§ Bouncing light clock is an hourglass
I've always disliked the "clocks" that are used in special relativity,
because a clock attempts to measure something absolute, rather than something
relative. So, we should rather use hour glasses. In an hour glass, we can
only measure intervals of time.
Now, when we have such an hourglass, we should fill the hourglass with
photons , because their speed of falling is invariant for all reference frames.
So, what we need is the ability to create an hourglass worth of photons which
we keep dripping down, once the photon at the funnel has reached the bottom.
This is exactly what the two mirror photon clock does --- it bounces a photon
between two mirrors. We can look at this as us "flipping" the hourglass
once the photon reaches the bottom of the hourglass.