We return to the next-to-last bus stop on the super-highway, called ‘supergravity’. Supergravity theory worked well, but not well enough. At some points the mathematical construction did not work perfectly. Also, not all particle types seemed to fit together in such a model, and not all infinities cancelled. In courageous perseverence, researchers next tried the same theories in spaces with many more dimensions than ours.

A two-dimensional space can be compared with the surface of a piece of paper, such as the pages you are reading now. Suppose you tear one page out now and roll it up to obtain a cylinder. For a little red spider that happened to be walking on your paper, this makes little difference. It takes a long time for the spider to walk one full circle around the cylinder. It probably would not even notice that it came back to where it started from. We say that its world is still two-dimensional. But, seen from some distance, the pipe is rather like a stick, having only one dimension. In the same vein, the world of the very tiniest particles could have more than three (space-like) dimensions. These tiny particles could be like our little red spider. They would not notice that some of their dimensions are ‘rolled up’. For us, these rolled-up dimensions have become invisible. This idea had already been suggested by Theodor Kaluza in 1919, and was further elaborated upon by Oskar Klein, in Stockholm, Sweden.