No fifth force in a scale invariant universe

We revisit the possibility that the Planck mass is spontaneously generated in scale invariant scalar-tensor theories of gravity, typically leading to a "dilaton." The fifth force, arising from the dilaton, is severely constrained by astrophysical measurements. We explore the possibility that nature is fundamentally Weyl-scale invariant and argue that, as a consequence, the fifth force effects are dramatically suppressed and such models are viable. We discuss possible obstructions to maintaining scale invariance and how these might be resolved.