Holography and Applied String Theory

Until the middle of the 1990’s, string theory had a reputation for being a rather ambitious formalism that, while being the most promising candidate for unifying gravity with the Standard Model, had provided few concrete, testable predictions or applications. This changed following Maldacena’s proposal, the AdS/CFT correspondence or gauge/gravity duality [36, 2], in 1997; building on holographic notions reaching back to the work of ’t Hooft and Susskind, the conjecture posits that certain quantum field theories, in say d spacetime dimensions, are equivalent to quantum theories of gravity in d + 1 dimensions obtained from string theory. This allows one to use string theory to obtain results, such as correlation functions or response functions related to experimental probes, that may be otherwise difficult to compute directly in a field theoretic approach. In particular, the correspondence relates some strongly coupled quantum field theories to the classical limits of certain string theories, e.g. classical gravity, allowing access to quantities where there was previously no tractable approach. Though the most precise formulations of the correspondence often involve particular field theories with large amounts of supersymmetry and in the large N limit, the hope is that the results obtained via holography will shed light on the structure of more realistic theories. Since its inception, gauge/gravity duality has sparked enormous activity in the numerous areas of physics, both in using the classical gravity to learn about strongly coupled quantum field theories (QFT) as well as using QFTs to learn about the nature of quantum gravity. Early applications of the correspondence focused more on its uses in traditional particle physics systems, such as the behavior of QCD (quantum chromodynamics) in the strongly coupled regimes found in heavy ion collisions being explored at RHIC (Relativistic Heavy Ion Collider) and the LHC (Large Hadron Collider), but a widely growing field of late has been applying the duality to condensed matter systems, which has seen a plethora of strongly coupled systems emerge in the past few decades that challenge conventional understandings. In the other direction, the correspondence provides a precise formulation of quantum gravity and can provide useful insight and intuition into the structure of quantum gravity. As quantum gravity has proved to be a rather formidable subject, any insights are welcome and may prove invaluable to our understanding of gravity. In this report we briefly discuss some of the major areas of current holographic research and the results presented during the workshop. Due to the widespread applicability of holographic techniques and numerous open questions regarding the nature of the duality, the topics covered in the workshop ranged over a large