quote from wikipedia: "According to the special theory of relativity, it is impossible to say in an absolute sense whether two distinct events occur at the same time if those events are separated in space, such as a car crash in London and another in New York. The question of whether the events are simultaneous is relative: in some reference frames the two accidents may happen at the same time, in other frames (in a different state of motion relative to the events) the crash in London may occur first, and in still other frames the New York crash may occur first. However, if the two events could be causally connected ("event A causes event B"), the causal order is preserved (i.e., "event A precedes event B") in all frames of reference" My sister is 2 years older than me and not born in the same place. Is there a frame where we are born at the same time ? my fiddling with lorentz transformations seems to say yes. But I must be wrong. How does the "causally connected" proviso prevent this?