In October 1984, it was announced that the Nobel Prize for Physics had been awarded to Carlo Rubbia and Simon van der Meer, for the discovery of the W and Z bosons at the UA1 experiment at CERN just the previous year. This was the capstone discovery in the establishment of the Standard Model of particle physics. The third generation of fermions had already been discovered (the tau lepton by Martin Perl in 1977, the bottom quark by Leon Lederman also in 1977), and the nature of the strong interactions had been elucidated by deep-inelastic scattering experiments at SLAC in the late 1960’s and early 1970’s. Unsuspected by many, particle physics was about to enter an extended period in which no truly surprising experimental results would emerge; subsequent particle experiments have only been able to confirm the Standard Model over and over again, including the eventual discovery of the top quark at Fermilab in 1995. (Astrophysics, of course, has provided substantial evidence for physics beyond the Standard Model, from neutrino oscillations to dark matter and dark energy.)
A month earlier, in September 1984, Michael Green and John Schwarz submitted a paper on anomaly cancellation in superstring theories. String theory had been around for a while, and it had been understood for ten years that it predicted gravity, and was a candidate “theory of everything.” But there were many such candidates, each of which had run into significant difficulties when taken seriously as a theory of quantum gravity. Most people who were paying attention had presumed that string theory would face the same fate, but the Green-Schwarz result convinced them otherwise. A brief article in Physics Today was entitled “Anomaly Cancellation Launches Superstring Bandwagon,” and theorists everywhere jumped to learn everything they could about the exciting new possibilities the theory offered.
So here we are, over twenty years later, still with no surprising new results from particle accelerators (although hopefully that will change soon), and still with strings dominating the landscape (if you will) of theoretical high-energy physics. And still, one hardly needs to mention, with no clear path to connecting string theory to low-energy phenomenology, nor indeed any likely experimental tests of any sort.
In the circumstances, it’s not surprising there would be something of a backlash against string theory. The latest manifestation of anti-stringy sentiment is in two new books aimed at popular audiences: Peter Woit‘s Not Even Wrong: The Failure of String Theory and the Continuing Challenge to Unify the Laws of Physics, and Lee Smolin’s The Trouble With Physics: The Rise of String Theory, The Fall of a Science, and What Comes Next. I haven’t read either book, so I won’t presume to review them, but I think we’ve heard the core arguments expressed on this blog and elsewhere. I’m a firm believer that it’s good to have such books out there; I’m happy to let the public in on our internecine squabbles, just as I’m happy to keep them updated on tentative experimental results and speculative theoretical ideas. It seems unduly patronizing to think that we can’t reveal anything to the wider world until everyone in the community agrees on it.
But I don’t actually agree with what the books are saying. Here is the main point I want to make with this post, trite though it may be: the reason why string theory is so popular in physics departments is because, in the considered judgment of a large number of smart people, it is the most promising route to quantizing gravity and moving physics beyond the Standard Model. I don’t necessarily want to rehash the reasons why people think string theory is promising — I’m not positing an objective measurement of the relative merits, but simply an empirical observation about people’s best judgments. Rather, I just want to emphasize that, when you get right down to it, people like string theory for intellectual reasons, not socio-psycho-political ones. It’s not a Vast String Theory Conspiracy, funded by shadowy billionaires who funnel money through Princeton and Santa Barbara to brainwash naive onlookers into believing the hype. It’s trained experts who think that this is the best way to go, based on the results they have seen thus far. And — here’s the punchline — such judgments could change, if new results (experimental or theoretical) came along to suggest that there were some better idea. The way to garner support for alternative approaches is not to complain about the dominance of string theory; it’s to make the substantive case that some specific alternative is more promising. (Which people are certainly trying to do, in addition to the socio-psycho-political commentating about which I am kvetching.)
That is, after all, the way string theory itself became popular. Green and Schwarz labored for years on a relatively lonely quest to understand the theory, before they were able to demonstrate anomaly cancellation. This one result got people psyched about the theory, and off it went. It’s not a matter of impressionable young physicists docilely obeying the dictates of their elders. Read Jacques Distler’s (absolutely typical) story about how he dived into string theory as a graduate student, despite the fact that his advisor Sidney Coleman wasn’t working on it. In a completely different field, listen to Nobel-winning economist Gary Becker on the response to his ideas (via Marginal Revolution):
“There was a sea change. I began to notice it in the 1970s and 1980s. A lot of the younger people coming out of Harvard, MIT and Stanford were very interested in what I was doing, even though their faculty were mainly – not entirely – opposed to the sort of stuff I was doing.”
This is just how academics act. They are stubborn and willful (even at a charmingly young age!), and ultimately more persuaded by ideas than by hectoring from their elders. And it’s not just the charmingly young — if good ideas come along, supported by exciting results, plenty of entrenched middle-aged fogeys like myself will be happy to join the party. If you build it, they will come.
There’s no question that academic fields are heavily influenced by fads and bandwagons, and physics is no exception. But there are also built-in mechanisms that work to protect a certain amount of diversity of ideas — tenure, of course, but also the basic decentralized nature of university hiring, in which different departments will be interested in varying degrees in hiring people in certain fields. Since the nature of science is that we don’t yet know the right answers to the questions we are currently asking, different people will have incompatible intuitions about what avenues are the most promising to pursue. Some people are impressed by finite scattering amplitudes, others like covariant-looking formulations, others don’t want to stray too far from the data. The thing is, these considered judgments are the best guide we have, even if they are not always right. Green and Schwarz were lonely, but they persevered. If you want to duplicate their success, find a surprising new result! You can’t ask a department to hire people in an area they don’t think is promising, just because it serves the greater goal of diversifying the field overall. Crypto-socialist pinko though I may be in the political arena, when it comes to intellectual life I’m a firm believer in the free market of ideas, and would tend to resist affirmative-action programs for underrepresented theories.
The bandwagons come and go, influenced by both data and new ideas. When I was in grad school in 1990, things were in a lull in fundamental physics generally, and students were escaping to Wall Street and elsewhere. The discovery by COBE of temperature anisotropies in the microwave background re-invigorated cosmology, and attracted a number of bright young theorists. The Second Superstring Revolution in the mid-90’s did the same for string theory. There’s every reason to believe that the LHC will do the same for phenomenology — the leading indicators are already easily visible.
The thing that has kept string theory alive is that interesting results have kept coming, from the 70’s (gravity!), to the 80’s (anomaly cancellation, five critical string theories), to the 90’s (branes, dualities, black hole entropy, AdS/CFT). The last few years haven’t witnessed their own “revolution” (unless you count the landscape), but it would seem a little impatient to give up on that basis alone. If nothing else, string theory is extraordinarily fruitful and robust. Indeed, the AdS/CFT correspondence says you can’t really separate field theory and string theory. Take an ordinary gauge theory in flat four-dimensional spacetime, and make it as supersymmetric as possible without adding gravity. Then make the coupling very strong, and the degrees of freedom rearrange themselves — just as the strong coupling in QCD makes the quarks and gluons rearrange themselves into pions and nucleons — into Type IIB superstrings living in a ten-dimensional spacetime. How amazing is that? It’s not proof that strings are connected to the real world (which, as people sometimes forget, is not manifestly maximally supersymmetric, and does in fact involve gravity), but it’s the kind of rich structure that keeps people optimistic that string theory is on the right track.
Of course, you do have to make the case that your personally favorite approach is a promising one, to the public and to colleagues in other specialties as well as to graduate students. This is not always a job that string theorists have done well. Some of them, I’ve heard rumors, can even occasionally be a mite arrogant. Let’s admit, this is something of an occupational hazard among academics; if universities fired all the arrogant people, the remaining faculty would be stuck teaching twenty courses a semester. And, while I think that an enormous landscape of stringy vacua might very well exist, I think that supporters of the idea have dramatically failed to take seriously the difficulty of actually calculating anything on that basis. Discussions about these crucial issues have all too often degenerated into sophomore-level philosophy-of-science debates, which haven’t done credit to either side. The truth is, we’re not doing science in a new way, it’s the same old way — trying to come up with the simplest possible consistent and coherent framework that explains the phenomena we observe.
And (to add one more “of course”), needless to say we need to keep our eyes on the prize, which really is explaining those phenomena. Sometimes people do get entranced with the math, which is fine, but as physicists the ultimate arbiter of interestingness is a connection to data. String theory hasn’t done that yet, and might not do it for a long while, but in the end will have to, one way or another. It’s hard! But string theory will either progress to the point where its connections to reality become increasingly manifest and specific, or people will lose interest and work on other things. That’s the way the system works.
Update: Interesting reports from the Strings 2006 meeting in Beijing from Victor Rivelles, Jonathan Shock, and Dennis Overbye.
Err, forgive the typos. Anyway, one might also note that Migdal was thinking about dual string theories for Yang-Mills as early as the mid-80s, if not before, and I don’t mean just the loop equations, but actual worldsheet actions. Similarly Polyakov was something of a prophetic voice in this field. It seems a bit disingenuous to claim them as the root of LQG as if this lends weight to LQG as opposed to string theory.
With reference to the comment that it is better to get a Nobel in physics than the Clay prize:
It took some 2000 years to get from Euclidean geometry to non-Euclidean geometry – this following the rigorous mathematical route. But without rigor, could we have conceived of non-Euclidean geometry? It took probably a similar 2000 years or more to get from the operational daily use of real numbers to the rigorous definition by Dedekind, et al. Did we gain any insight from rigor? From intuitive practice?
We might like to think that our mathematical imagination is fully liberated – but is it really? If this branch of physics (HEP) is indeed to be mathematical rather than experimental, can we afford to be purely rigorous? Can we afford to be purely pragmatic? The truth is that we are blind men, groping.
Dear anonymous,
The point is that while string theory is the natural realization of the gauge field-extended object duality in a background dependent context, LQG is its natural realization in a background independent context. I believe that the LOST uniqueness theorem, math-ph/0407006, gr-qc/0504147, is a precise statement of this. It says that if you want a diffeomorphism invariant quantization of a gauge theory on a manifold with no metric, in which the algebra of Wilson loops and electric flux is realized, there is a unique hilbert space to work in for each gauge group and manifold. It is the one we use in LQG.
I really wish people would stop seeing this as an either-or competition. There are two regimes, QFT on a spacetime with a fixed metric and diffeo invariant QFT on a manifold with no metric. Shouldn’t the same powerful idea have an expression in both contexts? I am also not being disingenuous, just reporting that the work of Polyakov and Migdal, which I learned from their talks while a grad student, was a major influence on our work in LQG.
If I am understanding Professor Smolin correctly, then I am detecting a subtle idea emerging from his comment #178 in regards to this issue of string theory backlash. More specifically, Smolin appears to be suggesting that a cozy symbiosis could possibly form between string theory and loop quantum gravity. Therefore – instead of declaring war – Smolin is proposing peace between these two opposing factions within theoretical physics. Cool idea!?!? 🙂
Best,
To Cynthia:
Writing a book entitled “The Trouble with Physics: The Rise of String Theory, the Fall of a Science, and What Comes Next” seems an odd sort of peace proposal…
Aron,
Overall, I echo your sentiment. On the surface – someone like Smolin – might be sending “black and white” messages which tend to be harsh in nature. However, on a deeper level, he might be revealing “mixed-gray” messages – as conveyed in comment#178 – which tend to soften this harsh nature.
“S” says “Your entire life is a gift from these physicists that seem like “barking up the wrong tree” to you.
Try saying this to a medical physicist when you go for an MRI, an X-ray etc.”
S,
Do I understand you correctly? Surely you’re not comparing X-Rays and Magnetic Resonance Imaging to String Theory? The whole point of the doctor having X-Rays and MRI is that these wonderful technologies facilitate repeatable medical observations, testable by experiment (how many ball bearings did little Howie swallow?).
Dear Cynthia and anon,
I have been making such a “peace proposal” for 20 years. Indeed, LQG developed directly out of work Louis Crane and I did in 84 & 85 to make a background independent formulation of string theory. I followed that up with so far around 18 papers exploring ways to do just that. The most recent of these is “A quantization of topological M theory”, hep-th/0503140, Nucl.Phys. B739 (2006) 169-185. My first book was devoted to the crisis of the landscape-just 6 years too early. My second book explicitly proposed that LQG and string theory were to be unified and that was what I was working on then (2001). Indeed, I never did see them as separate and have always been puzzled how anyone with a serious interest in solving quantum gravity could ignore either one.
The response of string theorists: roughly speaking, no interest. I got one invtation to give a talk at a conference in Japan, that was essentially it. I was surprised because many leading string theorist were saying all this time that the main problem was to find a background independent formulation of string of M theory. I am not claiming to have solved this problem but I do think I have made a sustained effort to solve it and have discovered a number of useful things that might lead to its solution-if it has one. Instead of interest, the most I got from leading string theorists was lectures explaining why it was permature to attack this problem.
My forthcoming book is not at all harsh. It is written with respect and affection towards all those working on all the different approaches. It attempts to contribute constructively to science. If someone takes my saying something positive about one approach as an attack on another, or gets emotional in response to an honest acounting of successes and failures, that is not my responsibility, it is a sign that a lot is happening here which is neither rational nor in the best interest of science.
Thanks,
lee
Island,
I’m unsure of whether I understand your comment. My apologies if I totally miss the point here. I suppose this comment is, in reality, respectfully directed to the larger community of physicists here at Cosmic Variance.
I’m the guy working the jackhammer. Somebody else needed someone to work the jackhammer. They didn’t create me, but they had something to do with the causes and conditions that put this jackhammer in my hands…
I need someone to get real about the consequences of small armies of smart physicists chasing pretty little imaginary strings at a time when the world needs practical concepts and innovative technologies to pull our collective butts out of the fire.
Again with all due respect to John Archibald Wheeler, (“delayed choice experiment”, starting on page 334 of Wheeler’s Geons, Black Holes & Quantum Foam), I will take a chance and speculate that one might have to be a physics professor to fail to see that “Singularity” and “SuperString” are sometimes just fancy ways of avoiding saying “God”. Or perhaps of avoiding introducing conciousness into a worldview and field of intuition.
I am obviously not a physicist. I am not an “Intelligent Design” Creationist either. But I will not take a world view represented to me as Science on Faith any sooner than I will let a Religionist impose Faith upon me from a pulpit.
From the point of view of an intelligent lay person with some degree of physical intuition, I see physicists pointing to an opaque “Singularity” in one direction and a suspiciosly obtuse and thornily difficult “String Theory” in another direction.
Guys and girls, it’s just not good enough.
I’m a musician. Some composers write thornily complex scores that look expremely impressive on paper. In many instances, I do not believe they even particularly care what they sound like in performance, any more than I care what a score of mine looks like.
Are there physicists like this?
Again, Listen to the Moon.
Owl, I was talking about the blog post that you referenced, which is essentially Wheeler’s interpretation of the anthropic physics that perplexes everyone on all sides:
My statement was meant to show that it is extremely arrogant to assume that we are enabled by the forces to our benefit, rather than to its benefit.
We are here to work, not watch.
What predominant characteristic does a flat yet expanding universe and humans have in common?
We both have the tendency to increase entropy… *EFFICIENTLY*
This is a conservation law.
Dear Lee,
Thanks for your reply. You will not be offended if I observe that someone so identified with loop methods might be felt not to be an entirely disinterested judge of string theory. I am sure your book will be an honest accounting; my worry is whether it will be an accurate accounting. The Bushes and the Clintons can give honest accounts of each other; they may not agree though.
More interestingly, can I ask you why you think loop methods, or more generally methods aiming to achieve manifest diffeomorphism invariance, should be useful for providing a non-perturbative definition of string theory? I am not an expert on LQG but understand its inspiration to be the manifestly covariant nature of GR.
Diffeomorphism invariance is a classical geometric property. One of the things I like about string theory is the way it is more than just geometry: for example, the nice description of spacetime topology change through extremal transitions. The degrees of freedom that allow this to happen (branes wrapped on vanishing cycles) are not simple `modes of the metric’. Another example is that of dualities: very different backgrounds are the *same state*. There is no fundamental distinction between winding modes and KK modes; but the latter is the only obvious geometric one.
So I don’t know what the `background-independent’ formulation of string theory is, but the background we are independent of seems to be far richer than just diffeomorphism invariance – it has at least to include different space-time topologies, be sensitive to non-geometric information and to know about all the dualities. I don’t know what this group should be, but it has to be a lot bigger than that of diffeomorphisms.
So, why should LQG methods work? Manifest diffeomorphism invariance seems too limited if we want to describe string theory. Diffeomorphisms don’t see dualities and don’t see topology change: why are they a useful starting point for a background-independent formulation?
Regards
anon
theOwl:
“Physics” is NOT equal to String Theory. MRI DID come out of a very fancy theory of its time (and one that crackpots STILL love to trash), Quantum Mechanics.
Dear Anon,
Thanks for your question about how loop related methods might be the basis for a background independent formulation of string theory.
As I explain carefully in my essay about background independence, diffeo invariance on a fixed differential structure is a very restricted notion of background independence. Indeed, much of spin foam models work in a more general setting, which is background independent in the sense that you mention.
One way to see this is that quantum theory without a background reduces to the study of algebras and their representations. The systematics of this are captured by the theory of modular tensor categories. Other examples of such categories are given by manifolds and cobordisms and also categories of surfaces of various dimensions, embedded in manifolds and their cobordisms. The basis of topological quantum field theories and spin foam models are in functors that relate these cobordism categories to categories of representations of quantum groups.
It seems natural to use the same set of mathematical ideas to capture background independent formulations of string theory and M theory. The inclusion of branes of various dimensions is straightforward. There have been ideas about how to incorporate the dualities (for example as generalized T-dualities). There are beautiful extensions in the higher category theories to study. The close relation between topological quantum field theories and gravitational theories that is revealed by this point of view seems also to extend to string and M theory.
As to how disinterested I may or may not be, can I emphasize again that I have spent myself all together many years of work on string theory. I am very interested in it and at times I have believed deeply in it. I actually think that I have a more objective and more correct and detailed understanding of exactly what has and has not been shown in string theory than many “string theorists”-because I had myself a number of times to think carefully through which program was worth the next few years of investment of time. And I have published such assessments before- hep-th/0303185. Let me know if I have anything there factually wrong. But just to make sure I did a great deal of consulting and fact checking with the present book.
Thanks,
Lee
S.
Y’all were able to come up with some pretty good experiments to test quantum mechanics, weren’t you?
Owl:
Yes, QM is quite well tested. General relativity is quite well tested. Don’t you find their logical inconsistency with each other a teeeeeny problem? Wouldn’t it be nice to have a logically self-consistent fundamental framework from which to see the world? One that would get rid of the singularities that you seem to not like all that much?
Since the discussion again seem to gravitate towards background independence and diffeomorphisms, let me make some trivial observations (and Jacques Distler does not need to listen).
4-diffeos are important for GR for the same reason that conformal transformations are important in perturbative string theory: it is the correct constraint algebra (in covariant formulations, a foliation gives you the Dirac algebra). In quantum theory, we always want representations of lowest-energy type; LQG looks weird precisely because this condition is not fulfilled. The lesson from CFT is that infinite-dimensional constraint algebras generically pick up extensions – the Virasoro algebra. It was because I wanted to do the same thing for 4-diffeos and GR that I discovered the multi-dimensional generalization of the Virasoro algebra and its representation theory.
From this POV it is obvious which ingredient both string theory and LQG are lacking: the correct quantum form of the correct constraint algebra of GR.
Jacques Distler wrote
Last years, young students were pursued to research in string theory. I know students doing a PhD in string theory because other research fields were -in practice- very limited to funding. I have received mails from people explaining me how abandoned physics because they were forced to do research in string theory and considering it a waste of time.
That string theory has been sistematically overhyped in both academia and media.
In what part of landscape? In real word (here) most of string theorists are just a kind of minimalist cult. Other physicists (experimental, Loop, QM, …) reported this many ways and in many sites.
“There is today a disconnect in the world of physics. Let me put it bluntly. There are physicists, and there are string theorists. Of course the string theorists are physicists, but the string theorists in general will not attend lectures on experimental physics. They will not be terribly concerned about the results of experiments. They will talk to one another.”
and “[…] they don’t listen to us.”
Glashow (The Nobel).
Juan R.
Center for CANONICAL |SCIENCE)
While a lot of like to think in “frames of reference,” it is hard to let go and explore other potentials within the issues of quantum gravity?
Lee’s positon has been “exemplitary” by bringing together perspectives on quantum gravity. I can understand his frustration, is a little different, then wanting to hear my proposal? 🙂
Yet in these string theoretics “mathematical assumptions” are correct?:)Qui! Non?
So the “foundational” ground work has been “laid out” regardless ,of what some people think about string theory?:)
Last years, young students were pursued to research in string theory. I know students doing a PhD in string theory because other research fields were -in practice- very limited to funding.
Most physics grad schools are full of grad students who go in thinking they want to do string theory and end up doing anything from condensed matter experiment to astrophysics to particle theory to biophysics. The thought of anyone being forced into string theory is laughable. There are neither enough advisors nor enough funding for everyone who would like to work in the field, and most of them aren’t talented enough in the first place.
Nobody cares about the students with no talent. The question is whether the talented ones have the opportunity to work on other approaches to quantum gravity if they recognize the behavior of string theorists as religious.
The actually talented ones are able to appreciate that string theory is by far the most promising approach to quantum gravity. None yet has been talented enough to find anything else that is more promising.
When it comes to quantizing gravity, string theory is no alternative to understanding the correct constraint algebra of GR on the quantum level.
You can’t make this stuff up department:
Q: The question is whether the talented ones have the opportunity to work on other approaches to quantum gravity if they recognize the behavior of string theorists as religious.
A: The actually talented ones are able to appreciate that string theory is by far the most promising approach to quantum gravity. None yet has been talented enough to find anything else that is more promising.
This is a textbook example of group think, when a group of people become convinced that agreement with them is a test of talent and intelligence. If anyone remembers the point of this thread, it was that some of us believe that Sean’s “market forces” don’t work because of such group think, leading to an inability to make objective scientific judgements of the relative risks and achievements of different approaches.
Should we then presume you think that the following people who work on approaches to quantum gravity other than string theory do so because they are lacking intelligence or talent: Ashtekar, Bjorken, Connes, Laughlin, Loll, Penrose, Rovelli, Sorkin, ‘t Hooft, Thiemann, Wen.
Sorry if this seems harsh, but you said it, not I.
I am just an amateur astronomer with the following observation.
From just reviewing Thorne’s “Black Holes and Time Warps” and,
in particular, chapter 11,’What is Reality’ in which “spacetime is viewed as curved on Sundays and flat on Mondays, and horizons are made from vacuum on Sundays and charge on Mondays, but Sunday’s experiments and Monday’s experiments agree in all details”, I can be a touch confused by this exchange. Are string theory and QLG all that far off in their predictions? And if they are, then won’t science run into some wall in the future if the wrong road is being taken just as it did with Rutherford’s solar system model of the atom?
Even with that last case,Rutherford’s model, wrong as it is to an astrophysicist or particle physicist, still serves a function to introductory chemistry and biology to this day, does it not?
I recall a similar period mentioned by Clifford Will in his “Was Einstein Right?” in which scientists were describing themselves as Brans-Dicke theorists three days a week and general relativists on the other three work days. Many of Dicke’s criticisms wound up being experiments that NASA took up and received quite a few funds for as I recollect. Dicke may have lost his battle, but regarding his view was a plus for science.
I also got the impression from Thorne that while Zel’dovich’s biggest fear was losing time when doing experiments, Wheeler’s biggest fear was overlooking any new idea. Wheeler was more deliberate. It seems that the different methodologies of Zel’dovich and Wheeler are still very much alive, the tortoise and the hare. Yet, my impression was that Zel’dovich, Wheeler, Hawking,Thorne and Landau all mixed well and respected each other.
I would like to think that such an atmosphere still can prevail?
the bonsai master
prunes for the sake of the tree
not his reflection