The mechanics of the very small are markedly different from the mechanics of the very large, yet they are different aspects of the same set of laws.
Yet, we lack a theory that enables us to explain what happened in the very early universe (roughly 1x10^-47 seconds), as present theories fail to have a solution for time itself in those moments. Without time, everything falls apart, and we are only able to say with some confidence what happens after that. In fact, we don’t actually have even words to describe it, as our language only has words for before and after, and in this case, before cannot be defined in terms of time.
The reasoning process in the video informs us that right through the 19th c till today, nobody has found any way to determine the velocity of light in any direction. All we have is the determination of the time taken by light in a round trip.
Now, let us imagine a light source at the centre of a ball, the inner wall of which is coated as a perfect mirror (loaded words, so some very smart construction is needed to actually build such a measuring device). Sensors loaded on the surface of the light ought to be able to measure the time taken by light to return from any and all directions. Will that solve the issue?
No, it won’t. It will only determine the time taken by each ray to make a round trip, and common sense as well as hard reality tells us that each ray is independent of the other, so the individual trips to the inner wall of the ball, and the return trips, have no bearing to each other. For each measurement, we are still stuck with a round trip.
Will fixing the velocity of light give us a single unified theory? Maybe not. But it establishes a more firm foundation for assessing the visible* universe, among other things.
*By ‘visible’, all ranges of electromagnetic spectra are included
Not perfect, of course, as it is obvious that if distortions in spacetime exist between our observation point and any distant observable source of light, then the velocity of light ceases to be a determinate metric towards establishing either distance or time of travel of the light from that object.
Can such distortions exist? Remarkably, they might even be common. Recent observations of our heliosphere have led to speculation that far distant orbiting objects, with highly elliptical orbits, might be extremely massy, yet so small that direct observation could need a different approach. Small, in this case, refers to a physical size similar to a ping-pong or golf ball, and such objects would have very similar properties as a ‘black hole’, or collapsed star, with extremely tiny event horizons, and also accretion disks. Orbiting in thinly filled space, such objects might not attract masses to themselves in any substantial fashion, but will very likely affect the passage of light from further objects.
Of course, observing the physical universe is hardly the only purpose of having a determinate value for light velocity. Much of our physics is based on the existence of a handful of constants, one of which is light velocity.