9 v 2 2 6 N ov 2 00 1 Why Nature has made a choice of one time and three space coordinates ?

We propose a possible answer to one of the most exciting open questions in physics and cosmology [1–11], that is the question why we seem to experience four-dimensional space-time with three ordinary and one time dimensions. We have known for more than 70 years that (elementary) particles have spin degrees of freedom, we also know that besides spin they also have charge degrees of freedom, both degrees of freedom in addition to the position and momentum degrees of freedom. We may call these ”internal degrees of freedom ” the ”internal space” and we can think of all the different particles, like quarks and leptons, as being different internal states of the same particle. The question then naturally arises: Is the choice of the Minkowski metric and the four-dimensional space-time influenced by the ”internal space”? Making assumptions (such as particles being in first approximation massless) about the equations of motion, we argue for restrictions on the number of space and time dimensions. (Actually the Standard model [12] predicts and