From a Relativistic Point Particle to String Theory
Using a classical action associated to a point-particle in (1+1)-dimensions the classical string theory is derived. In connection with this result two aspects are clarified: First, the point particle in (1+1)-dimensions is not an ordinary relativistic system, but rather a some kind of a relativistic top; and second, through the quantization of such a kind of top the ordinary string theory is not obtained, but rather a $\sigma$-model associated to a non-compact group which may be understood as an extended string theory.