general metric background. In this case the world sheet theory is formulated as a generalized two dimensional sigma model, and one derives consistency conditions (beta function equations) for the propagation of strings, in an expansion in the squared string length scale - the so-called $\alpha'$ expansion. For energy scales that are well below the string scale, this is a valid expansion and one can get useful information about the low energy limit of string theory in this way. However this expansion obviously breaks down at the string scale. In fact this is highlighted by the fact that the derivative expansion (as with generic higher derivative theories) has ghosts. These however appear at the string scale and are merely a sign that the theory has been pushed beyond its regime of validity. Thus any argument that is made about the interaction vertices of the theory - that is derived from the $\alpha'$ expansion - is invalid when the momentum flowing through those vertices is greater than the string scale. At such energies the low energy point field theory needs to be replaced by string (field?) theory.

The same is true for open string background calculations. Here one turns on a gauge 