String theory strutted onto the scene some 30 years ago as perfection itself, a promise of elegant simplicity that would solve knotty problems in fundamental physics — including the notoriously intractable mismatch between Einstein’s smoothly warped space-time and the inherently jittery, quantized bits of stuff that made up everything in it.

It seemed, to paraphrase Michael Faraday, much too wonderful *not* to be true: Simply replace infinitely small particles with tiny (but finite) vibrating loops of string. The vibrations would sing out quarks, electrons, gluons and photons, as well as their extended families, producing in harmony every ingredient needed to cook up the knowable world. Avoiding the infinitely small meant avoiding a variety of catastrophes. For one, quantum uncertainty couldn’t rip space-time to shreds. At last, it seemed, here was a workable theory of quantum gravity.

Even more beautiful than the story told in words was the elegance of the math behind it, which had the power to make some physicists ecstatic.

To be sure, the theory came with unsettling implications. The strings were too small to be probed by experiment and lived in as many as 11 dimensions of space. These dimensions were folded in on themselves — or “compactified” — into complex origami shapes. No one knew just how the dimensions were compactified — the possibilities for doing so appeared to be endless — but surely some configuration would turn out to be just what was needed to produce familiar forces and particles.

For a time, many physicists believed that string theory would yield a unique way to combine quantum mechanics and gravity. “There was a hope. A moment,” said David Gross, an original player in the so-called Princeton String Quartet, a Nobel Prize winner and permanent member of the Kavli Institute for Theoretical Physics at the University of California, Santa Barbara. “We even thought for a while in the mid-’80s that it was a unique theory.”

And then physicists began to realize that the dream of one singular theory was an illusion. The complexities of string theory, all the possible permutations, refused to reduce to a single one that described our world. “After a certain point in the early ’90s, people gave up on trying to connect to the real world,” Gross said. “The last 20 years have really been a great extension of theoretical tools, but very little progress on understanding what’s actually out there.”Many, in retrospect, realized they had raised the bar too high. Coming off the momentum of completing the solid and powerful “standard model” of particle physics in the 1970s, they hoped the story would repeat — only this time on a mammoth, all-embracing scale. “We’ve been trying to aim for the successes of the past where we had a very simple equation that captured everything,” said Robbert Dijkgraaf, the director of the Institute for Advanced Study in Princeton, New Jersey. “But now we have this big mess.”

Like many a maturing beauty, string theory has gotten rich in relationships, complicated, hard to handle and widely influential. Its tentacles have reached so deeply into so many areas in theoretical physics, it’s become almost unrecognizable, even to string theorists. “Things have gotten almost postmodern,” said Dijkgraaf, who is a painter as well as mathematical physicist.

The mathematics that have come out of string theory have been put to use in fields such as cosmology and condensed matter physics — the study of materials and their properties. It’s so ubiquitous that “even if you shut down all the string theory groups, people in condensed matter, people in cosmology, people in quantum gravity will do it,” Dijkgraaf said.

“It’s hard to say really where you should draw the boundary around and say: This is string theory; this is not string theory,” said Douglas Stanford, a physicist at the IAS. “Nobody knows whether to say they’re a string theorist anymore,” said Chris Beem, a mathematical physicist at the University of Oxford. “It’s become very confusing.”

String theory today looks almost fractal. The more closely people explore any one corner, the more structure they find. Some dig deep into particular crevices; others zoom out to try to make sense of grander patterns. The upshot is that string theory today includes much that no longer seems stringy. Those tiny loops of string whose harmonics were thought to breathe form into every particle and force known to nature (including elusive gravity) hardly even appear anymore on chalkboards at conferences. At last year’s big annual string theory meeting, the Stanford University string theorist Eva Silverstein was amused to find she was one of the few giving a talk “on string theory proper,” she said. A lot of the time she works on questions related to cosmology.

Even as string theory’s mathematical tools get adopted across the physical sciences, physicists have been struggling with how to deal with the central tension of string theory: Can it ever live up to its initial promise? Could it ever give researchers insight into how gravity and quantum mechanics might be reconciled — not in a toy universe, but in our own?

“The problem is that string theory exists in the landscape of theoretical physics,” said Juan Maldacena, a mathematical physicist at the IAS and perhaps the most prominent figure in the field today. “But we still don’t know yet how it connects to nature as a theory of gravity.” Maldacena now acknowledges the breadth of string theory, and its importance to many fields of physics — even those that don’t require “strings” to be the fundamental stuff of the universe — when he defines string theory as “Solid Theoretical Research in Natural Geometric Structures.”

**An Explosion of Quantum Fields**

This correspondence was never thought to be a perfect real-world model. The five-dimensional space in which it works has an “anti-de Sitter” geometry, a strange M.C. Escher-ish landscape that is not remotely like our universe.

But researchers were surprised when they dug deep into the other side of the duality. Most people took for granted that quantum field theories — “bread and butter physics,” Dijkgraaf calls them — were well understood and had been for half a century. As it turned out, Dijkgraaf said, “we only understand them in a very limited way.”

These quantum field theories were developed in the 1950s to unify special relativity and quantum mechanics. They worked well enough for long enough that it didn’t much matter that they broke down at very small scales and high energies. But today, when physicists revisit “the part you thought you understood 60 years ago,” said Nima Arkani-Hamed, a physicist at the IAS, you find “stunning structures” that came as a complete surprise. “Every aspect of the idea that we understood quantum field theory turns out to be wrong. It’s a vastly bigger beast.”

Researchers have developed a huge number of quantum field theories in the past decade or so, each used to study different physical systems. Beem suspects there are quantum field theories that can’t be described even in terms of quantum fields. “We have opinions that sound as crazy as that, in large part, because of string theory.”

This virtual explosion of new kinds of quantum field theories is eerily reminiscent of physics in the 1930s, when the unexpected appearance of a new kind of particle — the muon — led a frustrated I.I. Rabi to ask: “Who ordered that?” The flood of new particles was so overwhelming by the 1950s that it led Enrico Fermi to grumble: “If I could remember the names of all these particles, I would have been a botanist.”

Physicists began to see their way through the thicket of new particles only when they found the more fundamental building blocks making them up, like quarks and gluons. Now many physicists are attempting to do the same with quantum field theory. In their attempts to make sense of the zoo, many learn all they can about certain exotic species.

Conformal field theories (the right hand of AdS/CFT) are a starting point. You start with a simplified type of quantum field theory that behaves the same way at small and large distances, said David Simmons-Duffin, a physicist at the IAS. If these specific kinds of field theories could be understood perfectly, answers to deep questions might become clear. “The idea is that if you understand the elephant’s feet really, really well, you can interpolate in between and figure out what the whole thing looks like.”

Like many of his colleagues, Simmons-Duffin says he’s a string theorist mostly in the sense that it’s become an umbrella term for anyone doing fundamental physics in underdeveloped corners. He’s currently focusing on a physical system that’s described by a conformal field theory but has nothing to do with strings. In fact, the system is water at its “critical point,” where the distinction between gas and liquid disappears. It’s interesting because water’s behavior at the critical point is a complicated emergent system that arises from something simpler. As such, it could hint at dynamics behind the emergence of quantum field theories.Beem focuses on supersymmetric field theories, another toy model, as physicists call these deliberate simplifications. “We’re putting in some unrealistic features to make them easier to handle,” he said. Specifically, they are amenable to tractable mathematics, which “makes it so a lot of things are calculable.”

Toy models are standard tools in most kinds of research. But there’s always the fear that what one learns from a simplified scenario does not apply to the real world. “It’s a bit of a deal with the devil,” Beem said. “String theory is a much less rigorously constructed set of ideas than quantum field theory, so you have to be willing to relax your standards a bit,” he said. “But you’re rewarded for that. It gives you a nice, bigger context in which to work.”

It’s the kind of work that makes people such as Sean Carroll, a theoretical physicist at the California Institute of Technology, wonder if the field has strayed too far from its early ambitions — to find, if not a “theory of everything,” at least a theory of quantum gravity. “Answering deep questions about quantum gravity has not really happened,” he said. “They have all these hammers and they go looking for nails.” That’s fine, he said, even acknowledging that generations might be needed to develop a new theory of quantum gravity. “But it isn’t fine if you forget that, ultimately, your goal is describing the real world.”

It’s a question he has asked his friends. Why are they investigating detailed quantum field theories? “What’s the aspiration?” he asks. Their answers are logical, he says, but steps removed from developing a true description of our universe.

Instead, he’s looking for a way to “find gravity inside quantum mechanics.” A paper he recently wrote with colleagues claims to take steps toward just that. It does not involve string theory.

**The Broad Power of Strings**

Perhaps the field that has gained the most from the flowering of string theory is mathematics itself. Sitting on a bench beside the IAS pond while watching a blue heron saunter in the reeds, Clay Córdova, a researcher there, explained how what seemed like intractable problems in mathematics were solved by imagining how the question might look to a string. For example, how many spheres could fit inside a Calabi-Yau manifold — the complex folded shape expected to describe how spacetime is compactified? Mathematicians had been stuck. But a two-dimensional string can wiggle around in such a complex space. As it wiggled, it could grasp new insights, like a mathematical multidimensional lasso. This was the kind of physical thinking Einstein was famous for: thought experiments about riding along with a light beam revealed E=mc^{2}. Imagining falling off a building led to his biggest eureka moment of all: Gravity is not a force; it’s a property of space-time.

String theory has also made essential contributions to cosmology. The role that string theory has played in thinking about mechanisms behind the inflationary expansion of the universe — the moments immediately after the Big Bang, where quantum effects met gravity head on — is “surprisingly strong,” said Silverstein, even though no strings are attached.

Still, Silverstein and colleagues have used string theory to discover, among other things, ways to see potentially observable signatures of various inflationary ideas. The same insights could have been found using quantum field theory, she said, but they weren’t. “It’s much more natural in string theory, with its extra structure.”

Inflationary models get tangled in string theory in multiple ways, not least of which is the multiverse — the idea that ours is one of a perhaps infinite number of universes, each created by the same mechanism that begat our own. Between string theory and cosmology, the idea of an infinite landscape of possible universes became not just acceptable, but even taken for granted by a large number of physicists. The selection effect, Silverstein said, would be one quite natural explanation for why our world is the way it is: In a very different universe, we wouldn’t be here to tell the story.

This effect could be one answer to a big problem string theory was supposed to solve. As Gross put it: “What picks out this particular theory” — the Standard Model — from the “plethora of infinite possibilities?”

Silverstein thinks the selection effect is actually a good argument for string theory. The infinite landscape of possible universes can be directly linked to “the rich structure that we find in string theory,” she said — the innumerable ways that string theory’s multidimensional space-time can be folded in upon itself.

**Building the New Atlas**

At the very least, the mature version of string theory — with its mathematical tools that let researchers view problems in new ways — has provided powerful new methods for seeing how seemingly incompatible descriptions of nature can both be true. The discovery of dual descriptions of the same phenomenon pretty much sums up the history of physics. A century and a half ago, James Clerk Maxwell saw that electricity and magnetism were two sides of a coin. Quantum theory revealed the connection between particles and waves. Now physicists have strings.

“Once the elementary things we’re probing spaces with are strings instead of particles,” said Beem, the strings “see things differently.” If it’s too hard to get from A to B using quantum field theory, reimagine the problem in string theory, and “there’s a path,” Beem said.In cosmology, string theory “packages physical models in a way that’s easier to think about,” Silverstein said. It may take centuries to tie together all these loose strings to weave a coherent picture, but young researchers like Beem aren’t bothered a bit. His generation never thought string theory was going to solve everything. “We’re not stuck,” he said. “It doesn’t feel like we’re on the verge of getting it all sorted, but I know more each day than I did the day before – and so presumably we’re getting somewhere.”

Stanford thinks of it as a big crossword puzzle. “It’s not finished, but as you start solving, you can tell that it’s a valid puzzle,” he said. “It’s passing consistency checks all the time.”

“Maybe it’s not even possible to capture the universe in one easily defined, self-contained form, like a globe,” Dijkgraaf said, sitting in Robert Oppenheimer’s many windowed office from when he was Einstein’s boss, looking over the vast lawn at the IAS, the pond and the woods in the distance. Einstein, too, tried and failed to find a theory of everything, and it takes nothing away from his genius.

“Perhaps the true picture is more like the maps in an atlas, each offering very different kinds of information, each spotty,” Dijkgraaf said. “Using the atlas will require that physics be fluent in many languages, many approaches, all at the same time. Their work will come from many different directions, perhaps far-flung.”

He finds it “totally disorienting” and also “fantastic.”

Arkani-Hamed believes we are in the most exciting epoch of physics since quantum mechanics appeared in the 1920s. But nothing will happen quickly. “If you’re excited about responsibly attacking the very biggest existential physics questions ever, then you should be excited,” he said. “But if you want a ticket to Stockholm for sure in the next 15 years, then probably not.”

*This article was reprinted on TheAtlantic.com.*

"String theory has so far failed to live up to its promise as a way to unite gravity and quantum mechanics."

I have news for you, String theory has already successfully unified Quantum mechanics and Gravity.

Jules Henri Poincaré published E=mc2 in the leading Dutch physics journal in 1900. Einstein attributed the proof to himself more than 5 years later.

Patrice Ayme : September 15, 2016 at 5:16 pm

Could you please provide the exact and detailed citation?

Giotis says:

September 15, 2016 at 4:33 pm

You said: I have news for you, String theory has already successfully unified Quantum mechanics and Gravity.

Can you then tell us what is the mathematical description, according to that theory of the space-time corresponding to say, large, massive, spherical object which is in a quantum superposition of being located at two distant points in space?

I hope not lie -…..Giotis¡¡¡¡

Strange second life? Quite an euphemistic description of an ascension.

I think that string theory is just fine; so in 1to 100 years from now something may be known to understand it or prove it ,we'll see. Meanwhile if it turns out to be false then others will know not to take that direction while researching other unknowns, right. It's called building and testing a science so that we can trust the proven facts that we know and test them some more too make sure and if someone else finds a way to prove there is no use to continue in that way, then it will fail. Who is the famous genius that will do that? this is for the future to say. Just relax and wait for the next big find then you can have your Grand Canyon form in 1 day before your very eyes.You can also work really hard and bring proof to stop the silliness of These sting things. Either way the future still still learns from the study's of today. Science wins regardless; now you can see.

I'm not really seeing a broad outline in this article resolving the complete failure to find anything supporting String Theory from any of the runs at CERN–results which the author explicitly predicted.

String theory successfully unifies Gravity and Quantum Mechanics in the following sense:

For a particle physicist Gravity is the force mediated by a spin 2 massless particle called Graviton.

We already have a theory of Quantum Gravity which describes the quantum interactions of these particles called “perturbative Quantum Gravity”. This theory for example is used to calculate scattering amplitudes of gravitons when they collide.

The problem is that due to the very nature of gravitational interactions the aforementioned theories produce incurable divergences when gravitons at very high energies (called Ultra Violet UV regime) interact or when matter particles interact via Gravity. If this happens we say that the theory is “non renormilizable” meaning that it is ill and not well defined at arbitrary high energies. Despite the fact that this theory is not the fundamental one it is good enough to describe graviton interactions at low energies until some energy cutoff. Such theories are called “effective” field theories and their divergences signal the fact that above some energy scale they must be replaced by a new “UV finite” theory with new degrees of freedom that will tame these divergences. If this theory can indeed cure these divergences at arbitrary high energies then it is considered fundamental and people say that it provides the “UV completion” of the effective theory.

If you can find such theory you can say that you have successfully unified Quantum Mechanics and Gravity. This is what “perturbative String theory” does since the scattering amplitudes of String theory Gravitons (close loop of Strings) do not exhibit these divergences. Moreover at low energies they reduce to the effective theory of “perturbative Quantum Gravity”.

This is a very big achievement, no other can achieve this.

But Gravity is a very peculiar theory in the sense that it doesn’t describe only particle interactions in space-time; instead the Gravitational field itself is the dynamical space time. So a theory of Quantum Gravity should also be able to provide answers to more deep questions and phenomena related to the nature of space-time. These phenomena (e.g. the Black hole singularity or the singularity at the Big-Bang) are undetectable by a theory of perturbative nature and seemingly cannot be described by a perturbative String theory. To answer such questions people say than we need the “non perturbative” definition of String theory and we don’t have such definition yet (I exclude AdS/CFT because it is very difficult to decipher the CFT on the boundary in order to provide concrete answers for such questions in the bulk i.e. the answers to these questions are in the CFT but they are hidden and we don’t know how to extract them).

But people in this article are not talking about String theory as a theory of Quantum Gravity only, instead they are talking about String theory as a Theory of Everything (ToE) i.e. unified theory that can describe all particle interactions and matter content (The Standard model plus Gravity) in one complete Cosmology, the vacuum of our Universe.

This is a herculean task indeed but String theory has all the right necessary ingredients to accomplish it.

Despite a longstanding dissaffection for ST, I have to admit it's cool that its structure is so rich, offers so many new insights, and keeps so many smart people employed who otherwise might have ended up on street corners selling illegal prime numbers. On the other hand, this very expansiveness is addictive, preventing those involved from looking elsewhere. I mean, once you're in this endlessly interesting forest, who has the time to look elsewhere? Whether or not it hides a path to Truth, it at least provides a place for folks to hang out, conference all over the world, publish just about anywhere you want, and buy bread.

Maybe a new Cable Theory can make us gain more insight into the process of understanding QFT.

String theory would not be a reliable theory because it is background dependant that opposed the very notion that Einstine had first developed for theory of everything moreover it had many versions.So let's choose another approached.

is it possible that vibrating quantum fields of the field theory could be the same or similar to vibrating strings ? Are Entanglement of quantum fields or entanglement of strings the same ? Questions from a non physicist.

Just lol @ people who dismiss string theory yet can't even solve basic free body diagram problems in basic physics. NEWS FLASH: You do NOT know ANYTHING about PHYSICS nor the history of discovery. Nature is NOT obligated to reveal itself in your arbitrary time constraint.

I am still trying to wrap my head around quantum mechanics…

I agree with Giotis. String theory is elegant and the only theory that has a natural place for the spin 2 graviton as well as all the other particles of Standard Model. By replacing the Feynman diagrams made of point particles by soft strings lots of infinities go away. I sure hope that there will be some new insights that will simplify the horridly abstract math, but maintaining all the dualities.

The fundamental science (mathematics, physics, cosmology) are experiencing the crisis of understanding, which is all the more acute (Lee Smolin," The Trouble with Physics: The Rise of String Theory, the Fall of a Science, and What Comes Next").

Mathematicians confident that they will "close the physics" (L. Faddeev), but the mathematics itself has long been "lost certainty" (M.Kline "Mathematics. The Loss of Certainty"). Fundamental physics "rested" in the limits of knowledge, understanding "matter", "space", "time", the nature of "laws of nature", fundamental constants, information, consciousness.

Understand that means "to grasp the structure." (G.Gutner "Ontology of mathematical discourse"). Physics was the path of unification and geometrization. Today it is necessary to deepen this way – to the most distant ontological depth. The world picture of physics should be rich in meaningы of the "LifeWorld"(E.Husserl) as the picture of the world poets.

Perhaps it is time to try – regarding elementary particles – research that would parallel the development path that went from 'the periodic table (for elements)' to 'atomic and nuclear physics (for isotopes).' The periodic table is a table of 'what' – what elements might exist in nature. Atomic and nuclear physics later provided theory correlating with 'how' nature (in effect) chooses elements.

People might say that some work in string theory attempts to address the topic of 'how nature chooses elementary particles' before people adequately understood the topic of 'what elementary particles nature might include.'

The book "Models for Physics of the Very Small and Very Large" (recently published by Springer) shows an attempt to tackle the 'what elementary particles?' topic without emphasizing the 'how (for elementary particles)' topic. The work considers solutions to equations involving isotropic pairs of isotropic quantum harmonic oscillators. A subset of solutions correlates with the set of known elementary particles (and some of their properties and interactions). Other solutions correlate with possible elementary particles (and some properties and interactions). Thus, the work predicts elementary particles. To the extent that work and other work in the book pertain to nature, the book addresses aspects of 'how' regarding dark matter, dark energy, the rate of expansion of the universe, and other topics.

Assuming work in the book correlates with nature, people might want to explore the extent to which people can merge models in the work and aspects of string theory. Perhaps some synergies pertain. Likely, supersymmetry does not pertain.

Also, people might want to try to develop other models for 'what' elementary particles have yet to be found.

"Second life" implies that ST had a first life. Which apparently has ended. What was it's first life again, and when did it end?

@Giotis

Giotis said:

"String theory successfully unifies Gravity and Quantum Mechanics in the following sense:…"

> If 'successfully' means 'as an accumulation of assumptions', then I agree.

The problem sofar always remains the same : The renormalisations to tame infinities, make use of fairytale mathematics such as making disappear the mass contributions of the oscillators : 1+2+3+4+… = -1/12 and mistaking 1-1+1-1+1-1+… for an avarage resulting in 1/2.

If you do not indicate clearly how many steps the series take, then you have not specified any value, hence nothing to work with. And if you do specify the number of steps then you get a fixed positive number for the first series, and a 0 or a 1 for the second series (*).

It's not because Leibniz (*) said it, that we should accept it (and it's convenience to the problem at hand). It is just not an avarage.

Try putting apples in a bowl, and see if they turn out to be half an apple , or a negative apple…so much for making mass contributions disappear.

(*) : Leibniz (Gerhardt) pp.386-387; Hitt (p.143) translates the Latin into French.

>Now, the infinite series 1 − 1 + 1 − 1 + · · · has neither an even nor an odd number of terms, so it produces neither 0 nor 1; by taking the series out to infinity, it becomes something between those two options. There is no more reason why the series should take one value than the other, so the theory of "probability" and the "law of justice" dictate that one should take the arithmetic mean of 0 and 1, which is (0 + 1) / 2 = 1/2.

Conclusion :

Wishfull thinking does not successfully unify Gravity and Quantum Mechanics.

Hard work from the ground up might one day, by developing a theory which will avoid generating infinities right from the start.

P.S. If you respond, please stick to causal arguments on the claims made, and avoid personal attacks, thank you.

Best, Koenraad

Nice article. I personally agree with Giotas's comments. I'd like to add that for me string theory is an existence proof. It's a proof that the problem of quantum gravity has a solution. We don't know if it's this solution which is realized in the real world. But it's gratifying to know that a solution exists.

Meanwhile at "Not Even Wrong" … They seems to be questioning the credibility of this journalist: http://www.math.columbia.edu/~woit/wordpress/?p=8778#comments

So is there any physical evidence for this theory?

Same goes for Dark matter and dark energy.

Yes, consistent mathematical models can be created, but that doesn't cut it as evidence. Neither does "going viral" (i.e. talk, talk, talk).

I'm sorry, but this article contains nothing new. Every time a string theory article comes out we hear the same 8 or 9 people quoted saying the same things they were saying back in the late 90's when I was a string theory graduate student. It is always grandiose and packed with contentless hyperbole and is getting to be tedious self justification. Now the tone is shifting to how great string theory turns out to be for other unrelated fields. No doubt to justify continuing their playtime.

Here is a fact: take any broad enough base theory (in this case QFT), add Weyl and Kaluza's idea from almost 100 years ago about compactified extra dimensions, and you will have an endless mathematical playground already. Now do what string theorists have been doing for 25 years, namely read the mathematical popular literature and everytime some sexy math problem is popularized they add the math behind it into string theory so they can play. Number theory, algebraic topology, algebraic geometry, etc etc etc. You can hardly find an area of mathematics that doesn't have a string theory paper out there incorporating that math into "string theory" which essentially means they take well known math, and combining it with well known physics, to get something that, while true, is meaningless. The only strange thing is how everyone still falls for it. I guess the key is to give everything a new name and lingo so that people have no idea what you are talking about and are too embarrassed to admit it and challenge you.

There are plenty of stories of students jumping straight to string theory and skipping undergrad physics. This is stated as being amazing. A student being able to do string theory without learning physics is not miraculous. It doesn't show genius. It shows that the actual math is pretty easy and it has a tenuously meager relation to real physics. It is like inventing "modern music" which is an infinite all encompassing collection having no predefined rhythm, tune, or lyrics and then calling it miraculous when a child walks in and starts banging away with no prior music lessons.

Well. I had better go back to my elliptic curve zeta function renormalization on a betti-gruber-compacified argand dimension. I think I can solve the Brantford comjecture by simply adding extra cubic Bessel gauge structure on the dual graph of the principle bundle over our 47 dimensional space-time-time (i.e. a <2,45> fold geometry)

PS: Almost all math is deep and fun. All physics contains fascinating things to explore with that math. I so much _wish_ somebody would pay me to spend my days applying fun math to fun physics all day with no other responsibilities. That would be paradise. Unfortunately, most of us need to make it connect to reality (experimental reality) and we also have to teach, grade papers, advise students, sit of boards and committees, write grant applications, and try to find time to keep our marriages afloat. No relaxing IAS paradise for most of us. Pretty sweet gig you guys got. I can see why you popularize so much. I would to.

Why is string theory considered the *only* way to deal with particles of spin greater than one? There are others. In fact, effective theory works perfectly well for massive particles of any spin. Yes they are composite and yes there are lower spin components creeping in which is what causes all the trouble, but it seems difficult to believe we can't find another way to eliminate them without the seeming overkill of supersymmetry and 10 dimensions compactified on a vast number of possible calabi-yaus. In other words, what about re-examining the original motivation for string theory in the context of the 35 years of mathematical machinery we have developed?

Re: "explosion of new kinds of quantum field theories…"

What is the bar for real-world, observational prediction and experimentation before labelling hypotheses a theories?

"The Strange Second Life of String Theory" indeed!

It can now be used to make predictions for science articles.

Reading this I am reminded of the logical paradoxes- talking about oneself, about every thing, about the beginning and about the end, all involve antinomies. This is not to say that such exercises or discourses are not rewarding in other ways.

Loved the article.

Been brushing up on my pi 0.5s and spinors, and though I don't work directly with string theory am starting to appreciate it more and more. Getting ready to take back half of the bad things I've said about it here real soon. Kidding, kidding. But seriously, the more I study it the more its making sense to me and the application it has to my work in GU.

In fact, I'm starting to become a real fan of it, I have to say. I wouldn't mind seeing more in-depth articles on it here on Quanta.

One of the things I truly like about Quanta is the personalities you read about, basically making celebrities of the very people who should be our celebrities. I think. Yes, I love physics, I'll admit. And the amplihedron sketch as very cool too.

Everyone here that is saying String Theory has already solved the quantum gravity problem is fooling themselves. Though mathematically beautiful, String Theory has no scientific evidence whatsoever that it is true. I find it ironic that the majority of scientists, who are supposedly obsessed with critiquing their own ideas, couldn't imagine String Theory being false. Feynman didn't like it at all, saying:

"I don't like that they're not calculating anything. I don't like that they don't check their ideas. I don't like that for anything that disagrees with an experiment, they cook up an explanation."

And today, since we have come up completely empty on Supersymmetry, a prerequisite necessary for String Theory to be true, I too think that most (not all) String Theorists are guilty of some form of group-think.

String is dead. Cremated with publication of The Theory of Everything and The Origin of Everything.

"I have news for you, String theory has already successfully unified Quantum mechanics and Gravity."

I could do that with a piece of tape and generate the same number of testable predictions.

Jim Andreasen asks: "What is the bar for real-world, observational prediction and experimentation before labelling hypotheses a[s] theories?"

A hypothesis that expressed mathematically becomes a theory. No need to relate it to nature. Just to math.

I would guess this accords well with multiverse theory. Since there are an infinite number of universes, each math conjecture must have its own corresponding universe. So math can just substitute for nature.

Everybtime I read about the plethora of new theories, both past and present, I think of Pauli saying famously, "That theory isn't even wrong." in physics, experimental confirmation must reign supreme. Mathematics is, of course a differents matter.

A future succesfull theory is in desperate need of clearly defined causal prinicples.

In his essay on lessons from Einstein’s 1915 discovery of General Relativity, Lee Smolin [arXiv:1512.07551] addresses this and related issues, summarizing :

“The lesson is that the task of formulating a physical principle must come first. Only when we have one in hand do we have a basis to look for new mathematics to express the new principle."

"The start of the answer is that Einstein asked different questions than his contemporaries. Why? Because he had a deep need to tell a coherent story about the world.” That's hard to disagree with in my oppinion.

Both String Theory and LQG have advantages and disadvantages (*).

But further: Both are missing out on a true causal priniple for gravity and neither succeeds in intrinsically avoiding infinities in the formulation of the geometry of space and time. Infinitesimal calculus fails to describe the finite character of a necessary quantum-like description. One approach could be to redefine the calculus system in such a way that it automatically formulates a finite geometry.

In today's research in theoretical physics (and in other domains as well), we need more meta-cognitive thinking (google on 'Bloom's taxonomy revised').

It is vastly underrepresented in our educative system today, in any age-group.

And efficient creativity holds an enormous know-how which hasn't yet been introduced in theoretical physics learning program.

How to solve the problems in our problem solving methods ?

(Einstein was a meta-cognitive thinker : "You cannot solve the problem with the method that caused it.")

How can we develop more intelligent problem solving protocols ?

How to implement lateral thinking harmoniously with vertical thinking ?

Learning, knowing and understanding existing (complex and difficult) theories is not the same thing as being capable of improving, reformulating or inventing a new theory. The former requires classical 'intelligence', the latter requires meta-cognitive thinking combined with efficient creativity, context-deepening and pattern-recognition.

We need complementary skilled people working together, that's what yields added value. The best theories in physics sofar emanated from that :

Einstein-Grossmann and Faraday-Maxwell.

(*)

String theory :

+: It has the ambition to explain space ànd matter

-: It starts from spacetime as currently described, therefore de facto, it cannot look underneeth hyperbolic spacetime.

LQG :

+: It tries to describe what spacetime itself is constructed from.

-: It does not attempt to incorporate the nature of matter in the concept.So de facto not a unifying theory.