Hi how, in my master project I am working on extra dimensions and I am asking my self why is it common to start most of the theories with a space time symmetry given by SO(n,1) (n>4) and then compactify the obtained spectrum to SO(3,1)xG (where G is an abitrary symmetry group). Because I think there might be other groups which have the SO(3,1) as subgroup as well? In my question I said that "most of the theories" working in such a way. This means that all models I have seen, working in that fashion. I hope you understand my question, otherwise ask and i will do my best to explain my question further. best regards, timb00 P.s. : sorry for my bad English.