# Emitting flash of light at relativistic speed

Let's say we have two observers, one stationary and the other moving at relativistic speeds. The observers agree to emit a flash of light when their clock reads 10 seconds. I think it should be possible to arrange it so that the stationary observer is in the same location as the moving observer when the stationary observer's clock reads 10 seconds (although I don't think that this should matter). It seems like each observer should release their light before they observe the light from the other person, but this doesn't really seem possible physically. If one of the observers sees the other person's light before they emit their own, then it breaks the symmetry of relativity.

Can anyone give me some physical intuition for this?

• Draw the spacetime diagram and all will be clear. – WillO Oct 21 '15 at 1:30
• HOw do you propose that the observers synchronize their clocks? They have to be at the same location simultaneously to synchronize them. – Bill N Oct 21 '15 at 2:48
• So the problem is that the assumption that the observers can be in the same place when one their clocks is at 10 seconds is wrong as the clocks can't be synchronized at the start? If they start at the same place, then the resolution is that both observers do flash before they observe the light of the other person due to the transit time of the light, right – vukov Oct 21 '15 at 3:22

Since the moving clock will go more slowly than the stationary clock, the stationary observer will emit the flash first; if they are "in the same location", the moving observer will receive the flash (Doppler shifted), and later issue his flash (when his clock reads 10 seconds. Doesn't seem to present a logical problem.

• But from the moving observer's perspective, he is stationary, so by this logic he should also issue his flash before he receives the other observer's light. If this were true, then the observers could tell who was stationary and who was moving, which I don't think should the case. – vukov Oct 20 '15 at 23:51
• The key concept here is not time dilation, but "the relativity of simultaneity". – dmckee Oct 21 '15 at 0:27
• The moving observer, though he may be moving in a straight line at a uniform speed, had to accelerate to get to that speed, and has experienced the effects of acceleration - thus he IS the moving observer. – Norm Oct 22 '15 at 1:20
• I don't think it is so simple. If the two observers are able to synchronize their clocks, then the two observers really are identical. In this case, maybe they are not but this argument is not so convincing. – vukov Oct 22 '15 at 19:41