“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.”

Interview in The Hindu (2013)
Context: The improved understanding of the equations of hydrodynamics is general in nature; it applies to all quantum field theories, including those like quantum chromodynamics that are of interest to real world experiments. I think this is a good (though minor) example of the impact of string theory on experiments. At our current stage of understanding of string theory, we can effectively do calculations only in particularly simple — particularly symmetric — theories. But we are able to analyse these theories very completely; do the calculations completely correctly. We can then use these calculations to test various general predictions about the behaviour of all quantum field theories. These expectations sometimes turn out to be incorrect. With the string calculations to guide you can then correct these predictions. The corrected general expectations then apply to all quantum field theories, not just those very symmetric ones that string theory is able to analyse in detail.

as quoted by Michio Kaku, Hyperspace: A Scientific Odyssey Through Parallel Universes, Time Warps, and the 10th Dimension (1995)
Context: String theory is extremely attractive because gravity is forced upon us. All known consistent string theories include gravity, so while gravity is impossible in quantum field theory as we have known it, it is obligatory in string theory.

"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)

Physics and Philosophy (1942)

Foreward, written June 30, 1999, to Supersymmetry: Unveiling the Ultimate Laws of Nature (2000) by Gordon Kane

Physics and Geometry, a paper written for the Symposium on Theoretical Physics at the University of Helsinki, Helsinki, Finland on August 28, 2003 and at the Freydoon Mansouri Memorial Session of the 3rd International Symposium on Quantum Theory and Symmetries at the University of Cincinnati, Cincinnati, OH, on September 13, 2003. Report #EFI03-47.

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

"Edward Witten" interview, Superstrings: A Theory of Everything? (1992) ed. P.C.W. Davies, Julian Brown
Context: I think one has to regard it as a long term process. One has to remember that String theory, if you choose to date it from the Veneziano model, is already eighteen years old... that quantum electrodynamic theory towards which Planck was heading [in 1900], took fifty years to emerge.

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