  A new approach to the Kaluza theory and its relation to the gauge theory is
presented. Two degenerate metrics on the $4+d$-dimensional total manifold are
used, one corresponding to the spacetime metric and giving the fiber of the
gauge bundle, and the other one to the metric of the fiber and giving the
horizontal bundle of the connection. When combined, the two metrics give the
Kaluza metric and its generalization to the non-abelian case, justifying thus
his choice. Considering the two metrics as fundamental rather than the Kaluza
metric explains why Kaluza's theory should not be regarded as five-dimensional
vacuum gravity. This approach suggests that the only evidence of extra
dimensions is given by the existence of the gauge forces, explaining thus why
other kinds of evidence are not available. In addition, because the degenerate
metric corresponding to the spacetime metric vanishes along the extra
dimensions, the lengths in the extra dimensions is zero, preventing us to
directly probe them. Therefore this approach suggests that it is not justified
to search for experimental evidence of the extra dimensions as if they are
merely extra spacetime dimensions. On the other hand the new approach uses a
very general formalism, which can be applied to known and new generalizations
of the Kaluza theory aiming to achieve more and make different experimental
predictions.
