Quantum theory and the equivalent principle

The status of the gravitational principle of equivalence is obscure on the atomic scale, where quantum processes are relevant. On the assumption that a fundamental principle ought to apply to fundamental particles, the principle of equivalence is examined in a quantum context. Linearized gravitational perturbations are treated as a massless, spin-two, gauge field coupled to itself and to matter, and argue that a consistent theory of this type, based on an action principle, is impossible unless the gravitational coupling is universal. The argument derives from a set of consistency conditions connecting successive orders of the perturbation expansion. This consistency argument is illustrated for scalar electrodynamics, and it is proved that the scalar particles must couple to gravity with the same strength as photons. (author)