Outline

Abstract

Twenty years ago, an intriguing link emerged between two very different kinds of theories. On the one side of this link are quantum gauge theories, akin to the ones describing the electroweak and strong forces which govern nature on the microscopic scales of particle physics. On the other side of this link are theories of gravity, the force which governs nature on the macroscopic scales of cosmology and black holes. At the time, string theory was found to contain extended objects that are described by gauge fields in one limit and by gravity in another limit. Crucially, the limit in which the gravity description becomes appropriate corresponds to the limit in which the gauge theory becomes strongly coupled. The gauge/gravity duality [1–4] asserts that these two descriptions are in fact equivalent for all values of the gauge coupling, albeit typically only one side of the duality is tractable. At weak gauge coupling, the gauge theory is under perturbative control, whereas the dual gravitational degrees of freedom couple to the whole tower of string excitations and can no longer be treated classically. Conversely, at strong gauge coupling, the gauge degrees of freedom defy a perturbative treatment, but they rearrange themselves into a dual description in terms of classical gravity.