Are Strings Still Our Best Hope for a Theory of Everything?

Fifty-eight years after it first appeared, string theory remains the most popular candidate for the “theory of everything,” the unified mathematical framework for all matter and forces in the universe. This is much to the chagrin of its rather vocal critics. “String theory is not dead; it’s undead and now walks around like a zombie eating people’s brains,” the former physicist Sabine Hossenfelder said on her popular YouTube channel in 2024.

String theory is a “failure,” the mathematical physicist and blogger Peter Woit often says. His complaint is not that string theory is wrong — it’s that it’s “not even wrong,” as he titled a 2006 book. The theory says that, on scales of billionths of trillionths of trillionths of a centimeter, extra curled-up spatial dimensions reveal themselves and particles resolve into extended objects — strands and loops of energy — rather than points. But this alleged substructure is too small to detect, probably ever. The prediction is untestable.

A further problem is that uncountably many different configurations of dimensions and strings are permitted at those tiny scales; the theory can give rise to a limitless variety of universes. Amid this vast landscape of solutions, no one can hope to find a precise microscopic configuration that undergirds our particular macroscopic world.

These issues are profound indeed. Yet in my experience, the typical high-energy theorist in a prestigious university physics department still thinks string theory has a good chance of being correct, at least in part. The field has become siloed between those who deem it worth studying and those who don’t.

Recently, a new angle of attack has opened up. An approach called bootstrapping has allowed physicists to calculate that, under various starting assumptions about the universe, a key equation from string theory naturally follows. For some experts, these findings support the notion of “string uniqueness,” the idea that it is the only mathematically consistent quantum description of gravity and everything else.

Responding to one bootstrap paper on her YouTube channel, mere weeks after the “undead” comment, Hossenfelder said it was “string theorists do[ing] something sensible for once.” She added, “I’d say this paper strengthens the argument for string theory.”

Not everyone agrees, but the findings are reviving an important question. “This question of ‘Does string theory describe the world?’ has just been so taboo,” said Cliff Cheung, a physicist at the California Institute of Technology and an author of the paper discussed by Hossenfelder. Now, “people are actually thinking about it for the first time in decades.”

Getting wind of this work, I wanted to drill down on the logic and examine how the string hypothesis is faring these days.

The new bootstrapping work relates to the first bit of string theory to be discovered, in 1968. A young Italian physicist named Gabriele Veneziano reverse-engineered a formula to capture the behavior of particles called hadrons. Other researchers soon realized that his formula, now known as the Veneziano amplitude, implied that hadrons aren’t particles, but vibrating strings.

Further research showed that hadrons aren’t strings; they’re just stringlike. They’re made of pairs and trios of quarks that are bound together by stringy trails of gluons. Even as physicists came to understand these particles by developing a quantum field theory (QFT) — the standard language of particle physics, in which particles are ripples of energy in fields that permeate space-time — the string theory that emerged from Veneziano’s work stuck around. Physicists realized that it offered a deeper mathematical description of the quarks and gluons themselves, as well as all other elementary particles, including, most thrillingly, gravitons: the hypothetical quantum units of gravity. Vibrations of open-ended strings could give rise to the properties of all known particles. Join a string’s ends, forming a loop, to do the same for gravitons.

Those who came to be known as string theorists marveled at the beauty of the math. In QFT, point particles can take endlessly variable paths, which causes conceptual and technical headaches. But the paths of strings converge and split in finite, enumerable ways, simplifying calculations.

One catch was that these strings must have 10 space-time dimensions to wiggle around in, so string theorists posited that there must be six tiny extra directions curled up at each point in our familiar four-dimensional space-time.

For all the mathematical elegance, the hidden dimensions were a bitter pill, at least until a striking result in 1984 made strings much easier to swallow. Elementary particles are “chiral,” meaning they differ from their mirror images, but the chiral theories physicists attempt to write down are prone to mathematical inconsistencies called chiral anomalies. The string theorists John Schwarz of Caltech and Michael Green of Queen Mary University of London calculated that in string theory, all the terms that threaten to be anomalous cancel out. The self-healing power of the math triggered a string theory revolution. “The string theory community went into a level of obnoxiousness that has never before been seen in physics,” Eric Weinstein, a physicist turned financier turned podcaster, said recently about this period. “They became completely intolerable.”

Into the 1990s, string theorists uncovered a bewildering web of mathematical equivalences, or “dualities,” between different versions of string theory, and between strings and quantum fields in different dimensions. It made things more complicated, but more mathematical miracles emerged — calculations that went right that didn’t have to. In 1996, for instance, Andrew Strominger and Cumrun Vafa at Harvard University constructed a model of a black hole in string theory. (They stacked many “D-branes,” which are surfaces that open strings end on, until their gravity became inescapable.) They calculated the black hole’s entropy by counting the possible configurations of the D-branes and got the same expression that Stephen Hawking and Jacob Bekenstein had derived for black hole entropy in the early 1970s using thermodynamics. The Bekenstein-Hawking entropy law had been mysterious; string theory seemed to explain where it comes from.

No other theories satisfying the constraints of quantum mechanics and general relativity worked well enough to allow such explicit calculations. But string theory was still totally detached from empirical reality. In the early 2000s it was shown that there are at least 10⁵⁰⁰ different configurations of the six compact dimensions, each theoretically undergirding a universe with different properties. Any hopes of testing the theory and determining which configuration produces our universe faded. The “string wars” ensued — a vitriolic back-and-forth over whether string theory is legitimate science and whether string theorists deserve the power and prestige they’ve accrued. It’s become a forever war.

Enter the bootstrap. Physics usually involves proposing a theoretical model, making predictions based on that model, and then testing them experimentally. Bootstrapping, on the other hand, involves starting with a list of desirable logical and physical principles — symmetry principles, for example, and unitarity (the rule that the probabilities of possible outcomes must add up to 1) — and imposing these constraints to try to infer a theoretical model. When bootstrapping works, it can point to only one physical system that’s consistent with the assumptions.

In recent papers, researchers have bootstrapped the Veneziano amplitude, the formula for the scattering of two open strings, as the unique solution that follows from various sets of starting assumptions. “People have studied for decades, what does string theory imply, and we’re asking, ‘What implies string theory?’” Cheung said. With this approach, if assumptions X, Y, and Z are true of our universe, then string theory is true. This focuses the debate not on the pros and cons of string theory but on the reasonableness of the assumptions.

Bootstrappers assume that, even at the highest energies and shortest distances (known as “the UV,” a reference to short-wavelength light), it still makes sense to talk about individual quantum units (be they particles or strings) moving around on a flat space-time background. In other words, just as they do at lower energies, these quantum units would respect the properties of unitarity and Lorentz invariance — essentially the symmetry between observers moving at constant speeds through space-time. These principles serve as the pillars of quantum mechanics and relativity and are sacrosanct in the accessible domains of the universe, so it’s reasonable to assume they also hold in the UV.

On top of these baseline requirements, the bootstrappers must make further assumptions in order to arrive at a unique answer.

In the August 2025 paper “Strings From Almost Nothing,” Cheung and three collaborators assumed “ultrasoftness,” a mathematical statement about avoiding infinitesimal distances. They showed that if the universe’s fundamental objects are ultrasoft (along with one more technical assumption), high-energy particle states must fall into a restricted pattern. They found that only the Veneziano amplitude and the Virasoro-Shapiro amplitude, which describes the scattering of two closed strings, match that pattern. The upshot is that for the universe to be ultrasoft, string theory is the only way.

The outcome is nice, but it might not be very revealing, because ultrasoftness was a known property of string theory. When strings collide ever more energetically, they spin faster and stretch instead of concentrating energy at smaller and smaller points. Woit called the use of ultrasoftness to bootstrap string theory “sophistry.”

“What’s great is we are legalistic about our assumptions,” Cheung said. “If you want to break our conclusion, break the assumptions, and let’s go think about it. We don’t have to feel emotions about it.” This is the bootstrapping ethos.

The January 2026 paper “String Theory From Maximal Supersymmetry” is generally regarded as more striking. In it, Henriette Elvang at the University of Michigan and two collaborators start with assumptions about QFT and get the Veneziano amplitude as the unique answer at high energies. “To really be able to say, ‘I’m just using field theory; I’m not assuming anything about the higher-order theory,’ … and then string theory comes out of just field theory — that’s really, really fun, I think,” Elvang said.

The group’s main assumption was a property called “N = 4 supersymmetry,” which says that particles with different amounts of spin, or intrinsic angular momentum, form a single family with related interactions. We don’t see this highest level of symmetry in nature, but theorists often study it as a toy model, because calculations are easier and sometimes reveal insights or structures that extend to less symmetric QFTs.

Elvang and her co-authors showed that if a QFT has this highest level of supersymmetry (along with two other technical assumptions), at close range the particles must be strings — that is, for two particles scattering off each other, the Veneziano amplitude is the unique “UV completion.” (Note that these calculations involve the “tree-level” amplitude, which is an approximation of the full formula that omits rare versions of the scattering event.)

“It’s very nice work,” said Pedro Vieira, a physicist at the Perimeter Institute in Canada and the ICTP South American Institute for Fundamental Research in Brazil, who co-authored one of the first papers to bootstrap an element of string theory in 2021 but has since moved on to other questions. In his opinion, if string theory is the unique UV completion of maximally supersymmetric QFT (and he stresses that so far Elvang has only shown this for the tree-level amplitude), the same probably goes for the particles and fields of the real world. But that will be difficult or impossible to prove. The QFT we see in the present-day, low-energy universe simply isn’t symmetric enough for physicists to solve the necessary equations.

Still, as more starting points are shown to lead to stringiness in the UV, the story of string theory is evolving in a way critics must contend with.

Woit, when I asked for his take, called Elvang’s finding “not really surprising.” He noted that trying to understand certain limits of QFTs in terms of strings is “a very old idea with a lot of evidence for it.”

In my view, this is overly dismissive. It has indeed long been a central source of interest in string theory that certain toy QFT models can be re-expressed as string theories. Elvang’s paper reveals a novel aspect of the that connection, even if the scope and meaning of it all remains unclear.

Other researchers questioned the foundational assumptions of these recent papers, arguing that it might not even make sense to talk about fundamental objects scattering off each other at the high energies and short distances of the UV. Bootstrappers take such interactions for granted by assuming Lorentz invariance (the symmetries of a flat space-time fabric for particles to move around in). But Astrid Eichhorn, a physicist at Heidelberg University who studies an approach to quantum gravity called asymptotic safety, explained that the UV regime of quantum gravity might be “dominated by space-time configurations which are far from flat and around which fluctuations are large, so that flat-space scattering amplitudes are meaningless.” Latham Boyle, a physicist at the University of Edinburgh who studies an eclectic array of topics, also questioned the assumption that scattering necessarily makes sense in the UV.

Grant Remmen of New York University, an author of the ultrasoftness paper, pushed back. He argued that any UV-complete theory should say what happens in flat space: “Scattering amplitudes are a necessary ingredient that any full theory of quantum gravity should make a prediction for.” In other words, it should be possible to describe high-energy scattering, even if that’s only one aspect of the full theory.

In general, Boyle, who occasionally works on string theory but thinks string theorists have taken a wrong turn, expressed a perspective on the string uniqueness papers that I agree with. “I don’t think that the most likely implication is that string theory has to be true,” he said. But “these results, like many earlier results, do show that there’s something very special about string theory. So I wouldn’t at all be surprised if it turned out as one of the fundamental ingredients of nature.”

Asked to elaborate, he said he counted string theory among a collection of “special mathematical objects,” along with Penrose tilings, the four number systems (real and complex numbers, quaternions, and octonions), and certain symmetry groups. “When you begin asking questions in the space of those objects, you find that you always keep getting led back to these very special theories. And also these theories end up connecting to a lot of areas of math and physics. The literature is full of people getting led to them from all directions. So they have this special awesome quality. That can be a hint that they are on the right track.

“It’s certainly true,” Boyle added, “that the laws of physics have gotten more mathematically beautiful as we’ve understood them better.”

Several of the researchers I interviewed, including authors of the bootstrap papers, described themselves as agnostic about whether string theory is true in our universe. They prefer to map out the logical relationships between ideas — like supersymmetry, ultrasoftness, and string theory — but prefer not to read between the lines, perhaps to avoid the sins of their forebears: overenthusiasm and overinterpretation, for example, which tend to get people riled up.

But Vieira also told me about a steady drip of research over the years that raises a final point worth considering. Lines and other extended objects seem to be commonplace and even expected in QFT, which ostensibly describes pointlike objects. Symmetries and forces that operate along lines and over surfaces naturally arise. “I think that is something that has been accepted,” he said. “Whether these extended objects, should we call them strings? Do we need to call them strings? I don’t know. But I think understanding that to fully describe quantum theories and quantum objects, thinking just about points is not enough.”

From that perspective, the notion that string theory is true in some circumstances might be totally quotidian, even if the space of theories, fundamental objects, and mathematical relationships between them is not yet fully understood.