So my uncle recently asked me about a phenomenon that I have been unable to explain, but there that there must surely be a good answer for. A light clock is being observed on an airplane, with the light bouncing back and fourth between the plates of clock. The plates of the clock have the same velocity as the airplane, and the light is moving back and fourth at light speed. Why then, does the light not slowly move horizontally and escape the light clock? I have a few thoughts on the matter, but I would appreciate input so that I could come to a more concise answer.