“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

"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

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Edward Witten 20

American theoretical physicist 1951

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“It was found [in the 1970s], unexpectedly and without anyone really having a concept for it, that the rules of perturbation theory can be changed in a way that makes relativistic quantum gravity inevitable rather than impossible. The change is made by replacing point particles by strings. Then Feynman graphs are replaced by Riemann surfaces, which are smooth - unlike the graphs, which have singularities at interaction vertices. The Riemann surfaces can degenerate to graphs in many different ways. In field theory, the interactions occur at the vertices of a Feynman graph. By contrast, in string theory, the interaction is encoded globally, in the topology of a Riemann surface, any small piece of which is like any other. This is reminiscent of how non-linearities are encoded globally in twistor theory.”

Edward Witten (1951) American theoretical physicist

“Among the problems of the known string theories, as a theory of hadrons, was the fact that the spectrum of open strings contains massless spin 1 particles, and the spectrum of closed strings contains a massless spin 2 particle (as well as other massless particles), but there are no massless hadrons. In 1974, Joël Scherk and I decided to take string theory seriously as it stood, rather than forcing it to conform to our preconceptions. … Specifically, Scherk and Schwarz (1974) proposed trying to interpret string theory as a unified quantum theory of all forces including gravity. Neveu and Scherk (1972) had shown that string theory incorporates the correct gauge invariances to ensure agreement at low energies (compared to the scale given by the string tension) with Yang-Mills theory. Yoneya (1973,1974) and Scherk and Schwarz (1974) showed that it also contains gauge invariances that ensure agreement at low energies with general relativity.”

John Henry Schwarz (1941) American theoretical physicist

[Schwarz, J. H., The early history of string theory and supersymmetry, 2012, https://arxiv.org/abs/1201.0981]

“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)

John C. Baez (1961) American mathematician and mathematical physicist

[2008, http://www.edge.org/q2008/q08_5.html#baez, Should I be thinking about quantum gravity? (essay at the World Question Center), edge.org]

“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)

“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…”

Lee Smolin (1955) American cosmologist

"A perspective on the landscape problem" arXiv (Feb 15, 2012)

“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.”

Edward Witten (1951) American theoretical physicist

as quoted by John Horgan, The End of Science: Facing the Limits of Knowledge in the Twilight of the Scientific Age (1996)

“The incompatibility between gravity and quantum coherence represented by black holes should be solved by a consistent quantum theory that contains gravity as superstring theory. Despite many encouraging results in that sense, I question here the general feeling of a naive resolution of the paradox. And indicate non trivial physical possibilities towards its solution that are suggested by string theory and may be further investigated in its context.”

Daniele Amati (1931) Italian physicist

[The information paradox, arXiv preprint arXiv:hep-th/0612061v2, 14 December 2006, http://arxiv.org/abs/hep-th/0612061] (See also Thorne-Hawking-Preskill bet.)

“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

“String theory has a close relationship to quantum field theory — indeed in some sense it encompasses all interesting quantum field theories.”

Michael Green (physicist) (1946) British physicist

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