“Combine general relativity and quantum theory into a single theory that can claim to be the complete theory of nature. This is called the problem of quantum gravity.”

"Loop Quantum Gravity," The New Humanists: Science at the Edge (2003)

"Loop Quantum Gravity," The New Humanists: Science at the Edge (2003)

Source: The Fabric of Reality (1997), Ch. 2
Context: The quantum theory of parallel universes is not the problem, it is the solution. It is not some troublesome, optional interpretation emerging from arcane theoretical considerations. It is the explanation—the only one that is tenable—of a remarkable and counter-intuitive reality.

The Trouble With Physics: The Rise of String Theory, The Fall of a Science, and What Comes Next (2007)

[1992, Intersection Theory, Integrable Hierarchies and Topological Field Theory by Robbert Dijkgraaf, Fröhlich J., ’t Hooft G., Jaffe A., Mack G., Mitter P.K., Stora R. (eds.), New Symmetry Principles in Quantum Field Theory, NATO ASI Series (Series B: Physics), vol. 295, 95–158, Springer, Boston, MA, 10.1007/978-1-4615-3472-3_4]

[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.)

"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)

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.