Problem Set 4
1. In this problem we will re-visit a problem from Homework 3, this time with the use of rear
clock ahead. Alice sees two photons launched simultaneously in her frame as she rides on
a train. The train has 3 clocks mounted on the front, exact middle, and back of the train.
These clocks are synchronized in Alice's frame. The train is 10 light-seconds wide according
to Alice. When Alice's clocks read 10s, one photon is launched from the back of the train
and one photon is launched from the middle of the train. Bob watches from the ground
and sees Alice and the train moving at a speed of $\frac{1}{2}c$ to the right.
Alice's Frame:
10
back
7.5
front
10
10
B
$\frac{1}{2}c$
5 ls
5 ls
10 ls
(a) When the left clock reads 10s and the left photon is emitted, what do the middle and
right clocks read according to Bob?
(b) When the middle clock reads 10s and the middle photon is emitted, what do the left
and right clocks read according to Bob?
(c) What does the right clock read when the middle photon reaches the right wall? What
does the right clock read when the left photon reaches the right wall? Is your answer
accurate in Alice's frame? Bob's frame? Both?
(d) Which photon does Bob see reach the right wall first?
