Solar constraints on new couplings between electromagnetism and gravity
  The unification of quantum field theory and general relativity is a fundamental goal of modern physics. In many cases, theoretical efforts to achieve this goal introduce auxiliary gravitational fields, ones in addition to the familiar symmetric second-rank tensor potential of general relativity, and lead to nonmetric theories because of direct couplings between these auxiliary fields and matter. Here, we consider an example of a metric-affine gauge theory of gravity in which torsion couples nonminimally to the electromagnetic field. This coupling causes a phase difference to accumulate between different polarization states of light as they propagate through the metric-affine gravitational field. Solar spectropolarimetric observations are reported and used to set strong constraints on the relevant coupling constant k: k^2 < (2.5 km)^2. 
S.K. Solanki (1), O.Preuss (1), M.P. Haugan (2), A. Gandorfer (1,3),   H. P. Povel (3), P. Steiner (3), K. Stucki (3), P. N....