A new test of the Einstein equivalence principle
All metric theories of gravity, including general relativity, are based on Einstein’s equivalence principle (EEP). Despite its great importance, the EEP is tested directly only with a fairly low accuracy (10-4). It is nevertheless possible to test the EEP indirectly by connecting it with Galileo’s equivalence principle, which is tested with very high precision (10-13). This connection functions only as long as there is no rotation of the plane of linear polarisation of the light traveling over great distances (the so-called Cosmic Polarization Rotation, CPR). Consequently the search for the CPR has assumed great importance, and has been carried out using the polarisation of distant radio galaxies and of the cosmic microwave background, to date with negative results. Recently we have proposed a new method for seeking the CPR, using a special type of polarisation of the cosmic background.