Hi everyone, I have a problem with understanding a few lines from the book on Relativity I am using. Let me first quote my troubles, "Let us express these facts algebraically, for two observers, K and K', who are moving with uniform velocity relatively to each other, thus: K writes x = ct, and K' writes x' = ct', both using the same value for the velocity of light, namely, c, and each using his own measurements of length, x and x', and time, t and t', respectively. It is assumed that at the instant when the rays of light start on their path, K and K' are at the same place, and the rays of light radiate out from that place in all directions. Now according to equation x = ct, K who is unaware of his motion through the ether (Since he cannot measure it), may claim that he is at rest and that in time, t, K' mus have moved to the right and [vice versa (K' thinks same about K)" My question is, if K and K' are in the same place at an instant, how come either of them moves while the other stays in place? As is written, K thinks that K' moved, while K' thinks K moved, but wait a minute, since they're at the same place, don't they both move or stay?