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We can now have a look at the most evident example about the following question: when two proceedings could be assumed contemporaneous. The graphic on the right shows a train have a fixed speed v. The green observer is motionless and observes this speed of the train v. At the same time the train is considered unmoving for the blue observer, who stays inside the train and he moves with a same velocity v. 
There is also a third observer, the orange one, (at the next graphic) outside the train, who runs to right (that is to say to the direction of train) and with a speed of w, which is greater than v.
At a specific moment all the observers lie at the same direction. That moment the blue sends two luminous signals, two fireworks, to the right and to the left. We should now try to observe the phenomena in slow motion. The fireworks have a speed of c, which is of course much greater than the speeds u and w. Let us see what these observers should think about the motion:
Blue Observer: I stay inside the train and I do not conceive movement. Even if the train moves, I move with it. The two fireworks have with the same speed, that of light and they reach simultaneously in points A and B.
Green Observer: The fireworks have the same speed and at the same time the train moves to the right. Therefore, I observe that the firework moving to the left reaches first the point A and after a while the other firework reaches point B. In other words, the firework, which travels to the left, has to cover a smaller distance in comparison to the other one, because in the meanwhile the train has covered a distance.
Orange Observer: My speed, w is greater than u thus what I conceive is the train moving to the left! Consequently, the two fireworks (as before), even though they have the same speed, they do not reach simultaneously the points A and B. Indeed the firework moving to the right reaches first the point B.
What could we conclude? Phenomena are not synchronous anymore. The concept "time period" is not finite. This is the basis of the Special Theory of Relativity.
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