  The theory of general relativity, for which we celebrate the centennial at
this Symposium, is based on the Einstein equivalence principle. This principle
could be violated through a pseudoscalar-photon interaction, which would also
produce a rotation of the polarization angle for radiation traveling over very
long distances. Therefore, if we could show that this cosmic polarization
rotation does not exist, our confindence in general relativity would be greatly
increased. We review here the astrophysical searches for cosmic polarization
rotation, which have been made in the past 26 years using the polarization of
radio galaxies and of the cosmic microwave background. So far no rotation has
been detected within about 1 degree. We discuss current problems and future
prospects for cosmic polarization rotation measurements.
