  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.
