  We come now to the critical part of the theory. In my view the origin and the necessity for the electromagnetic field lie in the following  justification. The components \(\psi_{1},\psi_{2}\)  are, in fact, not uniquely determined by the tetrad but only to the  extent that they can still be multiplied by an arbitrary  “gauge-factor” \(e^{i\lambda}\) of  absolute value 1. The transformation of the \(\psi\) induced by a  rotation of the tetrad is determined only up to such a factor. In the special theory of relativity one must regard this gauge factor as a  constant, since we have here only a single point-independent tetrad.  This is different in the general theory of relativity. Every point  has its own tetrad, and hence its own arbitrary gauge factor, because the gauge factor necessarily becomes an arbitrary function of  position through the removal of the rigid connection between tetrads  at different points.