“Just around the same time that the string picture was formed, asymptotic freedom was discovered and made possible, in QCD, a more precise and successful theory of the strong interactions. Yet there has always been a striking analogy between QCD and string theory. If the hypothesis of quark confinement in QCD is true in its usual form, than a widely separated quark and antiquark are joined by a “color flux tube.” This has an obvious analogy to the notion of a meson as a string with charges at its ends, as assumed in string theory. Explaining this analogy would mean understanding quark confinement. This would be quite a nice achievement, since it is a longstanding sore point in theoretical physics that despite real experiments and computer simulations supporting the quark confinement hypothesis and despite a lot of ingenious work explaining qualitative criteria for quark confinement and why this notion is natural, there is no convincing, pencil and paper demonstration of quark confinement in QCD.”

— Edward Witten

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American theoretical physicist 1951

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“String theory was not invented to describe gravity; instead it originated in an attempt to describe the strong interactions, wherein mesons can be thought of as open strings with quarks at their ends. The fact that the theory automatically described closed strings as well, and that closed strings invariably produced gravitons and gravity, and that the resulting quantum theory of gravity was finite and consistent is one of the most appealing aspects of the theory.”

David Gross (1941) American particle physicist and string theorist

"Einstein and the Search for Unification", p. 10 https://books.google.com/books?id=rEaUIxukvy4C&pg=PA10, in The legacy of Albert Einstein: a collection of essays in celebration of the year of physics (2007)

“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.”

Edward Witten (1951) American theoretical physicist

"The Past and Future of String Theory" in The Future of Theoretical Physics and Cosmology: Celebrating Stephen Hawking's Contributions to Physics (2003) ed. G.W. Gibbons, E.P.S. Shellard & S.J. Rankin

“The universe has been expanding since the big bang, thus early on it was hot and dense. To trace the history of the universe we must understand the dynamics that operates when the universe was hot and particles were very energetic. Before the standard model we could not go back further than 200,000 years after the big bang. Today, especially since QCD simplifies at high energy, we can extrapolate to very eary times, where nucleons melt and quarks and gluons are liberated to form a quark-gluon plasma.”

David Gross (1941) American particle physicist and string theorist

"The Discovery of Asymptotic Freedom and the Emergence of QCD" http://www.nobelprize.org/nobel_prizes/physics/laureates/2004/gross-lecture.html, Nobel Lecture, p. 79, nobelprize.org (2004)

“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…”

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“It is very possible that a proper understanding of string theory will make the space-time continuum melt away…. String theory is a miracle through and through.”

Edward Witten (1951) American theoretical physicist

as quoted by K.C. Cole, "A Theory of Everything" New York Times Magazine (1987) Oct.18

“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.”

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“In the open string the gauge charges are carried by the Chan-Paton degrees of freedom at the endpoints. In the closed string the charges are carried by fields that move along the string.”

Joseph Polchinski (1954–2018) physicist working on string theory

[String theory: Volume 2, superstring theory and beyond, Cambridge University press, 1998, https://books.google.com/books?id=WKatSc5pjOgC&pg=PA59] (page 59)

“One possibility in this direction is to regard, classically, an electron as the end of a single Faraday line of force. The electric field in this picture from discrete Faraday lines of force, which are to be treated as physical things, like strings. One has then to develop a dynamics for such a string like structure, and quantize it…. In such a theory a bare electron would be inconceivable, since one cannot imagine the end of a piece of string without having the string.”

Paul Dirac (1902–1984) theoretical physicist

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“Unfortunately, so far… a truly background independent formulation of string theory has not been achieved… [It is] often called the search for M theory…”

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“Why is space-time doomed? There are many reasons, among which: In string theory we can change the dimension of space-time by changing the strength of the string force. Thus, the so-called II-A string theory, which semi-classically describes closed strings moving in ten-dimensional flat space for very weak coupling is dual for strong coupling to a theory, called M-theory, that at low energies is described by eleven-dimensional supergravity. By increasing the string coupling we can grow an extra dimension. How can the spatial continuum be fundamental if the number of spatial dimensions can be so changed?”

David Gross (1941) American particle physicist and string theorist

"Einstein and the Search for Unification", p. 11 https://books.google.com/books?id=rEaUIxukvy4C&pg=PA11, in The legacy of Albert Einstein: a collection of essays in celebration of the year of physics (2007)

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