... one thing that's worth mentioning, though, it that apart from the dream of understanding physics at a deeper level involving gravity, work in string theory has been useful in shedding lights on more conventional problems in quantum field theory and even in and as well with applications to mathematics. Apart from its intrinsic interest, those successes are one of the things that tend to give us confidence that we're on the right track. Because, speaking personally, I find it implausible that a completely wrong new physics theory would give rise to useful insights about so many different areas.

Whereas originally the hopes for string theory, and its descendants, were that some kind of uniqueness would be arrived at, whereby the theory would supply mathematical explanations for the measured numbers of experimental physics, the string theorists were driven to find refuge in the strong anthropic argument in an attempt to narrow down an absolutely vast number of alternatives. In my own view, this a very sad and unhelpful place for a theory to find itself.

String theory is an ambitious approach to the construction of a mathematical description of the physics that governs the properties of elementary particls and their interactions as well as the structure of space and time. It incorporates (and maybe even explains) well-established principles such as quantum mechanics and relativity. In fact, many string theorists (myself included) believe that string theory constitutes the third big physics revolution of the century, following relativity and quantum mechanics. It certainly requires conceptual advances every bit as bizarre and unexpected as was the case in the prior two revolutions.

Many of us believed in the possibility of a principled explanation for the laws of nature. We hoped to discover a short list of principles, which could be realized in a unique theory, which would retrodict the standard model and uniquely predict the physics to be discovered beyond it. The shocking implication of the results of Strominger reported in 1986 was that it was not to be, at least within the confines of string theory. ...String theory offered more, however... It offered the promise of a setting in which the different perturbative string theories are realized as expansions around solutions of a still more fundamental theory. ...That more fundamental theory would have to be background independent...

We believe string theory has a set of solutions, some of which might describe our world. Even leaving aside the question of few vacua or many, and organizing principles, perhaps the most basic question about the landscape is whether it will turn out to be more like mathematics, or more like chemistry.

My belief is based on the fact that string theory is the first science in hundreds of years to be pursued in pre-Baconian fashion, without any adequate experimental guidance.

What is very important to me is two points: A theory should be internally consistent and it should have some contact with observation. Well, I’m told by all the experts that this theory <nowiki>[</nowiki>String theory<nowiki>]</nowiki> is internally consistent, although they think up new interpretations every time I turn my back. But contact with reality? Nobody’s given me anything. I just watch. I’m somewhat unhappy that so many people are working on it. To me, as a physicist, it’s sort of sad that so many people at the same time work at something that doesn’t seem to have any contact with experiment. But that, to some extent, is due to the fact that we don’t have any great experimental puzzle to be thinking about. We have to supply the puzzles, and there aren’t that many puzzles right now.

I do feel strongly that this is nonsense! ... So perhaps I could entertain future historians by saying I think all this superstring stuff is crazy and is in the wrong direction. I think all this superstring stuff is crazy and is in the wrong direction. ... I don't like it that they're not calculating anything. ... why are the masses of the various particles such as quarks what they are? All these numbers ... have no explanations in these string theories – absolutely none! ... 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—a fix-up to say, “Well, it might be true.” For example, the theory requires ten dimensions. Well, maybe there's a way of wrapping up six of the dimensions. Yes, that's all possible mathematically, but why not seven? When they write their equation, the equation should decide how many of these things get wrapped up, not the desire to agree with experiment. In other words, there's no reason whatsoever in superstring theory that it isn't eight out of the ten dimensions that get wrapped up and that the result is only two dimensions, which would be completely in disagreement with experience. So the fact that it might disagree with experience is very tenuous, it doesn't produce anything.

I'm an experimentalist. I like to look at hard data. And there — string theory is a distant second to because it doesn't make any observable predictions. ... As an experimentalist, I like when a theory makes a prediction. My job is to not prove theorists right — it's to disprove everybody else.

... string theory has uncovered beautiful relations between physics and different parts of mathematics, mostly differential geometry, enumerative algebraic geometry and complex analysis. In fact string theory started like this. At the very beginning when and others were starting string theory motivated by the for strong interactions they found solutions to the duality equations for the scattering amplitudes in terms of s and these were nice mathematical functions like generalized s which are very natural in terms of complex analysis. It was then realized, by and others, that these models could be understood geometrically from the propagation of strings. It is a very powerful idea to “test” a given complex space using the space of complex curves inside or of maps from s to that target space. And physicists could use all the arsenal of which is quite powerful. This generated a very interesting group of people that do a kind of “physics motivated” mathematics which rejuvenated some parts of complex geometry. They adopt a rather free attitude towards mathematics, which is original and productive and had a very positive influence. It started as mathematics and had a very positive impact on mathematics up to now.

Replacing particles by strings is a naive-sounding step, from which many other things follow. In fact, replacing Feynman graphs by Riemann surfaces has numerous consequences: 1. It eliminates the infinities from the theory. ...2. It greatly reduces the number of possible theories. ...3. It gives the first hint that string theory will change our notions of spacetime. Just as in QCD, so also in gravity, many of the interesting questions cannot be answered in perturbation theory. In string theory, to understand the nature of the Big Bang, or the quantum fate of a black hole, or the nature of the vacuum state that determines the properties of the elementary particles, requires information beyond perturbation theory... Perturbation theory is not everything. It is just the way the [string] theory was discovered.

I would expect that a proper elucidation of what string theory really is all about would involve a revolution in our concepts of the basic laws of physics - similar in scope to any that occurred in the past.

… if one studies quantum gravity, something prevents you from localizing objects and particles with a better precision than one Planck length or so. String theory makes these guesses quantitative in various ways.

I don't think that any physicist would have been clever enough to have invented string theory on purpose... Luckily, it was invented by accident.