String Theory and the Microphysics of Black Holes

Lead Research Organisation: University of Cambridge
Department Name: Applied Maths and Theoretical Physics


A common theme in physics is the ability to describe Nature in terms of its simplest building blocks. But our current theory of gravity, though incredibly good at predicting phenomena at large scales from the motion of planets to the evolution of our Universe, has resisted attempts to make it consistent with the principles of quantum mechanics. Quantum theory has been equally successful at describing the behaviour of the fundamental constituents of Nature, such as electrons, and the forces that bind them, such as the strong nuclear force. But in order to describe gravitation in the language of quantum mechanics - what we would call a `quantum theory of gravity' - we need to work at scales where classical theory breaks down and quantum effects become manifest. Two scenarios where this occurs was during the creation of our Universe, and in the physics of black holes. Pioneering work led by Stephen Hawking proved that black holes are the laboratory for quantum gravity. Classically, black holes are objects from which nothing, not even light, can escape. Hawking showed, using generic quantum arguments, that black holes actually radiate particles at a certain temperature and they have a quantity that behave like entropy. The situation was like the thermodynamics of gases before the middle of the 19th century. The work of Boltzmann and others explained these properties microscopically - that is, in terms of the statistical mechanics of tiny gas particles They showed entropy was a measure of the `disorder' of the system, the number of microscopic states with the same macroscopic properties. A true quantum theory of gravity must provide a similar quantum explanation for the temperature and entropy of black holes. Since its resurgence two decades ago, string theory has emerged as the best candidate for a quantum theory of gravity. It asserts that the basic building blocks of the Universe are tiny vibrating strings. Gravity automatically arises from the quantum mechanics of the string. and is naturally unified with the other forces of Nature. Curiously, it predicts there are actually ten dimensions, not the usual four we see everyday. It also contains extended objects, called D-branes, upon which strings can attach. Certain configurations of D-branes look like black holes when seen in lower dimensions and following Hawking we can compute their entropy. Back in higher dimensions, counting the number of ways the D-branes can arrange themselves into that configuration, we can compute their entropy. The two calculations agree when a certain constraint, known as supersymmetry, is imposed. This is the first example of a truly quantum derivation of black hole properties. But recently new donut shaped black holes - `black rings' - have been discovered. A key goal of this work is to investigate exactly how string theory tells black rings apart from black holes and provide a quantum description of their entropy. An exciting new development gives rise to another definition of quantum gravity. There is strong theoretical evidence that quantum gravity on a certain background, called Anti-de Sitter (AdS) spacetime, is equivalent to a gauge theory much like the ones that describe the Standard Model of particle physics. It follows that black holes in this spacetime will have a fully quantum description in terms of states of this dual gauge theory. The main objective of this proposal is to completely classify all the black holes in AdS - distinguished by their shape, charge, and spin - that are consistent with the special constraint of supersymmetry. The next key aim is to count the states in the dual theory with these macroscopic properties and hence provide an exact quantum description of the black holes. These results would represent a further major milestone in the search for the quantum theory of gravity.


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