Communication by a real wave requires energy transfer. Yet it doesn't necessarily require modulation of pre-existing flux of energy.

Gravity in this universe is connected with the mass-energy, so something about the gravity is omnipresent and immutable just as much as mass-energy is conserved. In some way, every piece of mass (every quantum particle) is omnipresent through its inherited gravity. I don't see why should two or more omnipresent entities require time to start interacting, while I do understand why it may take time to finalize the "transaction" in the inertial medium...
So my idea is that all interactions start as instantaneous, and only appear retarded because of interference of medium.