A mystery of conformal coupling
  An origin and necessity of so called conformal (or,Penrose-Chernikov-Tagirov) coupling of scalar field to the metric of n-dimensional Riemannian space-time is discussed in brief. The corresponding general-relativistic field equation implies a one-particle (quantum mechanical) Schrodinger Hamiltonian which depends on the space-time dimensionality n, contrary to the Hamiltonian constructed by quantization of geodesic motion, which is the same for any value of n. In general, the Hamiltonians can coincide only for n = 4, the dimensionality of the ordinarily observed Universe. In view of the fundamental role of a scalar field in various cosmological models, this fact may be of interest for models of brane worlds where n > 4 . 
E.A. Tagirov