# Why is relativity of simultaneity so special?

I am currently learning about relativity of simultaneity right now, and I feel like this is a common phenomena that we experience every day (question at end). The example from Fundamentals of Physics by Halliday, Walker, and Resnick is as follows: "Suppose that one observer (Sam) notes that two independent events (event Red and event Blue) occur at the same time. Suppose also that another observer (Sally), who is moving at a constant velocity v with respect to Sam, also records these same two events. Will Sally also find that they occur at the same time?" Here is a picture of the problem below in which the two observers are Sam and Sally:

The answer to this question is that Sally will find that these two events do NOT occur at the same time. In a different scenario, instead of a light pulse being released as a result of the event occurring, a sound wave is released. Wouldn't Sam and Sally still record the same results as in the example above? What makes simultaneity so different in special relativity?

• The important part is that, after obtaining the best information and deducing backwards from all the observed facts, the two observers would disagree on whether two events happened at the same time or not. There is no such disagreement in old Newtonian physics. Sound waves are just not fundamental to the universe, and you could explain the discrepancy in sound waves by the properties of the medium in which sound waves travel. May 9 at 4:05
• I do not like this graphic. Is it really from HRW? It makes it sound as if the issue of simultaneity is a result of light waves chasing moving observers relative to stationary ones. The simultaneity issue works when light is not involved at all. The simultaneity issue is baked into the geometry of the universe. Light is not needed. There is an issue of light propagation, but that is different, and it is piled on top of the geometrical aspect. May 9 at 4:54
• @garyp I think the original problem did not include the light, it was just a question of mine. Perhaps I could have rephrased the question as, "Why is simultaneity unique to special relativity, do we not already see it in classical mechanics?" May 9 at 5:04
• Why is relativity of simultaneity so special? - Because it is Special relativity? ;) May 9 at 7:11
• It’s not at all “a common phenomenon that we experience every day”, because (unless we’re astronomers) we are never moving at a significant fraction of lightspeed relative to events in which we are interested. May 10 at 4:00