Origin of gauge invariance in string theory. II. The closed string
Gary T. Horowitz, Andrew Strominger · Physical review. D. Particles, fields, gravitation, and cosmology/Physical review. D. Particles and fields · 1986
A recently developed geometric approach to quantizing the open bosonic string is extended to the case of the closed string. Spacetime gauge invariance arises naturally from spacelike translations on the world sheet.