build in the foreseeable future. What's more, there are a great many more shapes that obscure supersymmetry than there are those that protect it. In the past, researchers (like me) who were deeply impressed by the mathematical beauty of supersymmetry, its capacity to address the puzzle with the Higgs mass, its natural unification of force strengths, its ability to simplify horrendously difficult calculations through justifiable approximations, and even its compelling proposals for dark matter were reluctant to study anything but the case of unobscured supersymmetry. But with no evidence for superpartners forthcoming from the Large Hadron Collider, this attitude is changing and researchers have been broadening their studies to include the much larger class of shapes that do not preserve supersymmetry. So, rather than ruling string theory out, the failure to find superpartners is instead shifting various research agendas within string theory itself.

Pulling back from the details, the challenge these developments underscore is that string theory does not make specific, unambiguous predictions that can be tested with current technology. Many of us had hoped that finding the superpartner particles would be the first in a series of breakthroughs that would, discovery by discovery, walk beyond the physics of the standard model and inch closer to string theory. But the goal of finding the superpartners was always a well-motivated hope and not a sharp prediction. Without equations that yield definitive values for the masses of the superpartners, there's ample opportunity for them to be too hefty for the reach of today's colliders and, more than likely, those of tomorrow.

To be sure, if colliders could reach nearly Planck-scale energies, we'd be in a very different situation, as direct probes of string physics would be possible. We would still need to understand the theory with greater depth to make detailed comparisons between calculations and data, but in that imagined setting experiment would guide theorizing much as it has across a significant stretch of the history of physics. Coming back to the reality of today's technology, at a remove of fifteen orders of magnitude from the Planck scale, such direct experimental contact is unlikely.

Does the lack of testable particle-physics predictions mean that string theory belongs to the realm of metaphysics? Detractors of string theory have argued, some with an oddly overheated zealotry, that the answer is yes. I find such assessments of little value. Any approach to quantum gravity, string theory or otherwise, will have core physics that is directly relevant to the Planck scale, and because this scale is so far beyond our technological reach, developing experimental tests will be extremely challenging. Such is the nature of the most ambitious theoretical research that walks along the very edge of knowledge. It surely does not mean the research fails to qualify as physics and should be abandoned.

But without experimental contact, how should we judge string theory?

We should use the very approach followed by those with the most on the line: young scientists deciding on their research careers as well as mature investigators striving to advance knowledge. As a matter of course, even without a formal assessment, these researchers are constantly evaluating the field's progress and achievements. And over the years, such evaluations have convinced some scientists that the advance toward experiment was too slow for their research expectations, and so they have left the field.

But others, indeed so many others that string theory has been berated for attracting too many of the highest-caliber scientists, have found that the pace of new theoretical discoveries and novel physical insights is so rapid and thrilling that they are propelled onward with vigor and excitement.

The point is, there is no right or wrong here. It is a matter of personal scientific taste. Those who want to work on projects that have a close and ongoing dialogue with experimental data should pass by string theory and focus their energies on other research fields. Those who want to explore unsolved problems at the interface of our deepest physical theories-quantum mechanics and general relativity-should focus on string theory (or any of the competing approaches). Everyone else should respect these choices. The best and brightest of our researchers deserve free rein in their careful deliberations regarding where to invest their talents.

In making such choices, some young researchers have nevertheless asked me when I expect string theory to confront data in a meaningful way. In the earlier days of string theory, my answer echoed the sentiment I articulate above. Namely, we all root for experimental contact, but as there are no crystal balls, researchers need to decide for themselves whether they can happily go forward even if such contact should fail to materialize during their productive years. My answer has since changed. I still note how important the question of experimental contact is, but I also note that a stunning breakthrough has shifted the character of the question. I will describe the result in more detail shortly, but here's the elevator pitch: We now have powerful evidence that—shockingly—string theory and quantum field theory are actually different languages for expressing one and the same physics. In consequence, the experimental luster of quantum field theory casts a newfound experimental glow on string theory. Make no mistake, to establish string theory experimentally we need to confirm a definitive string theoretic prediction. But the unexpected equivalence between string theory and the most rigorously tested framework in the history of science quantum field theory is strong evidence that by working on string theory we are pursuing physics, not metaphysics. ♦

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