ABSTRACT
We are familiar with the four space–time dimensions, which are noncompact. But from theoretical considerations we believe that there could be more than four space– time dimensions. The simplest possibility is to consider the extra dimensions to be compact with very small radius, so only the probes with energy close to the Planck energy (MPl ∼ 1019 GeV) could see these extra dimensions. Then the Newtonian gravity in our effective four-dimensional space–time would remain unaffected. When the dimensions higher than four are compactied, the components of the metric corresponding to the extra compact dimensions will become gauge elds in four dimensions. Any translation along the extra dimensions would then appear as gauge transformations in four dimensions. The graviton in higher dimensions would decompose into the graviton and gauge bosons in four dimensions (see section 11.2).
