I really think i'm not understanding this correctly and I haven't had a chance to think it through, but i'm confused about the issue of simultaneity in special relativity. As I understand it, a simultaneous event happens at exactly the same time in some reference frame. Special relativity says that a simultaneous does not have to be simultaneous in other reference frames. I think my issue is that I'm not sure how an observer determines when an event occurs. For example. consider two light bulbs on a line 100 meters apart, with observer A exactly half way between them, and observer B 1000 meters away from one bulb, and 1100 meters away from the other. Both are at rest, relative to the bulbs, and each other obviously. As I understand it, the flashing of the bulbs is a valid event, so suppose the bulbs are flashed so that observer A detects the flash at the same time. Then to observer A, the events are simultaneous. But to observer B, the events are then obviously not simultaneous, since there is a difference in distance from each bulb. Is it then valid to say that simultaneity is lost? even though A and B are in the same frame?