String Theory

String theory replaces the point particles of quantum field theory with one-dimensional extended objects whose quantised vibrational modes reproduce the known spectrum of matter and force carriers, including a massless spin-two state identified with the graviton. Consistency at the perturbative level requires supersymmetry and either ten spacetime dimensions (superstring theories) or eleven (M-theory, which unifies the five ten-dimensional superstring theories through a web of dualities). Central structural results include the AdS/CFT correspondence, which identifies a gravitational theory on an anti-de Sitter background with a conformal gauge theory on its boundary, the Kawai-Lewellen-Tye relations expressing graviton amplitudes as squares of gauge amplitudes, and the realisation that brane configurations in higher dimensions can reproduce four-dimensional Standard Model physics in low-energy effective descriptions such as Randall-Sundrum.