Relativity - Relativity of Simultaneity
Concept: Two events that are simultaneous in one frame of reference are NOT simultaneous in a second frame that is moving relative to the first, even if both are inertial frames.
To make this concept more concrete, we will look at one of Einstein's "thought experiments." Imagine a train moving at a constant velocity comparable to the speed of light. Joe is sitting in the middle of the train car. Bob is standing on a station platform being bypassed by the train. Two lightning bolts strike either end of the train car in which Joe is sitting. This event occurs at the moment Bob is equidistant from both lightning bolts.
Bob measures both lightning strikes reaching him at the same time. Therefore he concludes that the lightning bolts struck simultaneously. Joe agrees that the bolts were seen by Bob at the same time but disagrees that they occurred simultaneously.
From Joe's perspective, the lightning bolt which struck in the direction the train was moving struck first. Joe concludes that the lightning strikes occurred at slightly different times. Whether or not two events at different x-axis locations are simultaneous depends on the state of motion of the observer.
Additionally, each observer is correct in his or her own frame of reference. There is no basis for stating that Joe or Bob is more correct. Whether two events are simultaneous depends on the frame of reference.