By applying the equivalence principle to the propagation of scalar waves in a curved spacetime, the value 1/6 for the coupling constant between the field and the Ricci scalar is derived. No coupling to higher-order scalars constructed from the Riemann tensor is allowed.
No takes yet. Share an insight, caveat, or question.
Sonego et al. (1993) studied this question.
Synapse has enriched one closely related paper. Consider it for comparative context: