And which frame would both the detector AND the source be moving in? The only one I can think of would be an unspecified 'absolute rest frame'. Surely you don't mean a GLOBAL frame in which the CMB is at rest do you? Are you sure you understand frames of reference, Dale? Please Register or Log in to view the hidden image! Where have I ever mentioned 'the detector's rest frame'? I have repeatedly stated the detector goes through a sequence of three frames. Are you stating a comoving inertial frame is a non-inertial frame? That was the first frame. The accelerating frame of the detector WAS a non-inertial frame, the second frame of reference. Are you stating that the last frame of reference in which the last readings were made was a non-inertial frame? The detector was not accelerating in that frame, but there was a constant relative velocity measured by Doppler shift. You really don't know how frames work in General Relativity, do you? A non-inertial frame is NOT a 'rest frame'. The coordinates of the frame move through spacetime, spacetime does not 'move' while the coordinates of the detector are 'at rest'. I ask again, where are the coordinates of this frame fixed? The coordinates are not the coordinates of the detector's 'rest frame' in that case. How can you possibly use 'a single inertial frame' unless it is an unspecified third frame of reference in which the detector feels no force of acceleration? I specified the detector DOES feel a momentary force, but not throughout the exercise.