Consider the simple "bouncing photon" clock. In its proper frame, a photon
bounces back and forth, in one dimension, between two parallel mirrors -
registering one tick per round trip. As viewed from a moving frame this
photon travels a longer, "triangular" path through space. Since the photon
speed is numerically the same (c) as measured in both frames, the clock must
tick at a slower rate when viewed from the second frame (when compared to an
identical clock at rest in the second frame).
What is it about this effect that is not real, and only an appearance - must
we say that the invariance of vacuum light speed is not real, and only an
appearance?