Hello all. I wonder if you could help explain something to me. No matter how much I read I am not sure if my understanding of special relativity is correct. My confusion is with respect to a light clock constructed as one mirror above the other and a photon reflecting between the two mirrors. Scenario 1: There are two spacemen, each with an (initially) synchronised light clock and no relative velocity between the spacemen. If each spaceman looks at the other's clock, they would both agree that the cycle times of the photons in either light clock are identical and that the photons are “hitting” the mirrors in absolute synchrony. For the purpose of simplicity, let’s assume 1 full cycle takes 1 “second” in either clock. Scenario 2: Now assume one spaceman has constant velocity relative to the other. Again each has a light clock as mentioned. If each spaceman looks at his own clock he will continue to assert that a full cycle takes 1 second as he will be unaware of his own velocity. However, when spaceman 1 looks at spaceman 2’s clock he will disagree with spaceman 2. He will say that spaceman 2 has forward velocity and therefore, as light has constant velocity, the angulation of the photon’s path means that each cycle now takes longer than 1 second in spaceman 2’s clock. All well and fine. If this is true and the velocity of light is truly identical for both parties then by all accounts, the light should strike the mirror of spaceman 1 earlier than spaceman 2. But here is the problem I have… if the velocity is relative then the reciprocal should apply to spaceman 2: he will believe that he is stationary and spaceman 1 has velocity in the opposite direction. Therefore spaceman 2’s photon will hit his mirror earlier than spaceman 1. But if both can see the others photon they cannot both believe that their own photon hits their own mirror first! Thanks for your help NoEinstein!