Believe it or not, physicists want to keep it simple. That’s why many scientists, including Albert Einstein, believe physics could eventually converge into a single, overarching paradigm that describes the universe—a theory of everything.

Enter string theory. Very broadly speaking, string theory is a mathematical framework that replaces point-like particles with one-dimensional “strings” as the fundamental building blocks of matter. It was initially proposed as an explanation for a different phenomenon but quickly caught the attention of physicists working to unify quantum mechanics and general relativity—two extremely successful, equally valid theories that notoriously don’t get along.

Then followed two “superstring revolutions,” which saw impressive strides in mapping out the details of how string theory could capture the complexity of our universe. The fervor of string theory naturally leaked over to popular conversations—science enthusiasts of the 1990s and 2000s, I’m looking at you—producing famous documentaries such as PBS’s The Elegant Universe and a trove of popular and academic books.

With the turn of the century, however, the limelight over string theory appeared to fade. Granted, public opinion is fickle and blasé, and that non-academics don’t care doesn’t mean that the idea is dead. Still, if Google’s Ngram viewer is any guide, string theory’s clout has staggered over the past decade or so.

That’s the question we posed to physicists for this Giz Asks. Whatever happened to string theory? Clearly, physicists haven’t given up on a theory of everything. But is string theory the leading candidate—if it ever was? Or are there now better alternatives? Has it truly diminished in popularity and significance? If so, why?

The following responses may have been lightly edited and condensed for clarity.

Daniel Whiteson

Particle physicist, University of California, Irvine, and the ATLAS Collaboration at CERN.

String theory is not dead! The major objection is that its predictions are for things at a microscopic scale that we cannot yet test, so it has not provided a falsifiable prediction. But that doesn’t mean it never will. Group theory was a mathematical curiosity for a century before we discovered that it was essential for describing particle physics and quantum field theory. I’m not a fan of categorizing things as “science” or “not science,” because who knows what nerdy curiosity will lead to a discovery?

John H. Schwarz

Theoretical physicist, California Institute of Technology; discovered the Green-Schwarz mechanism, which ignited the first superstring revolution in 1984.

The subject took off [around 1984] and has been very active ever since. The annual string theory conferences are still going strong and typically have several hundred participants. […] A large portion of the theoretical particle physics community is now convinced that we are on the right track to discovering the correct unified theory of our universe (and many naysayers have repented).

That said, we also realize that there is much more that remains to be understood, and it may take a long time to obtain experimental evidence in support of this theory.

Peter Woit

Mathematician and physicist, Columbia University; author of Not Even Wrong and the eponymous blog.

The idea of string theory as a new fundamental unified theory has been dead for a long time. To simplify a bit, it needs ten space-time dimensions, but we only see four, so you have to get rid of six. Simple scenarios that do this don’t look like the real world; complicated ones can give you almost anything, so predict nothing.

The final nail in the coffin was the negative results at the LHC about supersymmetry, which is a crucial part of most scenarios and the only thing with any hope of being seen experimentally. The term “string theory” has come to be used to refer to a huge range of different ideas that grew out of trying to make a unified theory work. Most “string theorists” now work not on the failed unified theory but on very different subjects.

These have generated some important new ideas in mathematics but no new insight into fundamental physics in our world. In particular, while people study lots of “quantum gravities,” these are theories that don’t explain quantum gravity in our four-dimensional world.

Thomas Van Riet

Theoretical physicist, Leuven University in Belgium; specializing in supergravity, string theory, holography, and cosmology.

Yes, [string theory has diminished in popularity]. The reason is that 20 years ago science outreachers and grant writers promised the heavens. It never made any sense. We also knew 20 years ago that string theory has a so-called landscape of ground states and so has no unique predictions.

But the so-called alternatives cannot compete at all, and it remains a puzzle of sociological dynamics how they were able to frame themselves as alternatives. Let me be clear, it’s great that people study other options. But there is simply no reason to say they achieved a quantum mechanical description of gravity.

People say that without experiment we cannot call one theory better than another. That is plain wrong. There are many consistency checks, which are ridiculously hard to pass. Can you compute black hole entropy? String theorists were able to compute it in very idealized circumstances and reproduced Hawking’s famous formula for black hole entropy!

This is where science can progress without experimental input and it is a point that is largely misunderstood by philosophers: in physics we study the whole time unphysical things… But it means you create idealized circumstances so you are able to make computations and test a framework. In quantum gravity the very game of passing mathematical consistency is so strong that it makes the search for theories almost converge uniquely towards strings.

Similarly, string theory can be just a toy model. But even that is great! I can for instance, in this model, look at a space with a Big Bang singularity and ask myself, how does this theory deal with it? It must give an answer since it is a mathematically complete theory. So it has to tell you what the beginning of time looks like in this (toy?) model.

Carlo Rovelli

Theoretical physicist, Centre de Physique Théorique de Luminy in France; Rovelli is a founder of loop quantum gravity, a rivaling account to string theory.

Especially in the last 10 or 5 years, to the large physics community, the appeal of string theory has very much decreased. For three reasons. The first is that the theory has indicated that a number of physical predictions could be verified, such as supersymmetric particles observable by the [LHC], a negative cosmological constant, the production of mini black holes in particle physics experiments, modifications of Newton’s law at a short scale, and so on. None of these predictions has turned out to be correct.

These repeated failures do not completely kill the theory, which can always be “adjusted” to take care of the negative result. But they have decreased the confidence in the theory for the majority of scientists.

The second reason, probably even more important, is that the theory gained much attention in the 1980s and 1990s because it promised to solve open questions in particle physics. For instance, computing the free parameters of the Standard Model from first principles, understanding why there are three generations of particles, why those particular gauge groups, and so on. The theory never succeeded in this. This failure has also diminished the credibility of the string hypothesis.

The third reason is that after 50 years we do not yet have a well-defined formulation of string theory. We have a patchwork of related results but no clear overall theory defined by a well-posed set of equations.

Hiroshi Ooguri

Theoretical physicist, California Institute of Technology; key contributor to mathematical developments in string theory.

Unifying quantum mechanics with “general” relativity is 25 times more difficult than unifying it with “special” relativity. […] It took physicists 50 years to develop a concrete model based on quantum field theory to describe particle physics phenomena (the Standard Model) and another 40 years to confirm all of its predictions (the Higgs boson discovery).

Therefore, one could argue that it would take 1,250 years to construct a realistic model of the universe from string theory and another 1,000 years to verify it experimentally. Now, I don’t think it would really take that long, but this illustrates how difficult the problem is. Therefore, I would expect achieving such a unification to produce 25 times more rewarding results and have 25 times more impact on a broad range of physics and mathematics.

Currently, string theory is the only promising candidate for achieving unification. No other proposals have comparable impacts on a broad range of sciences or mathematical depth. So, in some sense, we theoretical physicists are developing a language to understand nature.

One thing I want to point out is that, of course, our ultimate goal is to unify general relativity and quantum mechanics and then test that experimentally. I hope that there’ll be a positive test, but if it is rejected, that’s also progress in science, so I would accept it.

Cumrun Vafa

Theoretical physicist, Harvard University; Vafa received the 2017 Breakthrough Prize in physics for his work in applying string theory to the study of black holes and the universe.

Puzzles often come in pairs, each serving as the other’s solution! This is what seems to be currently happening in physics. Recent cosmological observations by two groups, DESI and DES, in their 2024 and 2025 data have found evidence that the standard model of cosmology, which assumes the dark sector does not deplete over time, does not match the data. Currently our string-motivated model is the best model fitting their data!

In a parallel development there are experiments underway in Austria and the US that aim to directly detect the dark dimension [hypothetical extra dimension in string theory]… These experiments are within years of producing their first data.

So it seems that both the large-scale cosmological observation and the small-scale tabletop experiments are both on their way to confirming string theory predictions. We may be at the cusp of a major scientific discovery. We should have a clearer picture within 5 to 10 years.