String field theory as general relativity of loops

Invariance under general loop diffeomorphisms δX^μ((σ) = π{variant}^μ[X, σ] in used as a guide to construct a field theory of interacting closed strings. Local loop reparametrizations δX^μ (σ) = π{variant} [X, σ]π{variant}_σX^μ (σ) act on the tangent loop space, much as the local Lorentz rotations of general relativity. In terms of Fourier modes, the theory is obtained by gauging the semidirect sum of the Kac-Moody- extended Poincaré algebra with the Virasoro algebra. In the supersymmetric case, we discuss the relevant super-Kac-Moody (+ (super-) Virasoro algebra. The nilpotency of the corresponding BRST operator yields the superloop field equations.