Here is a thought experiment for which I cannot find a good answer or explanation. Let us imagine that in the void and on a (very long) line there are two lights switched off in points A and C. Two observers are waiting in point B, at the same distance of A and C.
The experiment is such as at 12 PM in the reference frame of the line, the lights are switched on and off, so that they are both emitting a photon towards the point B. As the line is very long, one of the observer has enough time to move to a point B' on the line while the two photons are going towards the observers. After he has moved to point B', the observer stops and waits, and so now the two observers are both again at rest with respect to the reference frame of the line.
Finally, at some time, the photons reach point B. They cross at point B, as they were equally distant from this point when they left, and as they travel with constant speed. But they also reach, at some (other?) time point B', where they cross each other, as for the second observer they were equally distant from him when they left and as they travel with constant speed.
So we now have two photons crossing each other at some different places in the same reference frame of the line, which seems very weird given their trajectory. What truly happens?