Fredagskollokvium: Gauge theories of gravitation and unification

This is an introduction to gauge approaches to gravitation, and a review of
developments towards geometrically unified theories during the last hundred
years. The interpretations of the "teleparallel" gauges are clarified, and the new 
"affinely" and "especially" conformal theories are explained. It is sketched, how 
we may, in terms of symmetries, understand intuitively, and formulate in a unified 
fashion, gravity, space, time and matter.
The equivalence principle can be understood as a statement of gauge
invariance. Soon after its introduction, Hermann Weyl proposed a unified theory
of gravitation and electromagnetism, based on an extension of the invariance
that incorporates also the relativity of magnitudes. Our claim is that all the
three main objections that have been since raised against the conformal
unification can be addressed in a gauge formulation of the unified theory: the 
problems with charges, second clocks and ghosts.