  In this paper we give a brief review of the pseudo-Riemannian geometry of the
five-dimensional homogeneous space for the conformal group O(4,2). Its topology
is described and its relation to the conformally compactified Minkowski space
is described. Its metric is calculated using a generalized half-space
representation. Compactification via Lie-sphere geometry is outlined. Possible
applications to Jaime Keller's START theory may follow by using its predecessor
- the 5-optics of Yu. B. Rumer. The point of view of Rumer is given extensively
in the last section of the paper.
  Keywords. Kaluza,Klein, Rumer, conformal symmetry, hyperbolic space, START,
fifth dimension, action coordinate, 5-optics
