String Theory as Quantum Gravity IV

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My long term goal is to understand how to reconcile Einstein's theory of gravity with quantum mechanics, with a special interest in the physics of black holes. To investigate this, I use string theory, which is currently the best-developed theoretical framework available for doing computations of interest in quantum gravitational physics. String theory is an appropriate toolset because it manages to encompass both the smooth spacetime geometry of gravity on large scales and the probabilistic uncertainty of quantum mechanics on small scales. Two related advances in string theory technology in the past nineteen years have helped make my long term goal more accessible.    The first advance was the discovery of higher-dimensional objects known as D-branes in string theory, which co-exist alongside the one-dimensional fundamental strings. D-branes are versatile theoretical beasts, because they gravitate and carry charges. Applications were quickly discovered which shed new light on longstanding fundamental questions about the physics of black holes. E.g. string technologies applied to a variety of special higher-dimensional black holes enabled the first quantitative derivation from first principles of the Bekenstein-Hawking entropy, a quantity akin to disorganization which encodes the large number of ways a black hole could be formed in gravitational collapse. String theory models with many moving parts also have the potential to address another more complicated question: Hawking's famous black hole information paradox, which concerns the fate of information that falls into black holes. Much progress has been made, but still more territory remains to be understood. Contributing to developing microscopic string theory models of black hole physics further is one of the twin themes of this research proposal.    The second advance was a surprising and powerful new discovery that certain gravitational theories with negative cosmological constant possessing one extra dimension of space can be physically equivalent to certain non-gravitational field theories. This is akin to how holograms work: you can reconstruct the whole three-dimensional image from only the two-dimensional hologram. String theory holography, also known as the AdS/CFT correspondence, has been applied to a wide variety of theoretical problems of interest for string and particle theorists, and has also produced partial progress on modelling aspects of the quark-gluon plasma and condensed matter phenomena. It has sparked discovery of new types of gravitational spacetimes, and strengthened the conceptual connections between information theoretic entanglement and the geometry of spacetime. Contributing to developing holography is the other main theme of this proposal.