The spacetime interval s between two events is s^2 = c^2*t^2 - x^2 where t is the time between the 2 events and x is the distance between the 2 events in a given frame of reference. What is the general condition on s such that two events cannot be simultaneous in any frame? I don't really understand the question.. What am i supposed to do? I mean...the shortest possible time between 2 events is 0, so picking a reference frame in which they are simultaneous, the spacetime interval between them would simply be the distance x between them or s = root (-x^2) which is not a real number....(even though I think the spacetie interval can be imaginary) So if the time between any 2 events is more than 0, then the spacetime interval would be greater than root (-x^2) since we'd be substracting -x^2 from a number larger than zero....so is the restriction that if s is less than root (-x^2), then 2 events can' tbe simultaneous in any frame? Can someone please tell me if im on the right track at all??