Hi, I’ve been teaching myself SR, currently reading Relativity for the Questioning Mind by Daniel F Styer. My math is not great, so I have been avoiding the heavy calculations for now. I will learn those too, but I wanted to understand the concepts first. For the most part I think I have got a good grasp on the basic concepts but then every now and then something comes up that stumps me. Like this thought experiment below. Looking at it from your FoR, imagine 3 rockets in a vertical line (A,B and C), separated by some distance. The rockets are at rest wrt each other and are travelling from left to right with some small velocity. (To approximate Euclidian space for now.) You see another rocket ship (D) travelling at an angle to the first 3 ships, which creates 3 events as shown below. Event 1, D passes in front of A http://img51.imageshack.us/img51/2075/56169348.jpg [Broken] Event 2, D passes in front of B http://img821.imageshack.us/img821/2290/38031069.jpg [Broken] Event 3, D passes a distance in front of C as it accelerated between events 2 and 3. http://img713.imageshack.us/img713/4276/84602899.jpg [Broken] The problem I am having is when I am looking at this from A’s FoR. D looks to pass A and B in straight line, but then suddenly veers off to the left and does not pass C in straight line path. A must think that D has turned off its original trajectory, as A, B and C have not accelerated so he knows that they are still at rest wrt each other. So he radios D and asks what happened. D says that he is following a laser and has not veered off his original trajectory, although he did accelerate after passing Ship B. To cut to the chase, A would know that if D has not moved from his original trajectory, the only way that D could have appeared to move from its initial straight line path is if A,B,C and D were all in motion. Or in other words, this scenario could not have happened if any FoR was 'at rest' All that seems fairly simple, but I thought it was not possible for any frame to say that they have absolute motion. So what am I missing please?