Is the relativity of simultaneity relative??? I've been thinking about this for a while and this is really troublesome... The short question: Why can’t we draw the conclusion that simultaneity is relative only when events involving light/electromagnetic radiation are considered? The explanation or the long question: In order to show that two events are relative, Einstein used the example of two lightning bolts occurring in the front, respectively in the back of a train. Alternatively, one can arrive at the same result by imagining that from a source placed in the centre of a train two light signals emerge towards the end of the train. In the reference frame of the train, the two light signals reach their destinations simultaneously, but from the reference frame of an external observer that watches the train passing by, the two events are not simultaneous (the light signal sent in the direction opposite to that in which the train is moving will arrive its destination first). However, the theory of relativity claims that all events simultaneous in one frame are not simultaneous in other frames, which move relatively to that frame. Thus, the fact that simultaneity of events is relative is not confined to events involving light, but – according to the special relativity – the simultaneity of all events, regardless of their nature, is relative. My question is what legitimates one to draw such a conclusion, since the relativity of simultaneity explicitly happens only to those events involving light (or other form of electromagnetic radiation). If, instead of light signals one chooses other kinds of signals (whose speed is not invariant), then one does not violate Galilean invariance, and hence, one does not obtain deviations from simultaneity, no matter how big the relative velocity of the considered frame. Why can’t we draw the conclusion that simultaneity is relative only when events involving light/electromagnetic radiation are considered? By no means do I question the validity of special relativity (I’m not that stupid), but I am stupid enough not to see why simultaneity is relative for all simultaneous events (like mechanical events, for example), not just for those involving light.