I have always had this difficulty with understanding the example of the light-clock: what keeps the light pulse bouncing to and fro between the mid-points of the top and bottom mirrors? Assuming the earth is inertial for most practical purposes, a coin dropped from a tower hits the ground right below my hand. If instead of a coin, it is a ball, it keeps for a while bouncing up and down on the very same point. But there is a physical cause for that: those effects, according to Galileo, happen because the coin and the ball, before being dropped, share the state of motion of the ground. If they were dropped from the tower on a racing train, that would not happen. If I emit sound upwards on a moving train, sound will bounce up and down right on my head. But again there is a similar physical cause for that: the air has been accelerated by the walls of the train until it has acquired the state of motion of the latter. Light is different. It does not acquire the state of motion of the source and it cannot be dragged along. First consequence, its speed is the same for all observers. I have no problems with that. I very much thank help received in this forum, thanks to which I understand why it is so. If the speed of light is measured as it is, it must be so. But, second, for the measurement instruments to render that result, they must work: the light pulse must keep bouncing with a purely vertical trajectory, without any deviation. But if the shell of the clock is accelerated, how does the light pulse learn so? How does it know that it must now take another direction? One explanation I have seen somewhere is: because of the principle of relativity. But that is only an idea. A wonderful idea, in my opinion, which is the basis of modern science and rightly so, but things do not happen just in order to fit into theories. Things happen because of a reason. It may happen that the reason is yet undiscovered. That is not so dramatic and science makes progress because it can live without the ultimate explanations for many phenomena. But there must lurk somewhere a reason, an explanation for everything and no doubt knowing it is "enlightening" and boosts further progress. In this case, don’t we know the reason why the light pulse "conforms to the principle of relativity"? My own humble understanding was that light remains in the mirror because it expands in all directions. So, if not one, another photon will hit the right place. But then the photons that did not hit the target would bounce around, thus rendering the instrument impractical. Given so, I thought: well, the light-clock thought experiment is just that, a pedagogical tool, it does not have to work itself. And I also thought: atomic clocks work analogously to light-clocks and they do not have such practical inconvenient. Is this explanation right? Sorry for the long post, but I wanted to refine the question. Thanks in advance.