Do extra dimensions exist?

That extra dimensions can unify gravity with other types of interactions is an old idea going back to the work of Kaluza and Klein in the 1920s. More recently it became clear that extra dimensions can arise dynamically in strongly interacting systems, by invoking another old concept, that of duality. The emergent circle of ideas has become known as holographic duality, and has uncovered unexpected links between gravity and strongly interacting gauge fields, gravity and hydrodynamics, and quantum spin systems and string theory.