is changing position along the 4th dimension that is Universal potential energy level that can be interpreted as time. This is a subjective interpretation of an entirely spatio-energetic phenomenon. Hence general relativity is also a subjective interpretation but of a physical phenomenon that is occurring in objective reality. The question is, what is universal time? Barbour makes the point that the only time "worthy of the name" is determined by the principle of least action. This is included in relativity. The "least action" is NO action, as with what happens with the internal function of light. There is none, therefore it is timeless. That would mean the only time "worthy of the name" is no time at all!! But that's not the case. Time is relative to the motion defining it, whether it's an atomic clock, or the cycles of the moon. It's like temperature. Absolute zero is the complete absence of motion, but that doesn't mean the only temperature worthy of the name is absolute zero. The problem with universal anything, is that it lacks definition. The more general any view/perspective is, the more rough grained and lacking detail it is, until it has no detail at all. That's the dichotomy. We can see the forest, or we can study the trees, but we can't do both at the same time. The absolute is both everything, or nothing. Anything inbetween is relative. It appears that in order to discuss this I will have to lay down some maths here.