Shift Equivalence Principle (SEP)
The Shift Equivalence Principle (SEP), states that within observerspace, since gravitational and motion shifts appear to share the same phenomenology, they should be considered interchangeable.
An incomplete implementation of this idea appears in Einstein's general theory, which allows us to calculate the gravitational shift associated with a gravitational differential by calculating the velocity that a free-falling body would undergo when crossing the region, and then calculating the resulting motion shift seen on the body at the end of its journey.
We can then imagine the object as being a container filled with light, that it released at the end of the experiment.
The released light will fall with the container, and when remitted, should have the same Doppler-shift as other light emitted by the body. If the light-energy is shifted by the same amount by the gradient regardless of how much time it takes to cross it, then we can calculate the same shift either as a motion shift or as a gravitational shift.
A philosophical justification for this equivalence can be obtained from observerspace arguments: If we look at a still photograph (or a set of still photographs) of an isolated object showing a polarised set of frequency shifts, we have no obvious way to tell whether these shifts exist because the body is moving, or because it is surrounded by a polarised gravitational field.
The redshift and blueshift seen at the rear and front of the body could be interpreted either as simple Doppler shifts or as evidence of a polarised gravitational field.
The aberration effects seen on the object could be interpreted either as the effects of velocity, or as the effects of lensing associated with a polarised gravitational field.
The duality between these two sets of arguments provides an additional equivalence principle over and above those used in C20th gravitational theory: The SEP increases the degree of data-redundancy in a universe (encouraging holographic approaches), it put additional strict constraints on the theory's predictions and equations of motion, so that we cannot simply arbitrarily declare the validity of a set from a different theory as Einstein did in 1916 (increasing the derivational purity of a theory), and the requirement that the principle cannot be seen to be broken also gives a theory an additional level of falsifiabiity over and above that of C20th physics.
SEP and local lightpeed constancy
The SEP provides an alternative mechanism for local lightspeed constancy: if two bodies recede at velocity v, then we can argue that the target body, receding at v, also exerts a gravitational effect on the light as it approaches, causing an effective change in the lightsignal's velocity, of v. the light therefore starts with a velocity of vEMITTER and arrives at a velocity of vRECEIVER.
SEP and local lightspeed constancy (advanced)
If we attempt to measure the variation of velocities of a signal passing between two bodies by inserting a physical piece of equipment into the signal path to act as a probe, then the probe will still measure the local speed of light to be vPROBE – the light will then traverse two gradients, VEMIITTER to vPROBE, and vPROBE to vDETECTOR. The insertion of a probe with a different state of motion to the existing objects, in order to measure a shift not previously measured, leads to a change in geometry (no new information without new geometry)