Kaluza-Klein Theories

In this short introduction to Kaluza-Klein (KK) theories, we emphasize on the possibility to obtain gauge fields from pure Einstein gravity in higher dimensions, a possibility introduced by Nordstrom (1914), Kaluza (1919), and Klein (1926). The original idea to introduce one extra space dimension to obtain electromagnetism and gravity in four dimensions offers a way to unify the two known forces of the 19th century and give them a common, geometrical origin. This idea is then taken further to show that non-Abelian gauge fields can be obtained, corresponding to the weak and strong forces, by introducing more spatial dimensions. A modern kind of Kaluza-Klein theory is represented by the theory of a universal extra dimension (UED), which can be seen as a generalization of the standard model of particle physics to five dimensions. It is shown that the four dimensional theory, as we observe it today, can be obtained, but with an extended particle spectrum that should be detectable at higher energies, depending on the size of the extra dimension. This work is part of a seminar course in theoretical physics given at KTH autumn 2005.