Revisiting the lesson of the "Relativity of Simultaneity" So, we have the "relativity of simultaneity", which is meant to show us that two relatively moving observers have different notions of the concept of "time", because (as in chapter 9 of Relativity), the person on the moving train will witness the lightning flashes in a different sequence than the person who is standing on the embankment. (The one who is on the train sees the forward flash before the rear flash, while the one who is on the embankment sees both flashes at the same moment.) The problem with this logic is that two relatively stationary observers (i.e. they exist within the same reference-frame) at different locations on the embankment may also witness lightning flashes in a different sequence. Say the strikes occur 100 meters apart, and each observer is located one meter away from each strike location. Well, it stands to reason, that, given any non-infinite, constant value for the speed of the propagation of light, each person will say that the flash event to which he is standing the closest happens before the other flash event. The point that I am trying to make here is that Einstein's thought experiment, used to demonstrate the different "meanings of time" for two relatively moving observers, does not ultimately make use of the relative motions between observers, but rather of the relative positions between observers and events (such as lightning strikes). In other words, in Einstein's thought experiment, the guy on the train only witnesses a different sequence of events because he happens to be closer to the forward flash-event, than is the guy on the embankment, at the moment that the oncoming wavefront crosses his line of sight. We can also easily rig Einstein's scenario so that both guys see both flashes simultaneously, which also negates the intended lesson of the entire chapter. (I leave this as a challenge to you.) The only lesson that I've learned from all of this is not that motion dictates how one understands "time", but only position! Or, perhaps we should stop confusing the concepts of "temporal sequence" and "time"! Only the former concept lies within the realm of scientific scrutiny. The latter is purely a philosophical question. I particulary like Kant's definition of time: it is the form of the internal intuition. I would just say that time is given by nothing other than the sensation of continuousness. That is, time does not change: it only endures.