Einstein showed in his Theory of Special Relativity that two observers cannot agree on simultaneous events. This can be understood by this diagram. On the left, a train carriage is shown with a person, Alice, inside. Alice turns on a light in the middle of the carriage and observes light beams arriving at the two ends of the carriage at the same time, T2. On the right we see the scenario from Bob's point on view on the platform as the train moves past with velocity v. He sees the two light beams emitted at the same time, just like Alice. However, because the train is moving to the right, the rear of the train intercepts the leftward light first, at time T1 < T2. Meanwhile, the light takes a little longer to strike the front of the train, which it does at time T3 > T1. So from Bob's perspective, the events which Alice saw to be simultaneous occur one after the other.