In general, I think it would be reasonable to say that there is no understanding in physics of what time is, other than a convenient parameter that can be used in mathematical models of the physical world that show good correspondence with experiments and with human perception. Attempts at a fundamental definition of time, either in physics, mathematics, or philosophy, have tended to get stuck in the problem that definitions of time always seem to appear to need to be stated in terms of time, and therefore end up as circular definitions without a terminating basis.
In modern physics, there seems to be mainly two conflicting views of time. The majority view is that time flows uniformly and continuously, and slows down as speed increases, such that time stops in the local reference frame for objects travelling at light speed. The minority view, particularly popular with philosophers, is that time does not exist at all, but is merely an artifact of human perception that exists solely in the human mind. In this view, time does not exist, because there is only the present.
Perhaps this is trivial and well-known, but what if we instead imagine that in the local reference frame there are two distinct time measures, extrinsic time that cannot be directly measured or observed but nevertheless governs the computation of the local physical evolution, and intrinsic time that can be observed locally, essentially as evolving changes of state of the local environment, that sometimes have a periodic character.
Let us for example imagine that the universe is a gigantic finite two-dimensional grid (or a three-dimensional grid sheet), for example shaped as a sphere or toroid, with objects that can move from one grid position to any adjacent grid position under control of some form of external clock signal that is locally generated outside the grid itself. Objects move in discrete jumps every time a local clock triggers according to local rules computed by the object itself, where the local rules are the same across the entire grid, perhaps such that each object can sense objects within a certain large number of grid positions, but always only moves in one jump.
I believe it’s well established that as long as the local rules are sufficiently complex, any local grid environment can exhibit Turing complete computational dynamics, and if the grid is sufficiently large and ticks for a sufficient number of clock ticks, it seems reasonable that complex systems could emerge, including structures exhibiting periodic behavior, adaptive self-replication, self-reflection, etc. I.e. life is emergent in this grid universe. Eventually structures may emerge that have an inherent capability of recognizing structure changes in their local environment, including periodic structure changes, which they may recognize as a measure of time in the grid universe. They would however, have no way of observing the external clock or the clock jumps, since these are only observable externally, outside the grid.
However, an external observer would see objects moving around according to a local clock signal with global regularity. Such an observer might see a large contiguous block of objects moving in the same direction. From an external point of view this is just a block moving at the speed of the clock signal, and not substantially different from other regions that are exhibiting more local organized behavior.
Within the grid, though, such a moving block would have to be considered a region where time stopped. Whenever a block of objects moves only in one direction, nothing else can change, and if any local clock structures were embedded in such a moving block they would remain stuck at the time they showed once they became embedded in the moving block.
Thus, intelligent life forms in the grid universe would eventually recognize the maximum speed of the grid, and recognize that time stops for anything moving at maximum grid speed.
With this view, what we call time can be properly defined only in terms of its relation to the extrinsic time, and we can dispense with absurd speculations about time travel to the “past” because this is physically impossible within the universe itself, and could only have meaning in a meta/extra-physical context with reference to the extrinsic time. Finally, both the majority and minority views of time are supported with minor modifications.
