We present a first quantization of the space-time embedding $X^{\mu }$ and the world-sheet metric $\rho$ of the open bosonic string. The world-sheet metric $\rho$ decouples from $S$-matrix elements in 26 dimensions. This formulation of the theory naturally includes 26-dimensional gauge transformations. The gauge invariance of $S$-matrix elements is a direct consequence of the decoupling of $\rho$. Second quantization leads to a string field $\Phi [X^{\mu },\rho ]$ with a gauge-covariant equation of motion.

• Received 21 April 1986

DOI:https://doi.org/10.1103/PhysRevLett.57.519