On the long plane flight to Italy I had the chance to read the recently published A Brief History of String Theory: From Dual Models to M-theory by philosopher of science Dean Rickles. The book deals with the history of string theory, beginning with its origins in the Veneziano model of strong interactions, and ending in the mid-90s with M-theory and the “Second Superstring Revolution”. It’s a good serious scientific history, explaining in technical detail exactly how the theory developed, with good explanations of the high points of crucial papers, together with some of the story of how they came about. While I’ve spent a lot of time in the past reading about much of this history, I learned a lot from the book, about string theory as well as other topics in particle physics that interacted with it. I’m strongly of the opinion that if you want to really understand a subject, you need to understand its history, so anyone who wants to really master string theory would do well to spend some time with this book.
There is something quite unusual about this though as a work of history, since while this subject is 45 years old, it is quite unclear how to evaluate its significance as science (arguments seem to still rage about this…). I can’t think of any other topic in modern science which has been the subject of such intense activity, with no one sure of how to evaluate it nearly a half century later. More succinctly, is this the history of a brilliant insight into the physical world or is it the history of a misguided failure? In the introduction Rickles worries that historians of science will find it too “Whiggish”, but maybe more of a problem is not knowing what the right final end-point will be. To a large degree Rickles adopts the point of view of many prominent string theorists, that this is a success story, whatever its problems might seem to be. My own point of view is different of course, and I’d claim that in recent years the viewpoint of the physics community as a whole has shifted, with this looking less and less like a success story and more and more like something else.
I can’t do justice to all that’s in the book, but for personal reasons I do want to focus on one part of the story and how Rickles treats it, one where I have a significant disagreement with him, and one that points out well the basic problem faced by this kind of history. The issue is the 1984 “First Superstring Revolution”, generally dated to the Green-Schwarz anomaly cancellation calculation of that summer. Rickles does a good job of explaining the background of this. He emphasizes that this didn’t come out of nowhere, that the issue of the problems posed by such chiral anomalies had been identified by Witten and others as of great importance in constructing unified theories.
What should one make of the significance of the discovery by Green and Schwarz of anomaly cancellation for SO(32) in type I string theory? The story of string theory as a success is that this convinced theorists that string theory was a very promising road to unification and unleashed a revolution. But I remember this differently (I had just finished by Ph.D. at Princeton and taken up a postdoc at Stony Brook). The idea that anomaly cancellation predicted a specific gauge group and dimension was obviously attractive, but the fact that the prediction was for the wrong gauge group (SO(32)) and the wrong dimension (10 space-time dimensions) looked to me (and many others) like a deadly problem. The flurry of activity leading to the heterotic string, E8, and Calabi-Yau compactifications was an impressive use of mathematical technology, but there was no sign of the Standard Model coming out of this in any natural way. It looked all too likely that this wasn’t explaining anything about particle physics, just parametrizing the choices of possible unified gauge theories in a very complicated way. Yes, there was also a theory of gravity there, but it was not obviously an attractive one.
Earlier today I was watching the video of John Schwarz’s general talk about string theory at the Simons Center yesterday. Schwarz gives much the same promotional talk he and other have given many times over the last 20 years (with little change since the addition of M-theory), making claims for success of exactly the sort that inform the point of view of the Rickles book. At the end of his talk, Dusa McDuff asked about parity violation in other parts of M-theory and Schwarz explained that the original 1984 motivation from Type I anomaly cancellation has long been abandoned:
Nowadays we have enough tricks up our sleeve that we can get parity violation out of anything.
There are now all sorts of ways of getting “string vacua” that might give a unified theory, many not using anomaly cancellation. The supposed breakthrough of 1984 is looking much more like a red herring, and the question of historical interest shifts from “how was this brilliant breakthrough accomplished?” towards “why did so many people not realize this obviously wasn’t going to work?”
Rickles on page 162 explicitly takes issue with the comments in my book emphasizing the important influence of Witten at this point, mischaracterizing me (and Smolin) as claiming it was “almost as if that community [theorists] had no decision-making power of its own”. This is far from any thing I think or wrote (a big section of my book explained the excellent reasons why any sensible person would take Witten’s opinions seriously). What I wrote was
By itself the news that gauge anomalies cancel in a version of type I superstring theory would probably not have had so dramatic an effect on the particle theory community, but the news that Witten was now devoting all his attention to this idea spread among theorists very quickly.
and I still think that’s quite accurate. Ten years ago I wrote about this in detail on the blog (see here, here and here), including a first-hand version of the story from Larry Yaffe, who was at Aspen and was the one who told Witten about the Green-Schwarz result. He reports:
Concerning reaction to the Green-Schwarz result, my recollection is that there was relatively little immediate buzz about it at Aspen. John had a fairly diffident style of presentation, and I don’t recall anyone jumping up and saying ‘this will change the course of physics!’. As best as I can reconstruct my own reaction, it seemed like a technically slick calculation and a nice result but it wasn’t, of course, addressing any of the conceptually hard questions about quantum gravity, and it seemed very far removed from the practical concerns of particle physics.
and
I think the speed with which others in the particle theory community jumped into string theory had a lot to do with Ed’s involvement and proselytizing, but I expect that even without his involvement, interest in string theory would have steadily grown, albeit slower.
Rickles ends his detailed history with M-theory, with the latter part of the book summarizing the recent history, again pretty much from the point of view of a string theory proponent, one on the defensive. I think he gets the multiverse issue quite wrong, characterizing the anthropic multiverse vs. search for a unique unified theory dichotomy among string theorists as:
It is more likely that the two stances will continue in parallel, as they appear to have done for some time, defined more by the personalities of those adopting them than by the physics.
I don’t think this has anything to do with personalities. The problem is that the anthropic multiverse point of view predicts nothing, and those unhappy with it are not unhappy because they have a personality that leads them to want or believe in uniqueness, but because they’re aware you need to make predictions to be doing science. But, one can’t expect historians to get right current events…
“Yes, there was also a theory of gravity there, but it was not obviously an attractive one.”
what makes this “not obviously an attractive one” ?
complicated is not attractive
Was “attractive” an intentional pun?
Dear Peter,
I essentially agree with your opinion about the string theory history, and that Witten’s involvement was very important. I was a graduate student at U. of Maryland in the period 1984-1989, and I did my PhD in string theory, with Warren Siegel, and the main reason I studied string theory was its claim that it can solve the problems of perturbative quantum gravity. This was based on the Mandelstam’s work on the finiteness of the superstring theory. The fact that satisfactory string phenomenology was not materialising was not such a big concern for people who were more interested in quantum gravity, because everybody thought that string phenomenology would eventually work out, since there was a lot of compactification possibilities. But, I remember R. Mohapatra, a prominent particle phenomenologist, saying in 1988 that it was impossible for him to get Standard Model out of string theory.
The reason why many people were, and still are interested in string theory is that it is a nice paradigm for a quantum gravity theory, so that for such people the fact that string phenomenology is not still giving results, is not important. I switched from string theory to canonical quantum gravity in the early 90´s, because of string theory inability to address non-perturbative quantum gravity and the absence of a background metric. The second string revolution in mid-90´s made a progress with the problem of non-perturbative quantum gravity effects, but the absence of a background metric independent formulation of string theory is the reason I am exploring LQG and spin foam formulations of quantum gravity.
I was at Harvard from 1984-1986 and saw Schwarz speak in 1985 I think. The general mood of the older theorists at Harvard was not that positive towards string theory, but that it was probably worth looking at. Sidney Coleman said I believe that since Witten was looking at it, and the math was insanely complex, there was no point in any one else looking at it. However, the younger guys were looking for things to do and it was pretty clear that GUT and BSM model building a la Georgi was not that productive anymore. There wasn’t any other really hopeful direction. Condensed matter seemed maybe more productive / interesting. People thought foundational navel gazing was a waste of time. No experimental results. Jaffe maybe had one student doing constructive field theory. Lots of theory people on sabbatical. The issues seem the same today as then, essentially. The telescopes and satellites seem like the way forward today.
I don’t want to start another string theory rules/sucks as a theory of QG debate. The point of my comment was about how things looked to most theorists in 1984: the idea of doing QG via string theory had been around for about ten years and was being promoted by Schwarz and a very few others. No one was buying. Like most people in the field I took a look at this and decided it wasn’t worth learning more about. It seemed to be a very complicated way of getting gravity. What got people interested post-1984 was the unification angle, that this would not just give gravity, but unify it with the Standard Model.
A nice tradition at CERN is the annual Xmas recital of the theory division, with funny and sometimes deep jokes about latest developments. In the 2001 edition, young string theorists were represented as talibans who talked by quoting sentences from the book by Witten. Privately, young string theorists openly talked about the real situation of the their field.
Being a graduate student in particle theory in the 80s, it was hard to escape the impact of string theory. But unlike later, it wasn’t a suffocating monopoly. Lots of other things were going on, like accelerator phenomenology and the burgeoning of astrophysics and cosmology in the Heavenly Accelerator.
The fatal blow for the field was the cancellation of the SSC in late 1993. Particle phenomenology ended, except for some specialized areas. From that point, string theory became truly the only game in town for fundamental theory, unless you were lucky enough to be working in astro/cosmo-particle physics, where real progress was and is being made.
On the West Coast, the impact of the anomaly cancellation calculation took into 1985 or even 86 to be fully felt. It was emphatically viewed as a particle/unification thing, not QG. String theory never lived up to the QG hype, of course, as it’s not coordinate-independent and can’t be extended beyond second order. After Kaplunovsky and others demonstrated the vacuum problem, the bottom fell out of string theory for a while. Its resurrection in the 1990s had a heavy component of marketing, through the NYTimes science section, for example. What actually happened and what was claimed were two quite different things. Theorists were constructing field theories with symmetries and other properties conjectured to arise from a conjectured “M theory.” No progress was being made toward a super-unification any longer, and the claims about gravity were a shell game. It was then and after that talk of “sociology” and the tyranny of Princeton became standard, in private, of course. It took nearly another decade and a half for Peter, and Lee Smolin, to say publicly what had been obvious to many for years: string theorists had developed their own cultish version of political correctness, one that bordered on pseudoscience and drove a deep wedge between string theory and the rest of physics.
… and who is this supreme being named SUSY? What’s she like? Sounds as if she’s into astrology herself. I prefer SUGRA in my QF-T. (Sorry, bad theory jokes from the early 80s.)
Hi Peter,
You may be interested in this recent issue of Studies in the History and Philosophy of Modern Physics…http://www.sciencedirect.com/science/journal/13552198/46/part/PA
CU Phil,
Thanks! I hadn’t seen that. The Ellis, Kragh and Smeenk articles in particular do a very good job of explaining the problems of the “multiverse” and of “eternal inflation”.
One thing they don’t much deal with (although Kragh does to some extent) is the way the anthropic string theory multiverse business emerged out of the failure of string theory. Physicists discuss this seriously, but it’s very clear they wouldn’t do so if the idea had emerged in some other way. Specifically, back in 1984, if Witten or any one else had promoted string theory using the anthropic multiverse justification people would treated this as pretty much crazy and obviously a waste of time. It was only after a generation of theorists had been sucked into spending decades working on this that some could be convinced to take this seriously.
It was Witten’s short preprint that appeared at the same time as the Green and Schwarz anomaly cancellation paper, or even right before it, and that used the words “In a stunning development” to describe the result of Green and Schwarz, that started the first superstring revolution. No doubt about that. No one had a clue what the anomaly cancellation result meant or why it was important.
Incidentally, it was Witten’s talk on string dualities at the annual string conference in 1995 that started the second revolution as well.