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.
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…