1. The problem statement, all variables and given/known data A very fast train (system S') travels on a straight track (system S) with speed 0.6c. When it enters a tunnel (at end A), which is 3 c*min long relative to S, observers in S and S' set their clocks to zero. The train will emerge from the tunnel at end B. At the midpoint of the tunnel, a transmitter on the train emits a radio signal towards observers at both ends of the tunnel. At what times are the signals received at ends A & B in S and in S'? 2. Relevant equations [itex]x' = γ(x-vt)[/itex] [itex]t' = γ(t - xv/c^2)[/itex] [itex]u'= (u-v)/(1 - uv/c^2)[/itex] 3. The attempt at a solution I'm sure I'm being stupid about this, but I have managed to confuse myself pretty well. For these conditions, the midpoint is 1.5 c*min into the tunnel to observers in S. Since the train is traveling at 0.6c, observers in S will observe the train reaching the midpoint 2.5 minutes after it enters. The train then releases a radio signal which travels at c to the observers in S, taking another 1.5 minutes to reach the end of the tunnel, for a total of 4 minutes elapsed. Is this correct? I feel like I'm missing something. For observers in S', the tunnel only appears to be 2.4 c*min due to length contraction, so they observe reaching the midpoint 2 minutes after entering the tunnel. They then set off the signal, which they observe traveling at c a distance of 1.2 c*min for a total of 3.2 minutes to reach the observers at the ends of the tunnel. Again, I'm not sure I'm going about this correctly, our lecturer has been taken ill, and we've had a disjointed string of guest lecturers, and I feel a bit turned around. Thanks for your help!