In the "apparentprogress__.docx" attached above, it is to be noted that we have two cases : an observer stationary with the plank of wood, and an observer moving uniformly to the left with respect to the plank of wood.

These are the two frames discussed.

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The reason why the light rays issued from the left and right ends of the plank of wood must meet in the middle, as shown in the animation, is only obvious when one considers an event placed in the middle that might be triggered only by the arrival of BOTH light rays at that one place at the same time, say as viewed by the stationary observer.

And so for the observer moving uniformly to the left, or any other observer for that matter, that same event would also have to triggered, or we would be left with two realities. That event can only trigger if, in all these other frames, the light arrives at the same place (in the middle) at the same time.

This is why the light must hasten along one limb, and lag along another, as viewed by the observer moving uniformly to the left, in order to reach the middle of the plank of wood "on time" so to speak.

This reality requirement for this standard setup is discussed here:

A Discussion on the Relativity of Simultaneity

http://dx.doi.org/10.5281/zenodo.5568917

October 2021

Xinghong Wang

We don't agree with the conclusion in Wang's paper, that “The relativity of simultaneity is not a valid theory". All that is demonstrated here, is that the light must reach the middle in all frames (which is does in our animations).