Thanks for the stimulating reply. I am still stuck though.
A realm of objects in eternal inertial motion is already as rock-bottom unchanging as you are going to get.
We can imagine space as a 3D euclidean space, divided into a mesh of invisible little points or cubes. Motion then has an absolute meaning, as moving with respect to this mesh.
But, this is a fiction. Motion is meaningful only relative to other frames of reference. That is fine, as objects are distributed throughout space, with their own separate velocities. But, if we were to claim that the *entire* universe is moving at some velocity, then, without this lattice of cubes to move against, this is exactly equivalent to saying that the universe is completely stationary.
But what if we treat time as a 1D line, analogous with space? Then, unlike with space, every object is at the same point, and moving through time at the same rate. Which, unless you imagine absolute points along this 1D line, analogous to the lattice of cubes in space, is also like saying that every object is motionless in time. Or, if you invoke relativity, then objects are only moving in time to the degree that relativistic effects are observed.
If we aren't moving through time, then how did I seem to start this at time t, and arrive at t + 20 seconds when I finished? Do we need the imaginary points, or some other unobservable, "unchanging backdrop thing"?
Or can we dispense with time altogether? Everything is just process, at rates relative to each other and nothing else, in an eternal present? I am ignorant as to whether physics actually requires an ontologically existent time, as opposed to a formal notion which makes the equations work.