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.

... 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.

After my two attempts at gently raising the issue of string theory, one afternoon I stepped into Feynman's office to ask him what he really thought.<p>"Can we talk a little about string theory?" I asked. … "Don't you think there are aspects of it that seem very promising?"<p>"Promising? What does it promise? Does it promise to tell you the mass of the proton? No. What does it promise to tell you?"<p>"Well, no one knows how to extract any quantitative predictions yet, but—"<p>"You're wrong. It does make a quantitative prediction. Do you know what that is?"<p>I looked at him. My mind was a blank.<p>"It requires that we live in ten dimensions. Is it reasonable to have a theory that requires ten dimensions? No. Do we see those dimensions? No. So it rolls them up into tiny balls or cylinders too small to detect. So the only prediction it makes is one that has to be explained away because it doesn't fit with observation."

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.

Even though it is, properly speaking, a postprediction, in the sense that the experiment was made before the theory, the fact that gravity is a consequence of string theory, to me, is one of the greatest theoretical insights ever.

... It was (in part) Gross’s excessive enthusiasm for string theory in the mid-80s that drove me (as an impressionable grad student at Princeton) away from theoretical physics (and into astronomy). String theory may have been a beautiful idea, but it made no predictions that could be tested experimentally in the then-foreseeable future. That’s not science. A quarter century later and the theoretical physics community has yet to wake up and realize that there is new physics right under their noses – just not the new physics they’ve been expecting (GUTs, strings, membranes, etc.). Galaxy dynamics are consistent with a single, universal force law, but this unexpected behavior has largely been ignored because it doesn’t fit with particle theorists’ dreams of super symmetric dark matter particles. That we do not understand the observed behavior makes it more interesting than the “expected” (but unobserved) new physics: who ordered this?

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.

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.

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.

Is our universe fundamentally a mess, or is there some simple and natural structure that all this could emerge from, or be parts of? One approach to answering these questions is String Theory (or, more generally, M-Theory), but string unification models have grown excessively in complexity while producing zero predictive progress. After several decades of extensive theoretical exploration leading nowhere, it is time to consider that the string program may have been a wrong turn. If we backtrack, imagining String Theory never happened, we can go in a new direction, building on the success of Grand Unified Theories and recent progress in Loop Quantum Gravity. The structures of GUTs and LQG rely heavily on Lie groups and are remarkably compatible. By considering the known Lie groups and fields of physics as parts of a larger geometric whole, we move towards Lie group unification.

Since the 1960s, particle theory had been split into two groups: those following the atomism of the quark theory and those who had followed the anti-atomism of that had led from the bootstrap program to the string theory. What happened in 1984 was that it was realised that string theory could combine and satisfy the aspirations of both approaches to fundamental physics. Thus, the community of gauge theorists, driven by the failure of the proton decay experiments to search for new ideas that could unify physics, all of a sudden encountered their old friends, the string theorists, in the middle of what might be called a desert of disappointed expectations.

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.

It's been said that string theory is part of the physics of the twenty-first century that fell by chance into the twentieth century. That's a remark that was made by a leading physicist about fifteen years ago. ...String theory was invented essentially by accident in a long series of events, starting with the Veneziano model... No one invented it on purpose, it was invented in a lucky accident. ...By rights, string theory shouldn't have been invented until our knowledge of some of the areas that are prerequisite... had developed to the point that it was possible for us to have the right concept of what it is all about.