Quantum physics and Einstein's theory of general relativity are the two solid pillars that underlie much of modern physics. Understanding how these two well-established theories are related remains a central open question in theoretical physics. Over the last two decades, efforts in this direction have led to a broad range of new physical ideas and mathematical tools. These have deepened our understanding not only of quantum gravity, cosmology, and particle physics, but also of intermediate scale physics, such as condensed matter systems, the quark-gluon plasma, and disordered systems. Ideas from string theory have also led to new insights and approaches to problems in many areas of mathematics. Indeed, the interface of quantum physics and gravity is a vibrant area of research that is expected to be extremely active in the coming decade. Researchers in the CTP have been at the forefront of many of these developments. CTP faculty members work on string theory foundations, the range of solutions of the theory, quantum cosmology, and the application of string-inspired ``holographic'' methods to strongly coupled field theories. The group in the CTP has close connections to condensed matter physicists, astrophysicists, and mathematicians both at MIT and other departments.

Even though we understand string theory better, there is still no clear fundamental description of the theory in a background-independent framework, and the set of solutions, or string vacua, is still poorly understood. The work of Washington Taylor and Barton Zwiebach combines physical insight with mathematical consistency to address these questions, and has led to the development of new mathematical results and ideas.

Taylor's recent work has given a new systematic understanding of certain types of complex manifolds used for string compactifications, and has identified specific constraints and generic properties that large classes of string theory solutions imply for quantum gravity theories in four and higher dimensions. This program has led to evidence that with additional dimensions and supersymmetry, the spectrum of any consistent quantum gravity theory must arise from a solution of string theory.

Zwiebach has been at the forefront of developments in double-field theory. This program has identified structures of generalized geometry that are relevant to the description of gravity in string theory and, despite advances, is still in its infancy.

Outside the direct application to questions of quantum gravity, string theory has in recent years spawned a rapidly-developing new area of application where "holographic" dualities relate theories of gravity in one spacetime to strongly-coupled quantum theories in a spacetime of one less dimension. Originally discovered in the context of string theory, these dualities appear to provide a rigorous mathematical equivalence between the two related theories. Such dualities give both a new perspective into quantum gravitational phenomena as encoded in quantum field theory, and a way to explore aspects of strongly coupled field theories using the gravitational dual. CTP faculty have played a pioneering role in several applications of holographic duality. Hong Liu and Krishna Rajagopal were at the forefront of efforts that used holography to find new insights into the physics of the quark-gluon plasma.

In recent years, a set of interesting new developments has begun to draw unexpected connections between a number of problems relating aspects of gravity, black holes, quantum information, and condensed matter systems. In particular, it is becoming clear that quantum entanglement, through holographic duality, underlies a characterization of spacetime geometry. These developments tie into the research activity of several CTP faculty members, including Aram Harrow and Hong Liu.

The string group in the CTP interacts broadly with the other groups within the CTP, and with the astrophysics group in the physics department. Faculty in other departments working in string-related areas include Isadore Singer (math). In addition to the regular MIT faculty, Ashoke Sen spends two months each year with the group as the Morningstar visiting professor.

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