Three men are riding on a train of proper length L that is moving at speed v = 3/5 c with respect to the ground. One of the men (A) is at the front of the train, one (B) is at the back, and one (O) is at the middle. A fourth man (O?) is standing on the ground next to the track. At the exact moment that O passes O?, two flash-bulb signals coming from A and B reach O and O?.
a) Who emitted the flash first, according to O? According to O??
b) Calculate the time difference between the two emissions, in both frames. Express your answers in terms of L and c. (Hint: Use the invariant spacetime interval between the events.)
c) Now there are two more observers on the ground, A? and B?, who are at the positions of A and B at the exact moment (according to the ground frame) that O and O? receive the flashes. How far apart are A? and B?, in the ground frame?
d) A? and B? (on the ground) read the clocks of A and B (on the train) as they whiz by. (Those clocks were synchronized in the train frame with the clock carried by O.) What are the readings of those two clocks?