This is a fairly simple question, but one that I'm a bit confused on. I have five lamps at positions $x_A,x_B,x_C,x_D,x_E$ which turn on at times $ct_A, ct_B, ct_C, ct_D,ct_E$ in the ground's rest frame.
The problem is, given a space-time diagram of these events, what order does the observer on the ground at $x=0$ see the lamps turn on? The lamps are at rest in the ground's rest frame, and so is the observer.
Intuition leads to three possible interpretations, and I'm not sure which is correct.
What order does the line $y=t$ intersect each of the lamps' events on the space-time diagram? That is, draw a horizontal line on the diagram, and move the line up, noting the order it reaches each event. This seems the simplest interpretation.
What order do the lines with gradient $-c$ intersect the $y$ axis of the space-time diagram for each event? This seems more logical, considering that the speed of light is finite, and the observer has to wait for the light from the lamp to reach them.
- The same as 2), but with lines of +c from the line $x=0$.