The reason time, space and gravity are relative due to acceleration is the same basic reason that sound is relative. As sound has a point of origin and is an energy transfer resulting in a wave, so to the gravitational wave has a point of origin and is an energy transfer resulting in a wave.
All mater and energy decay into a monopole wave field from all electromagnetic radiation fields creating three basic actions; Time is the basic change in energy from the electromagnetic field into the gravitational field, a monopole field and the endpoint of decay. Space is the volume of that changing field.
And the key to understand is the interaction of different gravitational fields is emitted gravitational field synchronization which creates gravity, and modifies relative time and space.