So, I think most of the folks here are familiar with Einstein's thought experiment that illustrates the relativity of simultaneity by using two lightning bolts and how the light reaches an observer on a moving train; that an observer on the ground observers the strikes simultaneously, but an observer on the train sees one before the other. But, something that kinda gets me about this, is this seems to imply that relativity of simultaneity only comes as a result of light-lag, rather than the events actually occurring at different times in the coordinate system. For example, if we're in a fluid, and two sound sources beep simultaneously, then an observer on the ground would hear them both at once, but an observer on a moving train would hear one before the other. This doesn't mean that simultaneity is relative based upon the speed of light of the fluid; it's merely a relic of the fact that sound has a finite speed. If the observer on the train properly takes this into account, then he'll realize that the two beeps really were simultaneous in his reference frame. So what sets the lightning bolt example apart? Light goes the same speed in every reference frame, true, but why does that make that difference? I do understand that it's not merely due to light-lag, but rather that SR truly effects how space and time flow; I'm just trying to make sense of the lightning bolt / train thought experiment in light of that knowledge (pun not intended, but relished nonetheless). Also for this reason, I do prefer the ladder-barn paradox to illustrate simultaneity rather than the lightning bolt illustration. Edit: To re-iterate and better illustrate; let's say you could teleport instantly to a given point. Now let's say we replace the lightning bolts with lights that are white, but then flash blue. Let's say someone argues; "Well, that doesn't mean that simultaneity is truly relative. As soon as you see the blue flash from the source in front of you, if you were to teleport to it, you would see it's white; ergo; you're merely seeing into the object's past because it took its light awhile to reach you; not because simultaneity is truly relative." While I know in reality, that relativity of simultaneity means that if you could teleport instantly to that object in your frame of reference; you'd arrive there and find it blue(?) (or rather, at least that less time passes for that object than if the above argument were correct). So what would be the correct response to the above argument?