All the problems in this chapter are qualitative and you will be able to solve them if you can read spacetime diagrams. Try them out to test your understanding of Special Relativity.

Reading the spacetime diagram

Street lamps

Five street lamps, numbered 1 through 5, are located on a straight line along the x-axis equal distance apart as shown in the figure. They turn on at points A, B, C, D, and E, respectively, on the spacetime diagram.

1. In what order do the lamps turn on in the ground-frame?

2. In what order does the light from the lamps reach the observer at the origin x = 0?

3. A car is moving to the right at constant speed relative to the ground. At t′ = t = 0, it is at x′ = x = 0. The space- and time-axes in the moving frame of the car are tilted with respect to those of the rest frame as shown in the spacetime diagram. In what order do the lamps turn on in the car-frame?

4. In what order does the light from the lamps reach the observer riding the car?

5. Where is the car when the light from street lamp 4 reaches it?