Jeff Harvey’s comment that it was Larry Yaffe who brought news of the Green-Schwarz anomaly cancellation result to Witten gave me the idea of contacting Larry to get a first-hand recollection of what the reaction was at Aspen back in 1984. He was a junior faculty member at Princeton at the time and I knew him since I had been a grad student there and we both were working on lattice gauge theory. I’ve always respected his work and had noticed that he was someone who had never joined the string bandwagon, so I took the opportunity to ask him for his views on string theory. I think they’re pretty reasonable and reflect the views of a lot of the sensible people in the particle theory community these days. He agreed to let me post them here:
“What Jeff Harvey related is correct: I was at Aspen when Green and Schwarz presented their anomaly cancellation result, and I told Ed and others about it a few days later when I got back to Princeton. (Of course, John and Michael may have sent Ed a copy of their paper completely independently. I don’t know about that. But he hadn’t seen it yet when I was asked “what’s the news from Aspen?”.)
As for whether it was Michael Green or John Schwarz who gave the seminar in Aspen, I think it was John — but I’m not 100% sure. (The different talks I’ve heard from John and Michael get mixed up in my memory.)
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. But the reaction back in Princeton was different: Ed certainly saw the significance immediately and I think others did as well (certainly quicker than I did). 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.
Since you asked about my views on string theory, I’ll try to give a summary. I think it is clear that:
String theory has been wildly over-hyped by some people. Even calling it a ‘theory’ is really a misnomer, given the lack of any adequate non-perturbative definition of string theory.
String theory has not yet made any convincing connection with the world we live in.
The predictive power (or the falsifiability) of string theory leaves much to be desired, especially in light of the emerging picture of the landscape of string theory vacua.
But at the same time:
The oft-repeated argument that string theory is the most promising framework we have for combining quantum mechanics and gravity remains true. Even though there is no real non-perturbative definition of string theory, I don’t think one can dispute this assertion. (As an aside, so-called “loop quantum gravity” is an interesting one-parameter family of statistical mechanics models, but has not been shown to have anything to do with gravity. Does it have a large-volume limit? Does it have long distance dynamics described by some effective field theory plus classical GR? Who knows…)
The perturbative consistency of string theory, combined with all the consistency checks of the (largely unproven) web of duality relations, are compelling hints that there is something deep and meaningful to string theory, even though it remains poorly understood.
String theory has made remarkable contributions to mathematics, allowing previously unforeseen connections to be found between very different areas. This has shown up in new (provable!) results in enumerative geometry, Gromov-Witten invariants, mirror symmetry, etc.
String theory has given partial insight into a few conceptual questions involving quantum gravity, such as (the absence of) black hole information loss, via the connection between BPS states and extremal black holes.
Improved understanding of gauge theories, especially strongly interacting theories, is emerging from string theory via “gauge-string” (or AdS/CFT) duality. Understanding is, as usual, frustratingly incomplete, but I think the message that non-gravitational ordinary field theories, and higher dimensional theories containing gravity, can be different representations of the *same* physics is revolutionary, and hints at some synthesis we are far from understanding. I think this point is already somewhat lessening the split in the theory community between ‘string theorists’ and ‘non-string theorists’.
Personally, I find this last point the most compelling reason to be interested in string theory, despite its lack of experimentally testable predictions. It is, of course, a matter of personal taste whether the ‘pro’ reasons to work on string theory outweigh the ‘cons’. Some people are comfortable working on an intellectual enterprise whose connection with the real world may never emerge during their lifetime. Some people aren’t — and that’s fine.”