The dynamical rest frame

To make the theory mathematically rigorous and let it couple properly to gravity, RSI describes the Eternal Consonance as a dynamical medium with a preferred rest frame. It does this with a unit timelike vector field, uμ, which represents the local 4-velocity of the medium's flow. A 4-velocity describes motion through both space and time.

In the medium's own rest frame, this field settles to the value (1, 0, 0, 0): it points purely along time. This gives a natural way to split any change in the field into two parts.

The time-like derivative measures change along the medium's flow. It governs how waves, such as light, move through the medium. The space-like derivative measures change across the flow. It sets the stiffness of space. Any departure from the Rest-State, such as the creation of matter, costs energy, and this provides the restoring force that defines the speed of light.